Process of wastewater treat. / Proces preciscavanja otpadnih voda

Wastewater Treatment

According to the Code of Federal Regulations (CFR) 40CFR Part 403, regulations were established in the late1970s and early 1980s to help publicly owned treatmentworks (POTW) control industrial discharges to sewers.These regulations were designed to prevent pass-throughand interference at the treatment plants and interferencein the collection and transmission systems.

Pass-through occurs when pollutants literally pass througha POTW without being properly treated, and cause thePOTW to have an effluent violation or increase the mag-nitude or duration of a violation.

Interference occurs when a pollutant discharge causes aPOTW to violate its permit by inhibiting or disruptingtreatment processes, treatment operations, or processesrelated to sludge use or disposal.

18.1 WASTEWATER OPERATORS

Like waterworks operators, wastewater operators arehighly trained and artful practitioners and technicians oftheir trade. Both operators are also required by the statesto be licensed or certified to operate a wastewater treat-ment plant.

When learning wastewater operator skills, there are anumber of excellent texts available to aid in the trainingprocess. Many of these texts are listed in Table 18.1.

18.1.1 THE WASTEWATER TREATMENT PROCESS: THE MODEL

Figure 18.1 shows a basic schematic of an example waste-water treatment process providing primary and secondarytreatment using the activated sludge process. This is themodel, prototype, and paradigm used in this book. Thoughit is true that in secondary treatment (which provides bio-chemical oxygen demand [BOD] removal beyond what isachievable by simple sedimentation), there are actuallythree commonly used approaches (trickling filter, acti-vated sludge, and oxidation ponds). For instructive andillustrative purposes, we focus on the activated sludgeprocess throughout this handbook. The purpose ofFigure 18.1 is to allow the reader to follow the treatmentprocess step-by-step as it is presented (and as it is actuallyconfigured in the real world) and to assist understandingof how all the various unit processes sequentially followand tie into each other.

We begin certain sections (which discuss unit processes)with frequent reference to Figure 18.1. It is important tobegin these sections in this manner because wastewatertreatment is a series of individual steps (unit processes)that treat the wastestream as it makes its way through theentire process. It logically follows that a pictorial presen-tation along with pertinent written information enhancesthe learning process. It should also be pointed out thateven though the model shown in Figure 18.1 does notinclude all unit processes currently used in wastewatertreatment, we do not ignore the other major processes:trickling filters, rotating biological contactors (RBCs), andoxidation ponds.

18.2 WASTEWATER TERMINOLOGY AND DEFINITIONS

Wastewater treatment technology, like many other techni-cal fields, has its own unique terms with their own meaning.Though some of the terms are unique, many are commonto other professions. Remember that the science of waste-water treatment is a combination of engineering, biology,mathematics, hydrology, chemistry, physics, and other dis-ciplines. Many of the terms used in engineering, biology,mathematics, hydrology, chemistry, physics, and othersare also used in wastewater treatment. Those terms notlisted or defined in the following section will be definedas they appear in the text.

18.2.1 TERMINOLOGY AND DEFINITIONS

Activated sludge the solids formed when micro-organisms are used to treat wastewater usingthe activated sludge treatment process. Itincludes organisms, accumulated food materi-als, and waste products from the aerobicdecomposition process.

Advanced waste treatment treatment technology usedto produce an extremely high quality discharge.

Aerobic conditions in which free, elemental oxygenis present. Also used to describe organisms,biological activity, or treatment processes thatrequire free oxygen.

Anaerobic conditions in which no oxygen (free orcombined) is available. Also used to describeorganisms, biological activity or treatment pro-cesses that function in the absence of oxygen.

18

© 2003 by CRC Press LLC

528

Handbook of Water and Wastewater Treatment Plant Operations

Anoxic conditions in which no free, elemental oxygenis present. The only source of oxygen is com-bined oxygen, such as that found in nitratecompounds. Also used to describe biologicalactivity of treatment processes that functiononly in the presence of combined oxygen.

Average monthly discharge limitation the highestallowable discharge over a calendar month.

Average weekly discharge limitation t he h ighes tallowable discharge over a calendar week.

Biochemical oxygen demand (BOD) the amount oforganic matter that can be biologically oxidized

under controlled conditions (5 days @ 20∞C inthe dark).

Biosolids (from 1977) solid organic matter recoveredfrom a sewage treatment process and used espe-cially as fertilizer (or soil amendment); usuallyused in plural (from Merriam-Webster’s Colle-giate Dictionary, 10th ed., 1998).

Note: In this text, biosolids is used in many places(activated sludge being the exception) toreplace the standard term sludge. The authorviews the term sludge as an ugly, inappropriatefour-letter word to describe biosolids. Biosolids

TABLE 18.1Recommended Reference and Study Material

1. Kerri, K.D. et al., Advanced Waste Treatment, A Field Study Program, 2nd ed., California State University, Sacramento, 1995.2. U.S. Environmental Protection Agency, Aerobic Biological Wastewater Treatment Facilities, EPA 430/9–77–006, Washington, D.C., 1977.3. U.S. Environmental Protection Agency, Anaerobic Sludge Digestion, EPA-430/9–76–001, Washington, D.C., 1977.4. American Society for Testing Materials, Section 11: Water and environmental technology, in Annual Book of ASTM Standards, Philadelphia, PA.5. Guidelines Establishing Test Procedures for the Analysis of Pollutants, Federal Register (40 CFR 136), April 4, 1995, Vol. 60, No. 64, p. 17160.6. HACH Chemical Company, Handbook of Water Analysis, 2nd ed., Loveland, CO, 1992.7. Kerri, K.D. et al., Industrial Waste Treatment: A Field Study Program, Vols. 1 and 2, California State University, Sacramento, CA, 1996.8. U.S. Environmental Protection Agency, Environmental Monitoring Systems Laboratory-Cincinnati, Methods for Chemical Analysis of Water and

Wastes, EPA-6000/4–79–020, revised March 1983 and 1979 (where applicable).9. Water Pollution Control Federation (now called Water Environment Federation), O & M of Trickling Filters, RBC and Related Processes, Manual

of Practice OM-10, Alexandria, VA, 1988.10. Kerri, K.D. et al., Operation of Wastewater Treatment Plants: A Field Study Program, Vols. 1 and 2, 4th ed., California State University,

Sacramento, 1993.11. American Public Health Association, American Water Works Association-Water Environment Federation, Standard Methods for the Examination

of Water and Wastewater, 18th ed., Washington, D.C., 1992.12. Kerri, K.D. et al., Treatment of Metal Wastestreams, 2nd ed., California State University, Sacramento, 1993.13. Price, J.K., Basic Math Concepts: For Water and Wastewater Plant Operators, Technomic Publ., Lancaster, PA, 1991.14. Haller, E., Simplified Wastewater Treatment Plant Operations, Technomic Publ., Lancaster, PA, 1999.15. Qaism, S.R., Wastewater Treatment Plants: Planning, Design, and Operation, Technomic Publ., Lancaster, PA, 1994.

Source: Spellman, F.R., Spellman’s Standard Handbook for Wastewater Operators, Vol. 1, Technomic Publ., Lancaster, PA, 1999.

FIGURE 18.1 Schematic of an example wastewater treatment process providing primary and secondary treatment using activated sludgeprocess. (From Spellman, F.R., Spellman’s Standard Handbook for Wastewater Operators, Vol. 1, Technomic Publ., Lancaster, PA, 1999.)

Sludgedisposal

Screenings

Influent

Grit

Sludgedewatering

Anaerobicdigester

Collectionsystem

Thickener

Screening andcomminution

Aeration Chlorinecontact tank

Activated sludge

Gritchamber

Primarysettling

Secondarysettling

Primary treatment Secondary treatment

Chlorine Effluent Air



529

is a product that can be reused; it has somevalue. Because biosolids has value, it certainlyshould not be classified as a waste product, andwhen biosolids for beneficial reuse isaddressed, it is made clear that it is not.

Buffer a substance or solution which resists changesin pH.

Carbonaceous biochemical oxygen demand (CBOD5)the amount of biochemical oxygen demand thatcan be attributed to carbonaceous material.

Chemical oxygen demand (COD) the amount ofchemically oxidizable materials present in thewastewater.

Clarifier a device designed to permit solids to settleor rise and be separated from the flow. Alsoknown as a settling tank or sedimentation basin.

Coliform a type of bacteria used to indicate possiblehuman or animal contamination of water.

Combined sewer a collection system that carries bothwastewater and storm water flows.

Comminution a process that shreds solids intosmaller, less harmful particles.

Composite sample a combination of individual sam-ples taken in proportion to flow.

Daily discharge the discharge of a pollutant measuredduring a calendar day or any 24-h period thatreasonably represents a calendar day for thepurposes of sampling. Limitations expressed asweight is total mass (weight) discharged overthe day. Limitations expressed in other units areaverage measurements of the day.

Daily maximum discharge the highest allowable val-ues for a daily discharge.

Detention time the theoretical time water remains ina tank at a given flow rate.

Dewatering the removal or separation of a portion ofwater present in a sludge or slurry.

Discharge monitoring report (DMR) the monthlyreport required by the treatment plant’sNational Pollutant Discharge Elimination Sys-tem (NPDES) discharge permit.

Dissolved oxygen (DO) free or elemental oxygen thatis dissolved in water.

Effluent the flow leaving a tank, channel, or treatmentprocess.

Effluent limitation any restriction imposed by theregulatory agency on quantities, dischargerates, or concentrations of pollutants that aredischarged from point sources into state waters.

Facultative organisms that can survive and functionin the presence or absence of free, elementaloxygen.

Fecal coliform a type of bacteria found in the bodilydischarges of warm-blooded animals. Used asan indicator organism.

Floc solids which join together to form larger particleswhich will settle better.

Flume a flow rate measurement device.Food-to-microorganism ratio (F:M) an act ivated

sludge process control calculation based uponthe amount of food (BOD or COD) availableper pound of mixed liquor volatile suspendedsolids.

Grab sample an individual sample collected at a ran-domly selected time.

Grit heavy inorganic solids such as sand, gravel, eggshells, or metal filings.

Industrial wastewater wastes associated with indus-trial manufacturing processes.

Infiltration/inflow extraneous flows in sewers; sim-ply, inflow is water discharged into sewer pipesor service connections from such sources asfoundation drains, roof leaders, cellar and yardarea drains, cooling water from air conditioners,and other clean-water discharges from commer-cial and industrial establishments. Defined byMetcalf & Eddy as follows:1

• Infiltration water entering the collectionsystem through cracks, joints, or breaks.

• Steady inflow water discharged from cellarand foundation drains, cooling water dis-charges, and drains from springs andswampy areas. This type of inflow is steadyand is identified and measured along withinfiltration.

• Direct flow those types of inflow that havea direct stormwater runoff connection to thesanitary sewer and cause an almost immedi-ate increase in wastewater flows. Possiblesources are roof leaders, yard and areawaydrains, manhole covers, cross connectionsfrom storm drains and catch basins, andcombined sewers.

• Total inflow the sum of the direct inflow atany point in the system plus any flow dis-charged from the system upstream throughoverflows, pumping station bypasses, andthe like.

• Delayed inflow stormwater that may requireseveral days or more to drain through thesewer system. This category can include thedischarge of sump pumps from cellar drain-age as well as the slowed entry of surfacewater through manholes in ponded areas.

Influent the wastewater entering a tank, channel, ortreatment process.


530


Inorganic mineral materials such as salt, ferric chlo-ride, iron, sand, gravel, etc.

License a certificate issued by the state board of water-works or wastewater works operators authorizingthe holder to perform the duties of a wastewatertreatment plant operator.

Mean cell residence time (MCRT) the average lengthof time a mixed liquor suspended solids particleremains in the activated sludge process. Mayalso be known as sludge retention time.

Mixed liquor the combination of return activatedsludge and wastewater in the aeration tank.

Mixed liquor suspended solids (MLSS) the suspend-ed solids concentration of the mixed liquor.

Mixed liquor volatile suspended solids (MLVSS) theconcentration of organic matter in the mixedliquor suspended solids.

Milligrams/Liter (mg/L) a measure of concentration.It is equivalent to parts per million.

National Pollutant Discharge Elimination System permit permit that authorizes the discharge oftreated wastes and specifies the condition,which must be met for discharge.

Nitrogenous oxygen demand (NOD) a measure ofthe amount of oxygen required to biologicallyoxidize nitrogen compounds under specifiedconditions of time and temperature.

Nutrients substances required to support living organ-isms. Usually refers to nitrogen, phosphorus,iron, and other trace metals.

Organic materials that consist of carbon, hydrogen,oxygen, sulfur, and nitrogen. Many organics arebiologically degradable. All organic com-pounds can be converted to carbon dioxide andwater when subjected to high temperatures.

Pathogenic disease causing. A pathogenic organism iscapable of causing illness.

Point source any discernible, defined, and discreteconveyance from which pollutants are or maybe discharged.

Part per million (ppm) an alternative (but numericallyequivalent) unit used in chemistry is milligramsper liter. As an analogy, think of this unit asbeing equivalent to a full shot glass in a swim-ming pool.

Return activated sludge solids (RASS) the concen-tration of suspended solids in the sludge flowbeing returned from the settling tank to the headof the aeration tank.

Sanitary wastewater wastes discharged from resi-dences and from commercial, institutional, andsimilar facilities that include both sewage andindustrial wastes.

Scum the mixture of floatable solids and water that isremoved from the surface of the settling tank.

Septic a wastewater that has no dissolved oxygenpresent. Generally characterized by black colorand rotten egg (hydrogen sulfide) odors.

Settleability a process control test used to evaluate thesettling characteristics of the activated sludge.Readings taken at 30 to 60 min are used tocalculate the settled sludge volume and thesludge volume index.

Settled sludge volume (SSV) the volume in percentoccupied by an activated sludge sample after30 to 60 minutes of settling. Normally writtenas SSV with a subscript to indicate the time ofthe reading used for calculation (SSV60) or(SSV30).

Sewage wastewater containing human wastes.Sludge the mixture of settleable solids and water that

is removed from the bottom of the settling tank.Sludge retention time (SRT) see mean cell residence

time.Sludge volume index (SVI) a process control calcu-

lation that is used to evaluate the settling qualityof the activated sludge. Requires the SSV30 andmixed liquor suspended solids test results tocalculate.

Storm sewer a collection system designed to carryonly storm water runoff.

Storm water runoff resulting from rainfall and snow-melt.

Supernatant the amber-colored liquid above thesludge that is in a digester.

Wastewater the water supply of the community afterit has been soiled by use.

Waste activated sludge solids (WASS) the concentra-tion of suspended solids in the sludge, which isbeing removed from the activated sludge process.

Weir a device used to measure wastewater flow.Zoogleal slime the biological slime which forms on

fixed film treatment devices. It contains a widevariety of organisms essential to the treatmentprocess.

18.3 MEASURING PLANT PERFORMANCE

To evaluate how well a plant or treatment unit process isoperating, performance efficiency or percent removal isused. The results can be compared with those listed in theplant’s operation and maintenance manual (O & M) todetermine if the facility is performing as expected. In thischapter sample calculations often used to measure plantperformance and efficiency are presented.



531

18.3.1 PLANT PERFORMANCE AND EFFICIENCY

Note: The calculation used for determining the per-formance (percent removal) for a digester isdifferent from that used for performance (per-cent removal) for other processes. Care must betaken to select the right formula

The following equation is used to determine plant perfor-mance and efficiency:

EXAMPLE 18.1

Problem:

The influent BOD is 247 mg/L and the plant effluent BODis 17 mg/L. What is the percent removal?

Solution:

18.3.2 UNIT PROCESS PERFORMANCE AND EFFICIENCY

Equation 18.1 is used again to determine unit process effi-ciency. The concentration entering the unit and the con-centration leaving the unit (i.e., primary, secondary, etc.)are used to determine the unit performance.

EXAMPLE 18.2

Problem:

The primary influent BOD is 235 mg/L and the primaryeffluent BOD is 169 mg/L. What is the percent removal?

18.3.3 PERCENT VOLATILE MATTER REDUCTION IN SLUDGE

The calculation used to determine percent volatile matter(%VM) reduction is more complicated because of thechanges occurring during sludge digestion:

(18.2)

EXAMPLE 18.3

Problem:

Using the digester data provided below, determine thepercent volatile matter reduction for the digester.

Data:

Raw sludge volatile matter = 74%

Digested sludge volatile matter = 54%

18.4 HYDRAULIC DETENTION TIME

The term detention time (DT) or hydraulic detention time(HDT) refers to the average length of time (theoreticaltime) a drop of water, wastewater, or suspended particlesremains in a tank or channel. It is calculated by dividingthe water or wastewater in the tank by the flow rate throughthe tank. The units of flow rate used in the calculation aredependent on whether the detention time is to be calcu-lated in seconds, minutes, hours or days. Detention timeis used in conjunction with various treatment processes,including sedimentation and coagulation and flocculation.

Generally, in practice, detention time is associatedwith the amount of time required for a tank to empty. Therange of detention time varies with the process. For exam-ple, in a tank used for sedimentation, detention time iscommonly measured in minutes.

The calculation methods used to determine detentiontime are illustrated in the following sections.

18.4.1 DETENTION TIME IN DAYS

Use Equation 18.3 to calculate the detention time in days:

(18.3)

% Removal

Influent Concentration Effluent Concentration 100

Influent Concentration

=

- ¥[ ]

(18.1)

%

%

Removal247 mg L 1 mg L 100

47 mg L=

- ¥

=

[ ]7

2

93

%

%

Removal235 mg L 1 9 mg L 100

mg L=

- ¥

=

[ ]6

235

28

%

% %

% % %

VM Reduction

VM VM

VM VM VMin out

in in out

=

-[ ] ¥

- ¥( )[ ]100

%.

.

%

VM Reduction.54

.74 .54=

- ¥- ¥

=

[ ]( )[ ]

0 74 0 100

0 74 0 0

59

HDT d Tank Volume ft 7.48 gal ft

Q gal d

3 3

( ) =( ) ¥( )


532 Handbook of Water and Wastewater Treatment Plant Operations

EXAMPLE 18.4

Problem:

An anaerobic digester has a volume of 2,400,000 gal.What is the detention time in days when the influent flowrate is 0.07 MGD?

Solution:

18.4.2 DETENTION TIME IN HOURS

(18.4)

EXAMPLE 18.5

Problem:

A settling tank has a volume of 44,000 ft.3 What is thedetention time in hours when the flow is 4.15 MGD?

18.4.3 DETENTION TIME IN MINUTES

EXAMPLE 18.6

Problem:

A grit channel has a volume of 1340 ft.3 What is thedetention time in minutes when the flow rate is 4.3 MGD?

Solution:

Note: The tank volume and the flow rate must be inthe same dimensions before calculating thehydraulic detention time.

18.5 WASTEWATER SOURCES AND CHARACTERISTICS

Wastewater treatment is designed to use the natural puri-fication processes (self-purification processes of streamsand rivers) to the maximum level possible. It is alsodesigned to complete these processes in a controlled envi-ronment rather than over many miles of a stream or river.Moreover, the treatment plant is also designed to removeother contaminants that are not normally subjected tonatural processes, as well as treating the solids that aregenerated through the treatment unit steps. The typicalwastewater treatment plant is designed to achieve manydifferent purposes:

1. Protect public health.2. Protect public water supplies.3. Protect aquatic life.4. Preserve the best uses of the waters.5. Protect adjacent lands.

Wastewater treatment is a series of steps. Each of thesteps can be accomplished using one or more treatmentprocesses or types of equipment. The major categories oftreatment steps are:

1. Preliminary treatment — Removes materials thatcould damage plant equipment or would occupytreatment capacity without being treated.

2. Primary treatment — Removes settleable andfloatable solids (may not be present in all treat-ment plants).

3. Secondary treatment — Removes BOD and dis-solved and colloidal suspended organic matter bybiological action. Organics are converted to sta-ble solids, carbon dioxide and more organisms.

4. Advanced waste treatment — Uses physical,chemical, and biological processes to removeadditional BOD, solids and nutrients (notpresent in all treatment plants).

5. Disinfection — Removes microorganisms toeliminate or reduce the possibility of diseasewhen the flow is discharged.

6. Sludge treatment — Stabilizes the solidsremoved from wastewater during treatment,inactivates pathogenic organisms, and reducesthe volume of the sludge by removing water.

The various treatment processes described above arediscussed in detail later.

DT dgal

d

0.07 MGD 1,000,000 gal MG

( ) =¥

=

2 400 000

34

, ,

HDT h

Tank Volume ft 7.48 gal ft h d

Q gal d

3 3

( ) =

( ) ¥ ¥

( ) 24

DT h44,000 ft 7.48 gal ft h d

4.15 MGD 1,000,000 gal MG

h

3 3

( ) =¥ ¥

¥

=

24

1 9.

HDT min

Tank Volume ft 7.48 gal ft min d

Q gal d

3 3

( ) =

( ) ¥ ¥

( ) 1440

(18.5)

DT min1340 ft 7.48 gal ft min d

4,300,000 gal d

3 3

( ) =¥ ¥

=

1440

3 36. min


Wastewater Treatment 533

18.5.1 WASTEWATER SOURCES

The principal sources of domestic wastewater in a com-munity are the residential areas and commercial districts.Other important sources include institutional and recre-ational facilities and storm water (runoff) and groundwater(infiltration). Each source produces wastewater with specificcharacteristics. In this section wastewater sources and thespecific characteristics of wastewater are described.

18.5.1.1 Generation of Wastewater

Wastewater is generated by five major sources: human andanimal wastes, household wastes, industrial wastes, stormwater runoff, and groundwater infiltration.

1. Human and animal wastes — Contains the solidand liquid discharges of humans and animals andis considered by many to be the most dangerousfrom a human health viewpoint. The primaryhealth hazard is presented by the millions ofbacteria, viruses, and other microorganisms(some of which may be pathogenic) present inthe wastestream.

2. Household wastes — Consists of wastes, otherthan human and animal wastes, discharged fromthe home. Household wastes usually containpaper, household cleaners, detergents, trash,garbage, and other substances the homeownerdischarges into the sewer system.

3. Industrial wastes — Includes industry specificmaterials that can be discharged from industrialprocesses into the collection system. Typicallycontains chemicals, dyes, acids, alkalis, grit,detergents, and highly toxic materials.

4. Storm water runoff — Many collection systemsare designed to carry both the wastes of thecommunity and storm water runoff. In this typeof system when a storm event occurs, the waste-stream can contain large amounts of sand,gravel, and other grit as well as excessiveamounts of water.

5. Groundwater infiltration — Groundwater willenter older improperly sealed collection sys-tems through cracks or unsealed pipe joints. Notonly can this add large amounts of water towastewater flows, but also additional grit.

18.5.2 CLASSIFICATION OF WASTEWATER

Wastewater can be classified according to the sources offlows: domestic, sanitary, industrial, combined, and stormwater.

1. Domestic (sewage) wastewater — Containsmainly human and animal wastes, household

wastes, small amounts of groundwater infiltra-tion and small amounts of industrial wastes.

2. Sanitary wastewater — Consists of domesticwastes and significant amounts of industrialwastes. In many cases, the industrial wastes canbe treated without special precautions. How-ever, in some cases, the industrial wastes willrequire special precautions or a pretreatmentprogram to ensure the wastes do not cause com-pliance problems for the wastewater treatmentplant.

3. Industrial wastewater — Consists of industrialwastes only. Often the industry will determinethat it is safer and more economical to treat itswaste independent of domestic waste.

4. Combined wastewater — Consists of a combi-nation of sanitary wastewater and storm waterrunoff. All the wastewater and storm water ofthe community is transported through one sys-tem to the treatment plant.

5. Storm water — Contains a separate collectionsystem (no sanitary waste) that carries stormwater runoff including street debris, road salt,and grit.

18.5.3 WASTEWATER CHARACTERISTICS

Wastewater contains many different substances that canbe used to characterize it. The specific substances andamounts or concentrations of each will vary, dependingon the source. It is difficult to precisely characterize waste-water. Instead, wastewater characterization is usuallybased on and applied to an average domestic wastewater.

Note: Keep in mind that other sources and typesof wastewater can dramatically change thecharacteristics.

Wastewater is characterized in terms of its physical,chemical, and biological characteristics.

18.5.3.1 Physical Characteristics

The physical characteristics of wastewater are based oncolor, odor, temperature, and flow.

1. Color — Fresh wastewater is usually a lightbrownish-gray color. However, typical waste-water is gray and has a cloudy appearance. Thecolor of the wastewater will change signifi-cantly if allowed to go septic (if travel time inthe collection system increases). Typical septicwastewater will have a black color.

2. Odor — Odors in domestic wastewater usuallyare caused by gases produced by the decompo-sition of organic matter or by other substances



added to the wastewater. Fresh domestic waste-water has a musty odor. If the wastewater isallowed to go septic, this odor will significantlychange to a rotten egg odor associated with theproduction of hydrogen sulfide (H2S).

3. Temperature — the temperature of wastewateris commonly higher than that of the water sup-ply because of the addition of warm water fromhouseholds and industrial plants. However, sig-nificant amounts of infiltration or storm waterflow can cause major temperature fluctuations.

4. Flow — the actual volume of wastewater iscommonly used as a physical characterizationof wastewater and is normally expressed interms of gallons per person per day. Most treat-ment plants are designed using an expected flowof 100 to 200 gallons per person per day. Thisfigure may have to be revised to reflect thedegree of infiltration or storm flow the plantreceives. Flow rates will vary throughout theday. This variation, which can be as much as50 to 200% of the average daily flow is knownas the diurnal flow variation.

Note: Diurnal means occurring in a day or daily.

18.5.3.2 Chemical Characteristics

In describing the chemical characteristics of wastewater,the discussion generally includes topics such as organicmatter, the measurement of organic matter, inorganic mat-ter, and gases. For the sake of simplicity, in this handbookwe specifically describe chemical characteristics in termsof alkalinity, BOD, chemical oxygen demand (COD), dis-solved gases, nitrogen compounds, pH, phosphorus, solids(organic, inorganic, suspended, and dissolved solids), andwater.

1. Alkalinity — This is a measure of the waste-water’s capability to neutralize acids. It is mea-sured in terms of bicarbonate, carbonate, andhydroxide alkalinity. Alkalinity is essential tobuffer (hold the neutral pH) of the wastewaterduring the biological treatment processes.

2. Biochemical oxygen demand — This is a mea-sure of the amount of biodegradable matter inthe wastewater. Normally measured by a 5-d testconducted at 20∞C. The BOD5 domestic wasteis normally in the range of 100 to 300 mg/L.

3. Chemical oxygen demand — This is a measureof the amount of oxidizable matter present inthe sample. The COD is normally in the rangeof 200 to 500 mg/L. The presence of industrialwastes can increase this significantly.

4. Dissolved gases — These are gases that aredissolved in wastewater. The specific gases andnormal concentrations are based upon the com-position of the wastewater. Typical domesticwastewater contains oxygen in relatively lowconcentrations, carbon dioxide, and hydrogensulfide (if septic conditions exist).

5. Nitrogen compounds — The type and amountof nitrogen present will vary from the rawwastewater to the treated effluent. Nitrogen fol-lows a cycle of oxidation and reduction. Mostof the nitrogen in untreated wastewater will bein the forms of organic nitrogen and ammonianitrogen. Laboratory tests exist for determinationof both of these forms. The sum of these twoforms of nitrogen is also measured and is knownas total kjeldahl nitrogen (TKN). Wastewaterwill normally contain between 20 to 85 mg/L ofnitrogen. Organic nitrogen will normally be inthe range of 8 to 35 mg/L, and ammonia nitro-gen will be in the range of 12 to 50 mg/L.

6. pH — This is a method of expressing the acidcondition of the wastewater. pH is expressed ona scale of 1 to 14. For proper treatment, waste-water pH should normally be in the range of6.5 to 9.0 (ideally 6.5 to 8.0).

7. Phosphorus — This element is essential to bio-logical activity and must be present in at leastminimum quantities or secondary treatmentprocesses will not perform. Excessive amountscan cause stream damage and excessive algalgrowth. Phosphorus will normally be in therange of 6 to 20 mg/L. The removal of phos-phate compounds from detergents has had asignificant impact on the amounts of phospho-rus in wastewater.

8. Solids — Most pollutants found in wastewatercan be classified as solids. Wastewater treatmentis generally designed to remove solids or to con-vert solids to a form that is more stable or canbe removed. Solids can be classified by theirchemical composition (organic or inorganic) orby their physical characteristics (settleable,floatable, and colloidal). Concentration of totalsolids in wastewater is normally in the range of350 to 1200 mg/L.

A. Organic solids — Consists of carbon, hydro-gen, oxygen, nitrogen and can be convertedto carbon dioxide and water by ignition at550∞C. Also known as fixed solids or losson ignition.

B. Inorganic solids — Mineral solids that areunaffected by ignition. Also known as fixedsolids or ash.



C. Suspended solids — These solids will notpass through a glass fiber filter pad. Can befurther classified as Total suspended solids(TSS), volatile suspended solids, and fixedsuspended solids. Can also be separated intothree components based on settling charac-teristics: settleable solids, floatable solids,and colloidal solids. Total suspended solidsin wastewater are normally in the range of100 to 350 mg/L.

D. Dissolved solids — These solids will passthrough a glass fiber filter pad. Can also beclassified as total dissolved solids (TDS),volatile dissolved solids, and fixed dissolvedsolids. TDS are normally in the range of250 to 850 mg/L.

9. Water — This is always the major constituentof wastewater. In most cases water makes up99.5 to 99.9% of the wastewater. Even in thestrongest wastewater, the total amount of con-tamination present is less than 0.5% of the totaland in average strength wastes it is usually lessthan 0.1%.

18.5.3.3 Biological Characteristics and Processes

(Note: The biological characteristics of water were dis-cussed in detail earlier in this text.)

After undergoing physical aspects of treatment (i.e.,screening, grit removal, and sedimentation) in preliminaryand primary treatment, wastewater still contains some sus-pended solids and other solids that are dissolved in thewater. In a natural stream, such substances are a sourceof food for protozoa, fungi, algae, and several varieties ofbacteria. In secondary wastewater treatment, these samemicroscopic organisms (which are one of the main reasonsfor treating wastewater) are allowed to work as fast asthey can to biologically convert the dissolved solids tosuspended solids that will physically settle out at the endof secondary treatment.

Raw wastewater influent typically contains millionsof organisms. The majority of these organisms are non-pathogenic, but several pathogenic organisms may also bepresent. (These may include the organisms responsible fordiseases such as typhoid, tetanus, hepatitis, dysentery, gas-troenteritis, and others.)

Many of the organisms found in wastewater are micro-scopic (microorganisms); they include algae, bacteria,protozoa (e.g., amoeba, flagellates, free-swimming cili-ates, and stalked ciliates), rotifers, and viruses.

Table 18.2 is a summary of typical domestic waste-water characteristics.

18.6 WASTEWATER COLLECTION SYSTEMS

Wastewater collection systems collect and convey waste-water to the treatment plant. The complexity of the systemdepends on the size of the community and the type of systemselected. Methods of collection and conveyance of waste-water include gravity systems, force main systems, vacuumsystems, and combinations of all three types of systems.

18.6.1 GRAVITY COLLECTION SYSTEM

In a gravity collection system, the collection lines aresloped to permit the flow to move through the system withas little pumping as possible. The slope of the lines mustkeep the wastewater moving at a velocity (speed) of 2 to4 ft/sec. Otherwise, at lower velocities, solids will settleout and cause clogged lines, overflows, and offensiveodors. To keep collection systems lines at a reasonabledepth, wastewater must be lifted (pumped) periodically sothat it can continue flowing downhill to the treatmentplant. Pump stations are installed at selected points withinthe system for this purpose.

18.6.2 FORCE MAIN COLLECTION SYSTEM

In a typical force main collection system, wastewater iscollected to central points and pumped under pressure tothe treatment plant. The system is normally used for con-veying wastewater long distances. The use of the forcemain system allows the wastewater to flow to the treatmentplant at the desired velocity without using sloped lines. Itshould be noted that the pump station discharge lines ina gravity system are considered to be force mains sincethe content of the lines is under pressure.

TABLE 18.2Typical Domestic Wastewater Characteristics

Characteristic Typical Characteristic

Color GrayOdor MustyDO >1.0 mg/LpH 6.5–9.0TSS 100–350 mg/LBOD 100–300 mg/LCOD 200–500 mg/LFlow 100–200 gal/person/dTotal nitrogen 20–85 mg/LTotal phosphorus 6–20 mg/LFecal coliform 500,000–3,000,000 MPN/100 mL

Source: Spellman, F.R., Spellman’s Standard Handbookfor Wastewater Operators, Vol. 1, Technomic Publ., Lan-caster, PA, 1999.



Note: Extra care must be taken when performingmaintenance on force main systems since thecontent of the collection system is under pressure.

18.6.3 VACUUM SYSTEM

In a vacuum collection system, wastewaters are collectedto central points and then drawn toward the treatment plantunder vacuum. The system consists of a large amount ofmechanical equipment and requires a large amount ofmaintenance to perform properly. Generally, the vacuum-type collection systems are not economically feasible.

18.6.4 PUMPING STATIONS

Pumping stations provide the motive force (energy) tokeep the wastewater moving at the desired velocity. Theyare used in both the force main and gravity systems. Theyare designed in several different configurations and mayuse different sources of energy to move the wastewater(i.e., pumps, air pressure or vacuum). One of the morecommonly used types of pumping station designs is thewet well/dry well design.

18.6.4.1 Wet Well–Dry Well Pumping Stations

The wet well–dry well pumping station consists of twoseparate spaces or sections separated by a common wall.Wastewater is collected in one section (known as the wetwell section); the pumping equipment (and in many cases,the motors and controllers) is located in a second sectionknown as the dry well. There are many different designs forthis type of system, but in most cases the pumps selectedfor this system are of a centrifugal design. There are a coupleof major considerations in selecting centrifugal design:

1. This design allows for the separation ofmechanical equipment (pumps, motors, con-trollers, wiring, etc.) from the potentially cor-rosive atmosphere (sulfides) of the wastewater.

2. This type of design is usually safer for workersbecause they can monitor, maintain, operate,and repair equipment without entering thepumping station wet well.

Note: Most pumping station wet wells are confinedspaces. To ensure safe entry into such spaces,compliance with Occupational Safety andHealth Administration’s 29 CFR 1910.146(Confined Space Entry Standard) is required.

18.6.4.2 Wet Well Pumping Stations

Another type of pumping station design is the wet welltype. This type consists of a single compartment that col-lects the wastewater flow. The pump is submerged in thewastewater with motor controls located in the space or

has a weatherproof motor housing located above the wetwell. In this type of station, a submersible centrifugalpump is normally used.

18.6.4.3 Pneumatic Pumping Stations

The pneumatic pumping station consists of a wet well anda control system that controls the inlet and outlet valueoperations and provides pressurized air to force or pushthe wastewater through the system. The exact method ofoperation depends on the system design. When operating,wastewater in the wet well reaches a predetermined leveland activates an automatic valve that closes the influentline. The tank (wet well) is then pressurized to a predeter-mined level. When the pressure reaches the predeterminedlevel, the effluent line valve is opened and the pressurepushes the wastestream out the discharge line.

18.6.4.4 Pumping Station Wet Well Calculations

Calculations normally associated with pumping stationwet well design (determining design lift or pumpingcapacity, etc.) are usually left up to design and mechanicalengineers. However, on occasion, wastewater operators orinterceptor’s technicians may be called upon to make cer-tain basic calculations. Usually these calculations dealwith determining either pump capacity without influent(e.g., to check the pumping rate of the station’s constantspeed pump) or pump capacity with influent (e.g., to checkhow many gallons per minute the pump is discharging).In this section we use examples to describe instances onhow and where these two calculations are made.

EXAMPLE 18.7: DETERMINING PUMP CAPACITY WITHOUT INFLUENT

Problem:

A pumping station wet well is 10 ¥ 9 ft. The operatorneeds to check the pumping rate of the station’s constantspeed pump. To do this, the influent valve to the wet wellis closed for a 5-min test, and the level in the well dropped2.2 ft. What is the pumping rate in gallons per minute?

Solution:

Using the length and width of the well, we can find thearea of the water surface:

10 ft ¥ 9 ft = 90 ft2

The water level dropped 2.2 ft. From this we can find thevolume of water removed by the pump during the test:

A D v¥ =

¥ =90 2 2 198 ft ft ft2 .



One cubic foot of water holds 7.48 gal. We can convertthis volume in cubic feet to gallons:

The test was done for 5 min. From this information,a pumping rate can be calculated:

EXAMPLE 18.8: DETERMINING PUMP CAPACITY WITH INFLUENT

Problem:

A wet well is 8.2 ¥ 9.6 ft. The influent flow to the well,measured upstream, is 365 gal/min. If the wet well rises2.2 in. in 5 min, how many gallons per minute is the pumpdischarging?

Solution:

Influent = Discharge + Accumulation

We want to calculate the discharge. Influent is known andwe have enough information to calculate the accumulation.

Using Equation 18.7:

Subtracting from both sides:

The wet well pump is discharging 343.4 gal each minute.

18.7 PRELIMINARY TREATMENT

The initial stage in the wastewater treatment process (fol-lowing collection and influent pumping) is preliminarytreatment. Raw influent entering the treatment plant maycontain many kinds of materials (trash). The purpose ofpreliminary treatment is to protect plant equipment byremoving these materials that could cause clogs, jams, orexcessive wear to plant machinery. In addition, theremoval of various materials at the beginning of the treat-ment process saves valuable space within the treatmentplant.

Preliminary treatment may include many differentprocesses. Each is designed to remove a specific type ofmaterial — a potential problem for the treatment process.Processes include: wastewater collections (influent pump-ing, screening, shredding, grit removal, flow measure-ment, preaeration, chemical addition, and flow equaliza-tion). The major processes are shown in Figure 18.1. Inthis section, we describe and discuss each of these pro-cesses and their importance in the treatment process.

Note: As mentioned, not all treatment plants willinclude all of the processes shown in Figure 18.1.Specific processes have been included to facil-itate discussion of major potential problemswith each process and its operation; this isinformation that may be important to the waste-water operator.

18.7.1 SCREENING

The purpose of screening is to remove large solids, suchas rags, cans, rocks, branches, leaves, roots, etc., from theflow before the flow moves on to downstream processes.

Note: Typically, a treatment plant will remove any-where from 0.5 to 12 ft3 of screenings for eachmillion gallons of influent received.

A bar screen traps debris as wastewater influent passesthrough. Typically, a bar screen consists of a series ofparallel, evenly spaced bars or a perforated screen placedin a channel (see Figure 18.2). The wastestream passesthrough the screen and the large solids (screenings) aretrapped on the bars for removal.

Note: The screenings must be removed frequentlyenough to prevent accumulation that will blockthe screen and cause the water level in front ofthe screen to build up.

The bar screen may be coarse (2 to 4-in. openings) orfine (0.75 to 2.0-in. openings). The bar screen may bemanually cleaned (bars or screens are placed at an angleof 30∞ for easier solids removal; see Figure 18.2) ormechanically cleaned (bars are placed at 45∞ to 60∞ angleto improve mechanical cleaner operation).

198 148133

7.48 gal1 ft

ft gal¥ =

1481 gal5 min

gal min= 296 21

296 2.

min.

365 gal

1 minDischarge Accumulation= +

Volume accumulated

gal

ft 9.6 ft 2.2 in.

1 ft

12 in.

gal

1 ft gal

Accumulation108 gal

1 min

gal min

3

= ¥ ¥ ¥

¥ =

= =

=

8 2

7 48108

5

21 6

21 6

.

.

min

.

.

Influent Discharge Accumulation

Discharge

= +

= +365 21 6gal min .

365 21 6

21 6 21 6

343 4

gal gal

gal

gal

min . min

. min .

. min

- =

+ -

=

Discharge gal min

Discharge



The screening method employed depends on thedesign of the plant, the amount of solids expected, andwhether the screen is for constant or emergency use only.

18.7.1.1 Manually Cleaned Screens

Manually cleaned screens are cleaned at least once pershift (or often enough to prevent buildup that may causereduced flow into the plant) using a long tooth rake. Solidsare manually pulled to the drain platform and allowed todrain before storage in a covered container.

The area around the screen should be cleaned fre-quently to prevent a buildup of grease or other materialsthat can cause odors, slippery conditions, and insect androdent problems. Because screenings may contain organicmatter as well as large amounts of grease they should bestored in a covered container. Screenings can be disposedof by burial in approved landfills or by incineration. Sometreatment facilities grind the screenings into small parti-cles; these particles are then returned to the wastewaterflow for further processing and removal later in the process.

18.7.1.1.1 Operational Problems

Manually cleaned screens require a certain amount ofoperator attention to maintain optimum operation. Failureto clean the screen frequently can lead to septic wastesentering the primary, surge flows after cleaning, and lowflows before cleaning. On occasion, when such opera-tional problems occur, it becomes necessary to increasethe frequency of the cleaning cycle. Another operationalproblem is excessive grit in the bar screen channel.Improper design or construction or insufficient cleaningmay cause this problem. The corrective action required iseither to correct the design problem or increase cleaningfrequency and flush the channel regularly. Another com-

mon problem with manually cleaned bar screens is theirtendency to clog frequently. This may be caused by exces-sive debris in the wastewater or the screen being too finefor its current application. The operator should locate thesource of the excessive debris and eliminate it. If thescreen is the problem, a coarser screen may need to beinstalled. If the bar screen area is filled with obnoxiousodors, flies, and other insects, it may be necessary todispose of screenings more frequently.

18.7.1.2 Mechanically Cleaned Screens

Mechanically cleaned screens use a mechanized rakeassembly to collect the solids and move them (carry them)out of the wastewater flow for discharge to a storage hop-per. The screen may be continuously cleaned or cleanedon a time or flow controlled cycle. As with the manuallycleaned screen, the area surrounding the mechanicallyoperated screen must be cleaned frequently to preventbuildup of materials, which can cause unsafe conditions.

As with all mechanical equipment, operator vigilanceis required to ensure proper operation and proper mainte-nance. Maintenance includes lubricating equipment andmaintaining it in accordance with manufacturer’s recom-mendations or the plant’s O & M manual.

Screenings from mechanically operated barscreens aredisposed of in the same manner as screenings from man-ually operated screens. These include landfill disposal,incineration, or the process of grinding into smaller par-ticles for return to the wastewater flow.

18.7.1.2.1 Operational ProblemsMany of the operational problems associated with mechan-ically cleaned bar screens are the same as those for manualscreens. These include septic wastes entering the primary,surge flows after cleaning, excessive grit in the bar screenchannel, and a screen that clogs frequently. Basically thesame corrective actions employed for manually operatedscreens would be applied for these problems in mechanicallyoperated screens. In addition to these problems, mechani-cally operated screens also have other problems. Theseinclude the cleaner failing to operate; and a nonoperatingrake, but operating motor. Obviously, these are mechanicalproblems that could be caused by jammed cleaning mech-anism, broken chain, broken cable, or a broken shear pin.Authorized and fully trained maintenance operators shouldbe called in to handle these types of problems.

18.7.1.3 Safety

The screening area is the first location where the operatoris exposed to the wastewater flow. Any toxic, flammableor explosive gases present in the wastewater can bereleased at this point. Operators who frequent enclosedbar screen areas should be equipped with personal airmonitors. Adequate ventilation must be provided. It is also

FIGURE 18.2 Bar screen. (From Spellman, F.R., Spellman’sStandard Handbook for Wastewater Operators, Vol. 1, Tech-nomic Publ., Lancaster, PA, 1999.)

Drain

Flow in



important to remember that, due to the grease attached tothe screenings this area of the plant can be extremelyslippery. Routine cleaning is required to minimize thisproblem.

Note: Never override safety devices on mechanicalequipment. Overrides can result in dangerousconditions, injuries, and major mechanicalfailure.

18.7.1.4 Screenings Removal Computations

Operators responsible for screenings disposal are typicallyrequired to keep a record of the amount of screeningsremoved from the wastewater flow. To keep and maintainaccurate screenings’ records, the volume of screeningswithdrawn must be determined. Two methods are commonlyused to calculate the volume of screenings withdrawn:

(18.6)

(18.7)

EXAMPLE 18.9

Problem:

A total of 65 gal of screenings are removed from thewastewater flow during a 24-h period. What is the screen-ings removal reported as cubic feet per day?

Solution:

First, convert gallons screenings to cubic feet:

Next, calculate screenings removed as cubic feet per day:

EXAMPLE 18.10

Problem:

During 1 week, a total of 310 gal of screenings wereremoved from the wastewater screens. What is the averagescreening removal in cubic feet per day?

Solution:

First, gallons screenings must be converted to cubic feetscreenings:

Next, calculate screenings removed as cubic feet per day:

18.7.2 SHREDDING

As an alternative to screening, shredding can be used toreduce solids to a size that can enter the plant withoutcausing mechanical problems or clogging. Shredding pro-cesses include comminution (comminute means cut up)and barminution devices.

18.7.2.1 Comminution

The comminutor is the most common shredding deviceused in wastewater treatment. In this device all the waste-water flow passes through the grinder assembly. Thegrinder consists of a screen or slotted basket, a rotatingor oscillating cutter, and a stationary cutter. Solids passthrough the screen and are chopped or shredded betweenthe two cutters. The comminutor will not remove solids,which are too large to fit through the slots, and it will notremove floating objects. These materials must be removedmanually.

Maintenance requirements for comminutors includealigning, sharpening and replacing cutters and correctiveand preventive maintenance performed in accordance withplant O & M manual.

18.7.2.1.1 Operational ProblemsCommon operational problems associated with comminu-tors include output containing coarse solids. When thisoccurs it is usually a sign that the cutters are dull ormisaligned. If the system does not operate at all, the unitis either clogged, jammed, a shear pin or coupling isbroken or electrical power is shut off. If the unit stalls orjams frequently, this usually indicates cutter misalign-ment, excessive debris in influent, or dull cutters.

Note: Only qualified maintenance operators shouldperform maintenance of shredding equipment.

18.7.2.2 Barminution

In barminution, the barminutor uses a bar screen to collectsolids that are shredded and passed through the bar screen

Screenings Removed ftScreenings ft

d3

3

d( ) =( )

Screenings Removed ftScreenings ft

Q MG3

3

MG( ) =( )

( )

65 gal

7.48 gal ft ft

3

3= 8 7. screenings

Screenings Removed ft8.7 ft

1 d

8.7 ft

33

3

d

d

( ) =

=

310 gal

7.48 gal ft ft

3

3= 41 4. screenings

Screenings Removed ft41.4 ft

7 d

5.9 ft

33

3

d

d

( ) =

=



for removal at a later process. In operation each device’scutter alignment and sharpness are critical factors in effec-tive operation. Cutters must be sharpened or replaced andalignment must be checked in accordance with manufac-turer’s recommendations. Solids, which are not shredded,must be removed daily, stored in closed containers, anddisposed of by burial or incineration.

Barminutor operational problems are similar to thoselisted above for comminutors. Preventive and correctivemaintenance as well as lubrication must be performed byqualified personnel and in accordance with the plant’sO & M manual. Because of higher maintenance require-ments the barminutor is less frequently used.

18.7.3 GRIT REMOVAL

The purpose of grit removal is to remove the heavy inor-ganic solids that could cause excessive mechanical wear.Grit is heavier than inorganic solids and includes, sand,gravel, clay, egg shells, coffee grounds, metal filings,seeds, and other similar materials.

There are several processes or devices used for gritremoval. All of the processes are based on the fact thatgrit is heavier than the organic solids, which should bekept in suspension for treatment in following processes.Grit removal may be accomplished in grit chambers or bythe centrifugal separation of sludge. Processes use gravityand velocity, aeration, or centrifugal force to separate thesolids from the wastewater.

18.7.3.1 Gravity and Velocity Controlled Grit Removal

Gravity and velocity controlled grit removal is normallyaccomplished in a channel or tank where the speed or thevelocity of the wastewater is controlled to about 1 footper second (ideal), so that grit will settle while organicmatter remains suspended. As long as the velocity is con-trolled in the range of 0.7 to 1.4 ft/sec the grit removalwill remain effective. Velocity is controlled by the amountof water flowing through the channel, the depth of thewater in the channel, the width of the channel, or thecumulative width of channels in service.

18.7.3.1.1 Process Control Calculations

Velocity of the flow in a channel can be determined eitherby the float and stopwatch method or by channel dimensions.

EXAMPLE 18.11: VELOCITY BY FLOAT AND STOP-

WATCH

Problem:

A float takes 25 sec to travel 34 ft in a grit channel. Whatis the velocity of the flow in the channel?

Solution:

EXAMPLE 18.12: VELOCITY BY FLOW AND CHANNEL DIMENSIONS

Note: This calculation can be used for a single chan-nel or tank or multiple channels or tanks withthe same dimensions and equal flow. If the flowthrough each unit of the unit dimensions isunequal, the velocity for each channel or tankmust be computed individually.

Problem:

The plant is currently using two grit channels. Each chan-nel is 3 ft wide and has a water depth of 1.2 ft. What isthe velocity when the influent flow rate is 3.0 MGD?

Solution:

Note: The channel dimensions must always be in feet.Convert inches to feet by dividing by 12 in./ft.

EXAMPLE 18.13: REQUIRED SETTLING TIME

Note: This calculation can be used to determine thetime required for a particle to travel from thesurface of the liquid to the bottom at a givensettling velocity. In order to compute the settlingtime, the settling velocity in feet per secondmust be provided or determined experimentallyin a laboratory.

Velocity, feet secondDistance Traveled, feet

Time Required, Seconds=

Vs

ft sec34 ft

25 ec

ft sec

( ) =

= 1 4.

Velocity, fps

Flow, MGD 1.55 cfs MGDChan. in Ser. Chan Width, ft Water D, ft

=

¥¥ ¥

#

VMGD

ft

ft sec3.0 MGD 1.55 ft

Channels 3 ft 1 ft

4.65 ft

ft sec

3

3

( ) =¥

¥ ¥

=

=

sec

.

sec

.

.

2 2

7 2

0 65

2



Problem:

The plant’s grit channel is designed to remove sand andhas a settling velocity of 0.085 ft/sec. The channel iscurrently operating at a depth of 2.2 ft. How many secondswill it take for a sand particle to reach the channel bottom?

Solution:

EXAMPLE 18.14: REQUIRED CHANNEL LENGTH

Note: This calculation can be used to determine thelength of channel required to remove an objectwith a specified settling velocity.

Problem:

The plant’s grit channel is designed to remove sand andhas a settling velocity of 0.070 ft/sec. The channel iscurrently operating at a depth of 3 ft. The calculatedvelocity of flow through the channel is 0.80 ft/sec. Thechannel is 35 ft long. Is the channel long enough to removethe desired sand particle size?

Solution:

Yes, the channel is long enough to ensure all of the sandwill be removed.

18.7.3.1.2 CleaningGravity type systems may be manually or mechanicallycleaned. Manual cleaning normally requires that the chan-nel be taken out of service, drained, and manually cleaned.Mechanical cleaning systems are operated continuouslyor on a time cycle. Removal should be frequent enoughto prevent grit carryover into the rest of the plant.

Note: Always ventilate the area thoroughly before andduring cleaning activities.

18.7.3.1.3 Operational Observations/ Problems/Troubleshooting

Gravity and velocity-controlled grit removal normallyoccurs in a channel or tank where the speed or the velocityof the wastewater is controlled to about 1 ft/sec (ideal), sothat grit settles while organic matters remains suspended.As long as the velocity is controlled in the range of 0.7 to1.4 ft/sec, the grit removal remains effective. Velocity iscontrolled by the amount of water flowing through the chan-nel, the depth of the water in the channel, by the width ofthe channel, or the cumulative width of channels in service.

During operation, the operator must pay particularattention to grit characteristics for evidence of organicsolids in the channel, for evidence of grit carryover intoplant, for evidence of mechanical problems, and for gritstorage and disposal (housekeeping).

Aerated grit removal systems use aeration to keep thelighter organic solids in suspension while allowing theheavier grit articles to settle out. Aerated grit removal maybe manually or mechanically cleaned; the majority of thesystems are mechanically cleaned.

During normal operation, adjusting the aeration rateproduces the desired separation. This requires observationof mixing and aeration and sampling of fixed suspendedsolids. Actual grit removal is controlled by the rate ofaeration. If the rate is too high, all of the solids remain insuspension. If the rate is too low, both grit and organicswill settle out.

The operator observes the same kinds of conditionsas those listed for the gravity and velocity-controlled sys-tem, but must also pay close attention to the air distributionsystem to ensure proper operation.

The cyclone degritter uses a rapid spinning motion(centrifugal force) to separate the heavy inorganic solidsor grit from the light organic solids. This unit process isnormally used on primary sludge rather than the entirewastewater flow. This critical control factor for the processis the inlet pressure. If the pressure exceeds the recom-mendations of the manufacturer, the unit will flood andgrit will carry through with the flow.

Grit is separated from flow, washed, and dischargeddirectly to a strange container. Grit removal performanceis determined by calculating the percent removal for inor-ganic (fixed) suspended solids.

The operator observes the same kinds of conditionslisted for the gravity and velocity-controlled and aeratedgrit removal systems, with the exception of the air distri-bution system.

Typical problems associated with grit removal includemechanical malfunctions and rotten egg odor in the gritchamber (hydrogen sulfide formation), which can lead tometal and concrete corrosion problems. Low recovery rateof grit is another typical problem. Bottom scour, over-aeration, or a lack of detention time normally causes this.

Settling Time, secondsLiquid Depth in Feet

Settling, Velocity, fps=

Settling Time sec.2 ft

.085 ft sec

sec

( ) =

=

2

0

25 9.

Required Channel Length

Channel Depth, ft Flow Velocity, fpsSettling Velocity, fps

=

¥

Required Channel Length ft ft sec

.070 ft sec

ft

( ) =¥

=

3 0 80

0

34 3

ft .

.



When these problems occur, the operator must make therequired adjustments or repairs to correct the problems.

18.7.3.2 Grit Removal Calculations

Wastewater systems typically average 1 to 15 ft3 ofgrit/MG of flow (sanitary systems average 1 to 4 ft3/MG;combined wastewater systems average from 4 to 15 ft3/MGof flow), with higher ranges during storm events.

Generally, grit is disposed of in sanitary landfills.Because of this practice, for planning purposes, operatorsmust keep accurate records of grit removal. Most often,the data is reported as cubic feet of grit removed permillion gallons of flow:

(18.8)

Over a given period, the average grit removal rate ata plant (at least a seasonal average) can be determined andused for planning purposes. Typically, grit removal is cal-culated as cubic yards because excavation is normallyexpressed in terms of cubic yards:

(18.9)

EXAMPLE 18.15

Problem:

A treatment plant removes 10 ft3 of grit in 1 d. How manycubic feet of grit are removed per million gallons if theplant flow was 9 MGD?

Solution:

EXAMPLE 18.16

Problem:

The total daily grit removed for a plant is 250 gal. If theplant flow is 12.2 MGD, how many cubic feet of grit areremoved per million gallons of flow?

Solution:

First, convert gallon grit removed to cubic feet:

Next, complete the calculation of cubic feet per milliongallons:

EXAMPLE 18.17

Problem:

The monthly average grit removal is 2.5 ft3/MGD. If themonthly average flow is 2,500,000 gal/d, how many cubicyards must be available for grit disposal pit to have a 90-d capacity?

Solution:

First, calculate the grit generated each day:

The cubic feet grit generated for 90 d would be:

Convert cubic feet grit to cubic yard grit:

18.7.4 PREAERATION

In the preaeration process (diffused or mechanical), weaerate wastewater to achieve and maintain an aerobic state(to freshen septic wastes), strip off hydrogen sulfide (toreduce odors and corrosion), agitate solids (to releasetrapped gases and improve solids separation and settling),and to reduce BOD. All of this can be accomplished byaerating the wastewater for 10 to 30 min. To reduce BOD,preaeration must be conducted from 45 to 60 min.

Grit Removed ftGrit Volume ft

MG3

3

MGQ

( ) =( )

( )

Grit Removal ydTotal Grit ft

7 ft3

3

3( ) =( )

( )2 3yd


MG

9 MGD

ft

3

3

3

MGQ

ft

MGD

( ) ( )( )

=

=

=

10

1 1

3

.

233 350 al

7.48 gal ft

3

gft=


MG

3

12.2 MGD

ft

3

3

3

MGQ

ft

MGD

( ) ( )( )

=

=

=

3

2 7

3

.

22 5

..

5 t

1 MG GD 6.25 ft

33f

M d¥ =

6.25 t

1 d

3fd ft¥ =90 562 5 3.

562.5

27 ft d

3

ft

ydy

3

3

321=



543

18.7.4.1 Operational Observations, Problems, and Troubleshooting

In preaeration grit removal systems, the operator is con-cerned with maintaining proper operation and must bealert to any possible mechanical problems. In addition, theoperator monitors DO levels and the impact of preaerationon influent.

18.7.5 CHEMICAL ADDITION

Chemical addition is made (either via dry chemical meter-ing or solution feed metering) to the wastestream toimprove settling, reduce odors, neutralize acids or bases,reduce corrosion, reduce BOD, improve solids and greaseremoval, reduce loading on the plant, add or remove nutri-ents, add organisms, and aid subsequent downstreamprocesses. The particular chemical and amount useddepends on the desired result. Chemicals must be addedat a point where sufficient mixing will occur to obtainmaximum benefit. Chemicals typically used in wastewatertreatment include chlorine, peroxide, acids and bases,miner salts (ferric chloride, alum, etc.), and bioadditivesand enzymes.


In adding chemicals to the wastestream to remove grit,the operator monitors the process for evidence of mechan-ical problems and takes proper corrective actions whennecessary. The operator also monitors the current chemicalfeed rate and dosage. The operator ensures that mixing atthe point of addition is accomplished in accordance withstandard operating procedures and monitors the impact ofchemical addition on influent.

18.7.6 EQUALIZATION

The purpose of flow equalization (whether by surge, diur-nal, or complete methods) is to reduce or remove the wideswings in flow rates normally associated with wastewatertreatment plant loading; it minimizes the impact of stormflows. The process can be designed to prevent flows abovemaximum plant design hydraulic capacity, reduce themagnitude of diurnal flow variations, and eliminate flowvariations. Flow equalization is accomplished using mix-ing or aeration equipment, pumps, and flow measurement.Normal operation depends on the purpose and require-ments of the flow equalization system. Equalized flowsallow the plant to perform at optimum levels by providingstable hydraulic and organic loading. The downside to flowequalization is the additional costs associated with con-struction and operation of the flow equalization facilities.


During normal operations, the operator must monitor allmechanical systems involved with flow equalization andmust watch for mechanical problems and take the appro-priate corrective action. The operator also monitors DOlevels, the impact of equalization on influent, and waterlevels in equalization basins; any necessary adjustmentsare also made.

18.7.7 AERATED SYSTEMS

Aerated grit removal systems use aeration to keep thelighter organic solids in suspension while allowing theheavier grit particles to settle out. Aerated grit removalmay be manually or mechanically cleaned; the majorityof the systems are mechanically cleaned.

In normal operation, the aeration rate is adjusted toproduce the desired separation, which requires observationof mixing and aeration and sampling of fixed suspendedsolids. Actual grit removal is controlled by the rate ofaeration. If the rate is too high, all of the solids remain insuspension. If the rate is too low, both the grit and theorganics will settle out.

18.7.8 CYCLONE DEGRITTER

The cyclone degritter uses a rapid spinning motion (cen-trifugal force) to separate the heavy inorganic solids orgrit from the light organic solids. This unit process isnormally used on primary sludge rather than the entirewastewater flow. The critical control factor for the processis the inlet pressure. If the pressure exceeds the recom-mendations of the manufacturer, the unit will flood andgrit will carry through with the flow. Grit is separated fromthe flow and discharged directly to a storage container.Grit removal performance is determined by calculating thepercent removal for inorganic (fixed) suspended solids.

18.7.9 PRELIMINARY TREATMENT SAMPLING AND TESTING

During normal operation of grit removal systems (withthe exception of the screening and shredding processes),the plant operator is responsible for sampling and testingas shown in Table 18.3.

18.7.10 OTHER PRELIMINARY TREATMENT PROCESS CONTROL CALCULATIONS

The desired velocity in sewers in approximately 2 ft/secat peak flow; this velocity normally prevents solids fromsettling from the lines. When the flow reaches the gritchannel, the velocity should decrease to about 1 ft/sec topermit the heavy inorganic solids to settle. In the example



calculations that follow, we describe how the velocity ofthe flow in a channel can be determined by the float andstopwatch method and by channel dimensions.

EXAMPLE 18.18: VELOCITY BY FLOAT AND STOPWATCH

Problem:A float takes 30 sec to travel 37 ft in a grit channel. Whatis the velocity of the flow in the channel?

Solution:

EXAMPLE 18.19: VELOCITY BY FLOW AND CHANNEL DIMENSIONS

Note: This calculation can be used for a single chan-nel or tank or for multiple channels or tankswith the same dimensions and equal flow. If theflow through each of the unit dimensions isunequal, the velocity for each channel or tankmust be computed individually.

Problem:

The plant is currently using two grit channels. Each chan-

nel is 3 ft wide and has a water depth of 1.3 ft. What is

the velocity when the influent flow rate is 4.0 MGD?

Solution:

Note: Because 0.79 is within the 0.7 to 1.4 level, theoperator of this unit would not make any adjust-ments.

Note: The channel dimensions must always be in feet.Convert inches to feet by dividing by 12 in./ft.

EXAMPLE 18.20: REQUIRED SETTLING TIME

Note: This calculation can be used to determine the timerequired for a particle to travel from the surfaceof the liquid to the bottom at a given settlingvelocity. To compute the settling time, settlingvelocity in feet per second must be provided ordetermined by experiment in a laboratory.

TABLE 18.3Sampling and Testing Grit Removal Systems

Process Location Test Frequency

Grit removal (velocity) Influent Suspended solids (fixed) VariableChannel Depth of grit VariableGrit Total solids (fixed) VariableEffluent Suspended solids (fixed) Variable

Grit removal (aerated) Influent Suspended solids (fixed) VariableChannel DO VariableGrit Total solids (fixed) VariableEffluent Suspended solids (fixed) Variable

Chemical addition Influent Jar test VariablePreaeration Influent DO Variable

Effluent DO VariableEqualization Effluent DO Variable

Source: Spellman, F.R., Spellman’s Standard Handbook for Wastewater Operators,Vol. 1, Technomic Publ., Lancaster, PA, 1999.

Velocity, feet secondDistance Traveled, ft

Time required, seconds=

Vs

ft sec37 ft

30 ec

ft sec

( ) =

= 1 2.

Velocity, fps

Flow, MGD 1.55 cfs MGD

# Chan in Ser Chan Width, ft Water Depth, ft

=

¥¥ ¥

VMGD

ft

ft sec4.0 MGD 1.55 ft

Channels 3 ft 1 ft

6.2 ft

ft sec

3

3

( ) =¥

¥ ¥

=

=

sec

.

sec

.

.

2 3

7 8

0 79

2

Settling Time, secondsLiquid Depth in ft

Settling, Velocity, fps=



Problem:

The plant’s grit channel is designed to remove sand andhas a settling velocity of 0.080 ft/sec. The channel iscurrently operating at a depth of 2.3 ft. How many secondswill it take for a sand particle to reach the channel bottom?

Solution:

EXAMPLE 18.21: REQUIRED CHANNEL LENGTH

Note: This calculation can be used to determine thelength of channel required to remove an objectwith a specified settling velocity.

Problem:

The plant’s grit channel is designed to remove sand andhas a settling velocity of 0.080 ft/sec. The channel iscurrently operating at a depth of 3 ft. The calculatedvelocity of flow through the channel is 0.85 ft/sec. Thechannel is 36 ft long. Is the channel long enough to removethe desired sand particle size?

Solution:

Yes, the channel is long enough to ensure all of the sandwill be removed.

18.8 PRIMARY TREATMENT (SEDIMENTATION)

The purpose of primary treatment (primary sedimentationor primary clarification) is to remove settleable organicand flotable solids. Normally, each primary clarificationunit can be expected to remove 90 to 95% settleable solids,40 to 60% TSS, and 25 to 35% BOD.

Note: Performance expectations for settling devicesused in other areas of plant operation is nor-mally expressed as overall unit performancerather than settling unit performance.

Sedimentation may be used throughout the plant toremove settleable and floatable solids. It is used in primarytreatment, secondary treatment, and advanced wastewatertreatment processes. In this section, we focus on primarytreatment or primary clarification, which uses large basinsin which primary settling is achieved under relatively qui-escent conditions (see Figure 18.1). Within these basins,mechanical scrapers collect the primary settled solids intoa hopper where they are pumped to a sludge-processingarea. Oil, grease, and other floating materials (scum) areskimmed from the surface. The effluent is discharged overweirs into a collection trough.

18.8.1 PROCESS DESCRIPTION

In primary sedimentation, wastewater enters a settling tankor basin. Velocity is reduced to approximately 1 ft/min.

Note: Notice that the velocity is based on minutesinstead of seconds, as was the case in the gritchannels. A grit channel velocity of 1 ft/secwould be 60 ft/min.

Solids that are heavier than water settle to the bottom,while solids that are lighter than water float to the top.Settled solids are removed as sludge and floating solidsare removed as scum. Wastewater leaves the sedimentationtank over an effluent weir and on to the next step intreatment. Detention time, temperature, tank design, andcondition of the equipment control the efficiency of theprocess.

18.8.1.1 Overview of Primary Treatment

1. Primary treatment reduces the organic loadingon downstream treatment processes by remov-ing a large amount of settleable, suspended, andfloatable materials.

2. Primary treatment reduces the velocity of thewastewater through a clarifier to approximately1 to 2 ft/min, so that settling and floatation cantake place. Slowing the flow enhances removalof suspended solids in wastewater.

3. Primary settling tanks remove floated greaseand scum, remove the settled sludge solids, andcollect them for pumped transfer to disposal orfurther treatment.

4. Clarifiers used may be rectangular or circular.In rectangular clarifiers, wastewater flows fromone end to the other, and the settled sludge ismoved to a hopper at the one end, either byflights set on parallel chains or by a single bot-tom scraper set on a traveling bridge. Floatingmaterial (mostly grease and oil) is collected bya surface skimmer.

Settling Time sec.3 ft

.080 ft sec

sec

( ) =

=

2

0

28 7.

Required Channel Length

Channel Depth, ft Flow Velocity, fps fps

=

¥

.0 080

R Lft

equired Channel ength ft ft sec

.080 ft sec

ft

( ) =¥

=

3 0 85

0

31 9

.

.



5. In circular tanks, the wastewater usually entersat the middle and flows outward. Settled sludgeis pushed to a hopper in the middle of the tankbottom, and a surface skimmer removes floatingmaterial.

6. Factors affecting primary clarifier performanceinclude:A. Rate of flow through the clarifierB. Wastewater characteristics (strength; tem-

perature; amount and type of industrialwaste; and the density, size, and shapes ofparticles)

C. Performance of pretreatment processesD. Nature and amount of any wastes recycled

to the primary clarifier7. Key factors in primary clarifier operation

include the following concepts:

18.8.2 TYPES OF SEDIMENTATION TANKS

Sedimentation equipment includes septic tanks, two storytanks, and plain settling tanks or clarifiers. All threedevices may be used for primary treatment; plain settlingtanks are normally used for secondary or advanced waste-water treatment processes.

18.8.2.1 Septic Tanks

Septic tanks are prefabricated tanks that serve as a combinedsettling and skimming tank and as an unheated–unmixedanaerobic digester. Septic tanks provide long settling times(6 to 8 h or more), but do not separate decomposing solidsfrom the wastewater flow. When the tank becomes full,solids will be discharged with the flow. The process is

suitable for small facilities (i.e., schools, motels, homes,etc.), but due to the long detention times and lack ofcontrol, it is not suitable for larger applications.

18.8.2.2 Two-Story (Imhoff) Tank

The two-story or Imhoff tank is similar to a septic tank inthe removal of settleable solids and the anaerobic diges-tion of solids. The difference is that the two story tankconsists of a settling compartment where sedimentation isaccomplished, a lower compartment where settled solidsand digestion takes place, and gas vents. Solids removedfrom the wastewater by settling pass from the settlingcompartment into the digestion compartment through aslot in the bottom of the settling compartment. The designof the slot prevents solids from returning to the settlingcompartment. Solids decompose anaerobically in thedigestion section. Gases produced as a result of the solidsdecomposition are released through the gas vents runningalong each side of the settling compartment.

18.8.2.3 Plain Settling Tanks (Clarifiers)

The plain settling tank or clarifier optimizes the settlingprocess. Sludge is removed from the tank for processingin other downstream treatment units. Flow enters the tank,is slowed and distributed evenly across the width anddepth of the unit, passes through the unit, and leaves overthe effluent weir. Detention time within the primary set-tling tank is from 1 to 3 h (2-h average).

Sludge removal is accomplished frequently on eithera continuous or intermittent basis. Continuous removalrequires additional sludge treatment processes to removethe excess water resulting from the removal of sludge,which contains less than 2 to 3% solids. Intermittentsludge removal requires the sludge be pumped from thetank on a schedule frequent enough to prevent large clumpsof solids rising to the surface but infrequent enough toobtain 4 to 8% solids in the sludge withdrawn.

Scum must be removed from the surface of the settlingtank frequently. This is normally a mechanical process,but may require manual start-up. The system should beoperated frequently enough to prevent excessive buildupand scum carryover but not so frequent as to cause hydrau-lic overloading of the scum removal system.

Settling tanks require housekeeping and maintenance.Baffles (devices that prevent floatable solids and scum fromleaving the tank), scum troughs, scum collectors, effluenttroughs, and effluent weirs require frequent cleaning to pre-vent heavy biological growths and solids accumulations.Mechanical equipment must be lubricated and maintainedas specified in the manufacturer’s recommendations or inaccordance with procedures listed in the plant O & Mmanual.

Retention Time hv gal 2 h d

gal d( ) = ( ) ¥

( )4

Q

S

Q

Surface Area

urface Loading Rate gal d ft

gal d

ft

2

2

( ) =

( )( )

Solids Loading Rate lb d ft

olids into Clarifier lb d

ft

2

2

( ) =

( )( )

S

Surface Area

Weir Overflow Rate gal d lineal ft

gal d

eir ength lineal ft

( ) =

( )( )

Q

W L



Process control sampling and testing is used to eval-uate the performance of the settling process. Settleablesolids, DO, pH, temperature, TSS and BOD5, as well assludge solids and volatile matter testing are routinelyaccomplished.

18.8.3 OPERATOR OBSERVATIONS, PROCESS PROBLEMS, AND TROUBLESHOOTING

Before identifying a primary treatment problem and pro-ceeding with appropriate troubleshooting effort, the operatormust be cognizant of what constitutes normal operation.(i.e., Is there a problem or is the system operating as perdesign?)

Several important items of normal operation can havea strong impact on performance. In the following section,we discuss the important operational parameters and nor-mal observations.

18.8.3.1 Primary Clarification: Normal Operation

In primary clarification, wastewater enters a settling tankor basin. Velocity reduces to approximately 1 ft/min.

Note: Notice that the velocity is based on minutesinstead of seconds, as was the case in the gritchannels. A grit channel velocity of 1 ft/secwould be 60 ft/min.

Solids that are heavier than water settle to the bottom,while solids that are lighter than water float to the top.Settled solids are removed as sludge and floating solidsare removed as scum. Wastewater leaves the sedimentationtank over an effluent weir and on to the next step intreatment. Detention time, temperature, tank design, andcondition of the equipment control the efficiency of theprocess.

18.8.3.2 Primary Clarification: Operational Parameters (Normal Observations)

1. Flow distribution — Normal flow distributionis indicated by flow to each in-service unitbeing equal and uniform. There is no indicationof short-circuiting. The surface-loading rate iswithin design specifications.

2. Weir condition — Under this condition, weirs arelevel, flow over the weir is uniform, and the weiroverflow rate is within design specifications.

3. Scum removal — The surface is free of scumaccumulations, and the scum removal does notoperate continuously.

4. Sludge removal — No large clumps of sludgeappear on the surface. The system operates asdesigned. The pumping rate is controlled to pre-

vent coning or buildup, and the sludge blanketdepth is within desired levels.

5. Performance — The unit is removing expectedlevels of BOD5, TSS, and settleable solids.

6. Unit maintenance — Mechanical equipment ismaintained in accordance with planned sched-ules; equipment is available for service asrequired.

To assist the operator in judging primary treatmentoperation, several process control tests can be used forprocess evaluation and control. These tests include thefollowing:

1. pH (normal range: 6.5 to 9.0)2. DO (normal range is <1.0 mg/L)3. Temperature (varies with climate and season)4. Settleable solids (influent is 5 to 15 mL/L; efflu-

ent is 0.3 to 5 mL/L)5. BOD (influent is 150 to 400 mg/L; effluent is

50 to 150 mg/L)6. Percent solids (4 to 8%)7. Percent volatile matter (40% to 70%)8. Heavy metals (as required)9. Jar tests (as required)

Note: Testing frequency should be determined on thebasis of the process influent and effluent vari-ability and the available resources. All shouldbe performed periodically to provide referenceinformation for evaluation of performance.

18.8.4 PROCESS CONTROL CALCULATIONS

As with many other wastewater treatment plant unitprocesses, process control calculations aid in determiningthe performance of the sedimentation process. Processcontrol calculations are used in the sedimentation processto determine:

1. Percent removal2. Hydraulic detention time3. Surface loading rate (surface settling rate)4. Weir overflow rate (weir loading rate)5. Sludge pumping6. Percent total solids (% TS)

In the following sections, we take a closer look at afew of these process control calculations and exampleproblems.

Note: The calculations presented in the following sec-tions allow you to determine values for eachfunction performed. Keep in mind that an opti-mally operated primary clarifier should havevalues in an expected range.



18.8.4.1 Percent Removal

The expected range of percent removal for a primary clar-ifier is:

18.8.4.2 Detention Time

The primary purpose of primary settling is to removesettleable solids. This accomplished by slowing the flowdown to approximately 1 ft/min. The flow at this velocitywill stay in the primary tank from 1.5 to 2.5 h. The lengthof time the water stays in the tank is called the hydraulicdetention time.

18.8.4.3 Surface Loading Rate (Surface Settling Rate and Surface Overflow Rate)

Surface loading rate is the number of gallons of wastewa-ter passing over 1 ft2 of tank/d. This can be used to com-pare actual conditions with design. Plant designs generallyuse a surface loading rate of 300 to 1200 gal/d/ ft2.

Other terms used synonymously with surface loadingrate are surface overflow rate and surface settling rate. Theequation for calculating the surface loading rate is asfollows:

(18.10)

EXAMPLE 18.22

Problem:

The settling tank is 120 ft in diameter and the flow to theunit is 4.5 MGD. What is the surface loading rate ingallons per day per square foot?

Solution:

EXAMPLE 18.23

Problem:

A circular clarifier has a diameter of 50 ft. If the primaryeffluent flow is 2,150,000 gal/d, what is the surface over-flow rate in gallons per day per square foot?

Solution:

18.8.4.4 Weir Overflow Rate (Weir Loading Rate)

Weir overflow rate (weir loading rate) is the amount ofwater leaving the settling tank per linear foot of weir. Theresult of this calculation can be compared with design.Normally weir overflow rates of 10,000 to 20,000 gal/d/ftare used in the design of a settling tank:

(18.11)

EXAMPLE 18.24

Problem:

The circular settling tank is 90 ft in diameter and has aweir along its circumference. The effluent flow rate is2.55 MGD. What is the weir overflow rate in gallons perday per foot?

Solution:

18.8.4.5 Sludge Pumping

Determination of sludge pumping (the quantity of solidsand volatile solids removed from the sedimentation tank)provides accurate information needed for process controlof the sedimentation process:

Settleable solids 90–95%Suspended solids 40–60%BOD 25–35%

Surface Loading Rate gal d ft

Q gal d

Settling Tank Area ft

2

2

( ) =

( )( )


Q gal d

Settling Tank Area ft

gal MGD

0.785 120 ft 120 ft

gal d ft

2

2

2

( )( )

( )=

=¥

¥ ¥

=

4 5 1 000 000

398

. , ,MGD

Surface Overflow Rate gal d ftQ gal d

Area ft

0.785 5 ft 50 ft

gal d ft

22

2

( ) ( )( )=

=¥ ¥

=

2 150 000

0

1096

, ,

Weir Overflow Rate gal d ft

Q gal d

Weir Length ft

2( ) =

( )( )

Weir Overflow Rate gal d ft

2.55 MGD 1,000,000 gal MG

3.14 90 ft

gal d ft

2( )

=¥

¥

= 9023



(18.12)

(18.13)

EXAMPLE 18.25

Problem:

The sludge pump operates 20 min/h. The pump delivers20 gal/min of sludge. Laboratory tests indicate that thesludge is 5.2% solids and 66% volatile matter. How manypounds of volatile matter are transferred from the settlingtank to the digester?

Solution:

Pump Time = 20 min/hPump Rate = 20 gal/min% Solids = 5.2%% VM= 66%

18.8.4.5.1 Percent Total Solids

EXAMPLE 18.26

Problem:

A settling tank sludge sample is tested for solids. Thesample and dish weigh 74.69 g. The dish weighs 21.2 g.After drying, the dish with dry solids now weighs 22.3 g.What is the percent total solids (% TS) of the sample?

Solution:

Sample + Dish ¥ Dish = Sample Weight74.69 g ¥ 21.2 g = 53.49 gDish + Dry Solids ¥ Dish = Dry Solids Weight22.3 g ¥ 21.2 g = 1.1 g

18.8.4.6 BOD and Suspended Solids Removal

To calculate the pounds of BOD or suspended solids (SS)removed each day, you need to know the milligrams perliter of BOD or suspended solids removed and the plant

flow. Then you can use the milligrams per liter to poundsper day equation:

(18.14)

EXAMPLE 18.27

Problem:

If 120 mg/L suspended solids are removed by a primaryclarifier, how many pounds per day of suspended solidsare removed when the flow is 6,230,000 gal/d?

Solution:

EXAMPLE 18.28

Problem:

The flow to a secondary clarifier is 1.6 MGD. If theinfluent BOD concentration is 200 mg/L and the effluentBOD concentration is 70 mg/L, how many pounds ofBOD are removed daily?

Solution:

Calculate the milligrams per liter of BOD removed:

Next calculate the pounds per day of BOD removed:

18.8.5 PROBLEM ANALYSIS

In primary treatment (as is also clear in the operation ofother unit processes), the primary function of the operatoris to identify causes of process malfunctions, develop solu-tions, and prevent recurrence. In other words, the operator’sgoal is to perform problem analysis or troubleshooting onunit processes when required and to restore the unit pro-cesses to optimal operating condition. The immediate goalin problem analysis is to solve the immediate problem.The long-term goal is to ensure that the problem does notpop up again, causing poor performance in the future.

Solids Pumped lb d Pump Rate

Pump Time 8.34 lb gal Solids

( ) = ¥

¥ ¥ %

Volume of Solids lb d Pump Rate

Pump Time 8.34 % Solids % VM

( ) = ¥

¥ ¥ ¥

Volume of Solids lb d gal min

min h h d

lb gal

lb d

( )

( )

= ¥

¥ ¥

¥ ¥

=

20

20 24

8 34 0 052 0 66

2748

. . .

1.1 g

53.49 g¥ =100 2% %

SS Removed mg L lb gal= ¥ ¥MGD 8 3.

SS Removed lb d mg L MGD

8.34 lb gal

lb d

( ) = ¥ ¥

=

120 6 25

6255

.

BOD removed lb d mg L mg L

mg L

( ) = ¥

=

200 70

130

BOD removed lb d mg L MGD

8.34 lb gal

lb d

( ) = ¥ ¥

=

130 1 6

1735

.



In this section, we cover a few indicators and obser-vations of operational problems with the primary treatmentprocess. The observations presented are not all-inclusive,but highlight the most frequently confronted problems.

1. Poor suspended solids removal (primary clarifier)Causal factors:A. Hydraulic overloadB. Sludge buildup in tanks and decreased vol-

ume and allows solids to scour out tanksC. Strong recycle flowsD. Industrial waste concentrationsE. Wind currentsF. Temperature currents

2. Floating sludgeCausal factors:A. Sludge becoming septic in tankB. Damaged or worn collection equipmentC. Recycled waste sludgeD. Primary sludge pumps malfunctionsE. Sludge withdrawal line pluggedF. Return of well-nitrified waste-activated sludgeG. Too few tanks in serviceH. Damaged or missing baffles

3. Primary sludge solids concentration too lowCausal factors:A. Hydraulic overloadB. Overpumping of sludgeC. Collection system problemsD. Decreased influent solids loading

4. Septic wastewater or sludgeCausal factors:A. Damaged or worn collection equipmentB. Infrequent sludge removalC. Insufficient industrial pretreatmentD. Septic sewage from collection systemE. Strong recycle flowsF. Primary sludge pump malfunctionG. Sludge withdrawal line pluggedH. Sludge collectors not run often enoughI. Septage dumpers

5. Primary sludge solids concentrations too highCausal factors:A. Excessive grit and compacted materialB. Primary sludge pump malfunctionC. Sludge withdrawal line pluggedD. SRT is too longE. Increased influent loadings

18.8.6 EFFLUENT FROM SETTLING TANKS

Upon completion of screening, degritting, and settling insedimentation basins, large debris, grit, and many settle-able materials have been removed from the wastestream.

What is left is referred to as primary effluent. Usuallycloudy and frequently gray in color, primary effluent stillcontains large amounts of dissolved food and other chem-icals (nutrients). These nutrients are treated in the nextstep in the treatment process, secondary treatment, whichis discussed in the next section.

Note: Two of the most important nutrients left toremove are phosphorus and ammonia. Whilewe want to remove these two nutrients from thewastestream, we do not want to remove toomuch. Carbonaceous microorganisms in sec-ondary treatment (biological treatment) needboth phosphorus and ammonia.

18.9 SECONDARY TREATMENT

The main purpose of secondary treatment (sometimesreferred to as biological treatment) is to provide BODremoval beyond what is achievable by primary treatment.There are three commonly used approaches, and all takeadvantage of the ability of microorganisms to convertorganic wastes (via biological treatment) into stabilized,low-energy compounds. Two of these approaches, thetrickling filter (and its variation, the RBC) and the activatedsludge process, sequentially follow normal primary treat-ment. The third, ponds (oxidation ponds or lagoons), canprovide equivalent results without preliminary treatment.

In this section, we present a brief overview of thesecondary treatment process followed by a detailed dis-cussion of wastewater treatment ponds (used primarily insmaller treatment plants), trickling filters, and RBCs. Wethen shift focus to the activated sludge process, the sec-ondary treatment process, which is used primarily in largeinstallations and is the main focus of the handbook.

Secondary treatment refers to those treatment pro-cesses that use biological processes to convert dissolved,suspended, and colloidal organic wastes to more stablesolids that can either be removed by settling or dischargedto the environment without causing harm.

Exactly what is secondary treatment? As defined bythe Clean Water Act (CWA), secondary treatment pro-duces an effluent with nor more than 30 mg/L BOD and30 mg/L TSS.

Note: The CWA also states that ponds and tricklingfilters will be included in the definition of sec-ondary treatment even if they do not meet theeffluent quality requirements continuously.

Most secondary treatment processes decompose solidsaerobically, producing carbon dioxide, stable solids, andmore organisms. Since solids are produced, all of thebiological processes must include some form of solidsremoval (settling tank, filter, etc.).



Secondary treatment processes can be separated intotwo large categories: fixed film systems and suspendedgrowth systems.

Fixed film systems are processes that use a biologicalgrowth (biomass or slime) that is attached to some formof media. Wastewater passes over or around the media andthe slime. When the wastewater and slime are in contact,the organisms remove and oxidize the organic solids. Themedia may be stone, redwood, synthetic materials, or anyother substance that is durable (capable of withstandingweather conditions for many years), provides a large areafor slime growth and an open space for ventilation, andis not toxic to the organisms in the biomass. Fixed filmdevices include trickling filters and RBCs.

Suspended growth systems are processes that use abiological growth that is mixed with the wastewater. Typ-ical suspended growth systems consist of various modifi-cations of the activated sludge process.

18.9.1 TREATMENT PONDS

Wastewater treatment can be accomplished using ponds.Ponds are relatively easy to build and manage, can accom-modate large fluctuations in flow, and can also providetreatment that approaches conventional systems (produc-ing a highly purified effluent) at much lower cost. It is thecost (the economics) that drives many managers to decideon the pond option. The actual degree of treatment pro-vided depends on the type and number of ponds used.Ponds can be used as the sole type of treatment or theycan be used in conjunction with other forms of wastewatertreatment (i.e., other treatment processes followed by apond or a pond followed by other treatment processes).

18.9.1.1 Types of Ponds

Ponds can be classified (named) based upon their locationin the system, the type wastes they receive, and the mainbiological process occurring in the pond. First we look atthe types of ponds according to their location and the typewastes they receive: raw sewage stabilization ponds (seeFigure 18.3), oxidation ponds, and polishing ponds. In thefollowing section, we look at ponds classified by the typeof processes occurring within the pond: Aerobic Ponds,anaerobic ponds, facultative ponds, and aerated ponds.

18.9.1.1.1 Ponds Based on Location and Types of Wastes They Receive

The types of ponds based on location and types of wastesthey receive include raw sewage stabilization ponds, oxi-dation ponds, and polishing ponds.

18.9.1.1.1.1 Raw Sewage Stabilization PondsThe raw sewage stabilization pond is the most commontype of pond (see Figure 18.3). With the exception ofscreening and shredding, this type of pond receives noprior treatment. Generally, raw sewage stabilization pondsare designed to provide a minimum of 45 d detention timeand to receive no more than 30 lb of BOD/d/acre. Thequality of the discharge is dependent on the time of theyear. Summer months produce high BOD removal, butexcellent suspended solids removals.

The pond consists of an influent structure, pond berm,or walls and an effluent structure designed to permit selec-tion of the best quality effluent. Normal operating depthof the pond is 3 to 5 ft.

The process occurring in the pond involves bacteriadecomposing the organics in the wastewater (aerobicallyand anaerobically) and algae using the products of the

FIGURE 18.3 Stabilization pond processes. (From Spellman, F.R., Spellman’s Standard Handbook for Wastewater Operators, Vol. 1,Technomic Publ., Lancaster, PA, 1999.)

Anaerobic digestion

(settled solids)

Solids

IN

Photosynthesis(Algae-producing oxygen)

Aerobic decomposition(bacteria producing CO2)

CO

2

O2

Pond surface

Pond bottom


552


bacterial action to produce oxygen (photosynthesis).Because this type of pond is the most commonly used inwastewater treatment, the process that occurs within thepond is described in greater detail below.

When wastewater enters the stabilization pond severalprocesses begin to occur. These include settling, aerobicdecomposition, anaerobic decomposition, and photosyn-thesis (see Figure 18.3). Solids in the wastewater will settleto the bottom of the pond. In addition to the solids in thewastewater entering the pond, solids, which are producedby the biological activity, will also settle to the bottom.Eventually this will reduce the detention time and theperformance of the pond. When this occurs (usually 20 to30 years) the pond will have to be replaced or cleaned.

Bacteria and other microorganisms use the organicmatter as a food source. They use oxygen (aerobic decom-position), organic matter, and nutrients to produce carbondioxide, water, stable solids (which may settle out), andmore organisms. The carbon dioxide is an essential com-ponent of the photosynthesis process occurring near thesurface of the pond.

Organisms also use the solids that settled out as foodmaterial. Because the oxygen levels at the bottom of thepond are extremely low the process used is anaerobicdecomposition. The organisms use the organic matter toproduce gases (hydrogen sulfide, methane, etc.), which aredissolved in the water; stable solids; and more organisms.

Near the surface of the pond a population of greenalgae will develop that can use the carbon dioxide pro-duced by the bacterial population, nutrients, and sunlightto produce more algae and oxygen, which is dissolvedinto the water. The DO is then used by organisms in theaerobic decomposition process.

When compared with other wastewater treatment sys-tems involving biological treatment, a stabilization pondtreatment system is the simplest to operate and maintain.Operation and maintenance activities include collectingand testing samples for DO and pH, removing weeds andother debris (scum) from the pond, mowing the berms,repairing erosion, and removing burrowing animals.

Note: DO and pH levels in the pond will vary through-out the day. Normal operation will result in veryhigh DO and pH levels because of the naturalprocesses occurring.

Note: When operating properly the stabilization pondwill exhibit a wide variation in both DO andpH. This is due to the photosynthesis occurringin the system.

18.9.1.1.1.2 Oxidation Ponds

An oxidation pond, which is normally designed using thesame criteria as the stabilization pond, receives flows thathave passed through a stabilization pond or primary set-tling tank. This type of pond provides biological treatment,

additional settling, and some reduction in the number offecal coliform present.

18.9.1.1.1.3 Polishing Ponds

A polishing pond, which uses the same equipment as astabilization pond, receives flow from an oxidation pondor from other secondary treatment systems. Polishingponds remove additional BOD, solids and fecal coliformand some nutrients. They are designed to provide 1 to 3 ddetention time and normally operate at a depth of 5 to10 ft. Excessive detention time or too shallow a depth willresult in algae growth, which increases influent, suspendedsolids concentrations.

18.9.1.1.2 Ponds Based on the Type of Processes Occurring within the Ponds

The type of processes occurring within the pond may alsoclassify ponds. These include the aerobic, anaerobic, fac-ultative, and aerated processes.

18.9.1.1.2.1 Aerobic Ponds

In aerobic ponds, which are not widely used, oxygen ispresent throughout the pond. All biological activity isaerobic decomposition.

18.9.1.1.2.2 Anaerobic Ponds

Anaerobic ponds are normally used to treat high strengthindustrial wastes. No oxygen is present in the pond andall biological activity is anaerobic decomposition.

18.9.1.1.2.3 Facultative Ponds

The facultative pond is the most common type pond (basedon processes occurring). Oxygen is present in the upperportions of the pond and aerobic processes are occurring.No oxygen is present in the lower levels of the pond whereanoxic and anaerobic processes are occurring.

18.9.1.1.2.4 Aerated Ponds

In the aerated pond, oxygen is provided through the useof mechanical or diffused air systems. When aeration isused, the depth of the pond and the acceptable loadinglevels may increase. Mechanical or diffused aeration isoften used to supplement natural oxygen production or toreplace it.

18.9.1.2 Process Control Calculations (Stabilization Ponds)

Process control calculations are an important part of waste-water treatment operations, including pond operations. Moresignificantly, process control calculations are an importantpart of state wastewater licensing examinations — you sim-ply cannot master the licensing examinations withoutbeing able to perform the required calculations. Wheneverpossible, example process control problems are providedto enhance your knowledge and skills.



18.9.1.2.1 Determining Pond Area in Acres

(18.15)

18.9.1.2.2 Determining Pond Volume in Acre Feet

(18.16)

18.9.1.2.3 Determining Flow Rate in Acre Feet per Day

(18.17)

Note: Acre-feet (ac-ft) is a unit that can cause confu-sion, especially for those not familiar with pondor lagoon operations. The measurement of1 ac-ft is the volume of a box with a 1-acre topand 1 ft of depth — but the top does not haveto be an even number of acres in size to useacre-feet.

18.9.1.2.4 Determining Flow Rate in Acre-Inches per Day

(18.18)

18.9.1.2.5 Hydraulic Detention Time in Days

(18.19)

Note: Hydraulic detention time normally ranges from30 to 120 d for stabilization ponds.

EXAMPLE 18.29

Problem:

A stabilization pond has a volume of 53.5 ac-ft. What isthe detention time in days when the flow is 0.30 MGD?

Solution:

Determine the flow rate in acre-feet per day:

Determine the detention time:

18.9.1.2.6 Hydraulic Loading in Inches per Day

(Overflow Rate)

(18.20)

(18.21)

Note: Population loading normally ranges from 50 to500 people per acre.

18.9.1.2.7 Organic Loading

Organic loading can be expressed as pounds of BOD peracre per day (most common), pounds BOD5 per acre-footper day, or people per acre per day.

(18.22)

Note: Normal range of organic loading is 10 to 50 lbBOD/d/acre.

EXAMPLE 18.30

Problem:

A wastewater treatment pond has an average width of380 ft and an average length of 725 ft. The influent flowrate to the pond is 0.12 MGD with a BOD concentrationof 160 mg/L. What is the organic loading rate to the pondin pounds per day per acre?

Solution:

Aacre

rea acresArea ft

43,560 ft

2

2( ) =( )

vac ft

acre-feet ac-ftv ft

43,560 ft -

3

2[ ]( ) =( )

Q ac ft d Q ft MG, . - MGD ac-= ( ) ¥ 3 069

Q acre inches d Q

inches MG

- MGD

acre-

( ) = ( ) ¥

.36 8

Hft

ft dDT d

Pond Volume ac-

Influent Flow ac-( ) = ( )

( )

Q ft d ft MG

ft d

ac- MGD 3.069 ac-

ac-

( ) = ¥

=

0 03

0 92

.

.

DTft d

d

d53.5 acre

0.92 ac-

( ) =

= 58 2.

Hydraulic Loading in. d

Influent Flow acre-

Pond Area acres

( ) =

( )( )

inches d

Population Loading people acre d

Population Served by System people

Pond Area acres

( ) =

( )( )

Organic L, lbs BOD Acre Day

BOD mg L Infl. flow, MGD 8.34Pond Area, Acres

=

¥ ¥

,

725 380 2 6 32

6 32

ft1 acre

43,560 ft acre

0.12 MGD 160 mg L 8.34 lb gal 106.1 lb d

160.1 lb d

25.3 lb d acre

ft

acre

¥ ¥ =

¥ ¥ =

=

.

.



18.9.2 TRICKLING FILTERS

Trickling filters have been used to treat wastewater sincethe 1890s. It was found that if settled wastewater waspassed over rock surfaces, slime grew on the rocks and thewater became cleaner. Today we still use this principle, butin many installations we use plastic media instead of rocks.

In most wastewater treatment systems, the trickling filterfollows primary treatment and includes a secondary settlingtank or clarifier as shown in Figure 18.4. Trickling filters arewidely used for the treatment of domestic and industrialwastes. The process is a fixed film biological treatmentmethod designed to remove BOD and suspended solids.

A trickling filter consists of a rotating distribution armthat sprays and evenly distributes liquid wastewater overa circular bed of fist-sized rocks, other coarse materials,or synthetic media (see Figure 18.5). The spaces betweenthe media allow air to circulate easily so that aerobicconditions can be maintained. The spaces also allowwastewater to trickle down through, around, and over themedia. A layer of biological slime that absorbs and con-

sumes the wastes trickling through the bed covers themedia material. The organisms aerobically decompose thesolids and produce more organisms and stable wastes thateither become part of the slime or are discharged backinto the wastewater flowing over the media. This slimeconsists mainly of bacteria, but it may also include algae,protozoa, worms, snails, fungi, and insect larvae. Theaccumulating slime occasionally sloughs off (sloughings)individual media materials (see Figure 18.6) and is col-lected at the bottom of the filter, along with the treatedwastewater, and passed on to the secondary settling tankwhere it is removed.

The overall performance of the trickling filter isdependent on hydraulic and organic loading, temperature,and recirculation.

18.9.2.1 Trickling Filter Definitions

To clearly understand the correct operation of the tricklingfilter, the operator must be familiar with certain terms. Thefollowing list of terms applies to the trickling filter process.

FIGURE 18.4 Simplified flow diagram of trickling filter used for wastewater treatment. (From Spellman, F.R., Spellman’s StandardHandbook for Wastewater Operators, Vol. 1, Technomic Publ., Lancaster, PA, 1999.)

FIGURE 18.5 Schematic of cross-section of a trickling filter. (From Spellman, F.R., Spellman’s Standard Handbook for WastewaterOperators, Vol. 1, Technomic Publ., Lancaster, PA, 1999.)

Influent Bar racks Gritchamber

Primarysedimentaion

Tricklingfilter

Settlingtank

Chlorinecontact

tankSc

reen

ings

Gri

t

Slud

ge

Effluent

Cl2 or NaOCl

Return effluent

Waste sludge

Rock Bed

Underdrainsystem

Rotating arm

Influent spray

Influent

Effluent

Rock bed



We assume that other terms related to other units within thetreatment system (plant) are already familiar to operators:

Biological towers a type of trickling filter that is verydeep (10 to 20 ft). Filled with a lightweightsynthetic media, these towers are also know asoxidation or roughing towers or (because oftheir extremely high hydraulic loading) super-rate trickling filters.

Biomass the total mass of organisms attached to themedia. Similar to solids inventory in the acti-

vated sludge process, it is sometimes referredto as the zoogleal slime.

Distribution arm the device most widely used toapply wastewater evenly over the entire surfaceof the media. In most cases, the force of thewastewater being sprayed through the orificesmoves the arm.

Filter underdrain the open space provided under themedia to collect the liquid (wastewater andsloughings) and to allow air to enter the filter.It has a sloped floor to collect the flow to acentral channel for removal.

Hydraulic loading the amount of wastewater flowapplied to the surface of the trickling filter media.It can be expressed in several ways: flow persquare foot of surface per day, flow per acre perday, or flow per acre-foot per day. The hydraulicloading includes all flow entering the filter.

High-rate trickling filters a classification (see Table18.4) in which the organic loading is in therange of 25 to 100 lb BOD/1000 ft3 of media/d.The standard rate filter may also produce ahighly nitrified effluent.

Media an inert substance placed in the filter to providea surface for the microorganism to grow on.The media can be field stone, crushed stone,slag, plastic, or redwood slats.

Organic loading the amount of BOD or COD appliedto a given volume of filter media. It does notinclude the BOD or COD contributed to anyrecirculated flow and is commonly expressed aspounds of BOD or COD per 1000 ft3 of media.

Recirculation the return of filter effluent back to thehead of the trickling filter. It can level flow

FIGURE 18.6 Filter media showing biological activities thattake place on surface area. (From Spellman, F.R., Spellman’sStandard Handbook for Wastewater Operators, Vol. 1, Tech-nomic Publ., Lancaster, PA, 1999.)

Zoogleal Slime

Sloughing

Oxygen Air

Organic matter

Influent flow

Media

TABLE 18.4Trickling Filter Classification

Filter Class Standard Intermediate High Rate Super High Rate Roughing

Hydraulic Loading (gal/d/ft2) 25–90 90–230 230–900 350–2100 >900Organic Loading BOD/1000 ft3 5–25 15–30 25–300 Up to 300 >300Sloughing frequency Seasonal Varies Continuous Continuous ContinuousDistribution Rotary Rotary fixed Rotary fixed Rotary Rotary FixedRecirculation No Usually Always Usually Not usuallyMedia depth (ft) 6–8 6–8 3–8 Up to 40 3–20Media type Rock Rock Rock Plastic Rock

Plastic Plastic Plastic PlasticWood Wood Wood Wood

Nitrification Yes Some Some Limited NoneFilter flies Yes Variable Variable Very few Not usuallyBOD removal 80–85% 50–70% 65–80% 65–85% 40–65%TSS removal 80–85% 50–70% 65–80% 65–85% 40–65%

Source: Spellman, F.R., Spellman’s Standard Handbook for Wastewater Operators, Vol. 1, Technomic Publ., Lancaster,PA, 1999.



variations and assist in solving operational prob-lems such as ponding, filter flies, and odors.

Roughing filters a classification of trickling filters(see Table 18.4) in which the organic is inexcess of 200 lb BOD/1000 ft3 of media/d. Aroughing filter is used to reduce the loading onother biological treatment processes to producean industrial discharge that can be safely treatedin a municipal treatment facility.

Sloughing the process in which the excess growthsbreak away from the media and wash throughthe filter to the underdrains with the wastewater.These sloughings must be removed from theflow by settling.

Staging the practice of operating two or more tricklingfilters in series. The effluent of one filter is usedas the influent of the next. This practice canproduce a higher quality effluent by removingadditional BOD or COD.

18.9.2.2 Trickling Filter Equipment

The trickling filter distribution system is designed tospread wastewater evenly over the surface of the entiremedia. The most common system is the rotary distributor,which moves above the surface of the media and spraysthe wastewater on the surface. The force of the waterleaving the orifices drives the rotary system. The distrib-utor arms usually have small plates below each orifice tospread the wastewater into a fan-shaped distribution sys-tem. The second type of distributor is the fixed nozzlesystem. In this system, the nozzles are fixed in place abovethe media and are designed to spray the wastewater overa fixed portion of the media. This system is used frequentlywith deep bed synthetic media filters.

Note: Trickling filters that use ordinary rock arenormally only about 3 m in depth because ofstructural problems caused by the weight ofrocks, which also requires the construction ofbeds that are quite wide (in many applications,up to 60 ft in diameter). When synthetic mediais used, the bed can be much deeper.

No matter which type of media is selected, the primaryconsideration is that it must be capable of providing thedesired film location for the development of the biomass.Depending on the type of media used and the filter clas-sification, the media may be 3 to 20 or more ft in depth.

The underdrains are designed to support the media,collect the wastewater and sloughings and carry them outof the filter, and provide ventilation to the filter.

Note: In order to ensure sufficient airflow to the filter,the underdrains should never be allowed to flowmore than 50% full of wastewater.

The effluent channel is designed to carry the flow fromthe trickling filter to the secondary settling tank.

The secondary settling tank provides 2 to 4 h of deten-tion time to separate the sloughing materials from thetreated wastewater. Design, construction, and operationare similar to the primary settling tank’s. Longer detentiontimes are provided because the sloughing materials arelighter and settle more slowly.

Recirculation pumps and piping are designed to recir-culate (and thus improve the performance of the tricklingfilter or settling tank) a portion of the effluent back to bemixed with the filter influent. When recirculation is used,pumps and metering devices must be provided.

18.9.2.3 Filter Classifications

Trickling filters are classified by hydraulic and organicloading. The expected performance and the constructionof the trickling filter are also determined by the filterclassification. Filter classifications include: standard rate,intermediate rate, high rate, super high rate (plasticmedia), and roughing rate types. Standard rate, high rate,and roughing rate are the filter types most commonly used.

The standard rate filter has a hydraulic loading of 25 to90 gal/d/ft3 and a seasonal sloughing frequency. It doesnot employ recirculation and typically has a 80–85% BODremoval rate and 80 to 85% TSS removal rate.

The high rate filter has a hydraulic loading of 230 to900 gal/d/ft3 and a continuous sloughing frequency. Italways employs recirculation and typically has a 65 to80% BOD removal rate and 65 to 80% TSS removal rate.

The roughing filter has a hydraulic loading of>900 gal/d/ft3 and a continuous sloughing frequency. Itdoes not normally include recirculation and typically hasa 40 to 65% BOD removal rate and 40 to 65% TSS removalrate.

18.9.2.4 Standard Operating Procedures

Standard operating procedures for trickling filters includesampling and testing, observation, recirculation, mainte-nance, and expectations of performance.

Collection of influent and process effluent samples todetermine performance and monitor process condition oftrickling filters is required. DO, pH, and settleable solidstesting should be collected daily. BOD and suspendedsolids testing should be done as often as practical to deter-mine the per cent removal.

The operation and condition of the filter should beobserved daily. Items to observe include the distributormovement, uniformity of distribution, evidence of operationor mechanical problems, and the presence of objectionableodors. In addition to the items above the normal observa-tion for a settling tank should also be performed.



Recirculation is used to reduce organic loading,improve sloughing, reduce odors, and reduce or eliminatefilter fly or ponding problems. The amount of recirculationis dependent on the design of the treatment plant and theoperational requirements of the process. Recirculation flowmay be expressed as a specific flow rate (i.e., 2.0 MGD).In most cases, it is expressed as a ratio (e.g., 3:1, 0.5:1.0,etc). The recirculation is always listed as the first numberand the influent flow listed as the second number.

Note: Since the second number in the ratio is always1.0, the ratio is sometimes written as a singlenumber (dropping the 1.0)

Flows can be recirculated from various points follow-ing the filter to various points before the filter. The mostcommon form of recirculation removes flow from the filtereffluent or settling tank and returns it to the influent of thetrickling filter as shown in Figure 18.7.

Maintenance requirements include lubrication ofmechanical equipment, removal of debris from the surfaceand orifices, as well as adjustment of flow patterns andmaintenance associated with the settling tank.

Expected performance ranges for each classificationof trickling filter. The levels of BOD and suspended solidsremoval are also dependent on the type of filter.

18.9.2.5 General Process Description

The trickling filter process involves spraying wastewaterover a solid media such as rock, plastic, or redwood slats(or laths). As the wastewater trickles over the surface ofthe media, a growth of microorganisms (bacteria, protozoa,fungi, algae, helminthes or worms, and larvae) develops.This growth is visible as a shiny slime very similar to theslime found on rocks in a stream. As the wastewater passesover this slime, the slime adsorbs the organic (food) matter.This organic matter is used for food by the microorgan-isms. At the same time, air moving through the openspaces in the filter transfers oxygen to the wastewater. Thisoxygen is then transferred to the slime to keep the outerlayer aerobic. As the microorganisms use the food andoxygen, they produce more organisms, carbon dioxide, sul-fates, nitrates, and other stable by-products; these materialsare then discarded from the slime back into the wastewaterflow and are carried out of the filter. The process is shownin the following equation:

(18.23)

The growth of the microorganisms and the buildup ofsolid wastes in the slime make it thicker and heavier. Whenthis slime becomes too thick, the wastewater flow breaksoff parts of the slime. These must be removed in the finalsettling tank.

In some trickling filters, a portion of the filter effluentis returned to the head of the trickling filter to level outvariations in flow and improves operations (recirculation).

18.9.2.5.1 Overview and Brief Summary of Trickling Filter Process

The following list provides an overview of the tricklingfilter process:

1. A trickling filter consists of a bed of coarsemedia, usually rocks or plastic, covered withmicroorganisms.

2. The wastewater is applied to the media at acontrolled rate, using a rotating distributor armor fixed nozzles. Organic material is removedby contact with the microorganisms as thewastewater trickles down through the mediaopenings. The treated wastewater is collectedby an underdrain system.

3. The trickling filter is usually built into a tankthat contains the media. The filter may besquare, rectangular, or circular.

4. The trickling filter does not provide any actualfiltration. The filter media provides a largeamount of surface area that the microorganismscan cling to and grow in a slime that forms onthe media as they feed on the organic materialin the wastewater.

5. The slime growth on the trickling filter mediaperiodically sloughs off and is settled andremoved in a secondary clarifier that followsthe filter.

6. Key factors in trickling filter operation includethe following concepts:

FIGURE 18.7 Common form of recirculation. (From Spellman, F.R., Spellman’s Standard Handbook for Wastewater Operators,Vol. 1, Technomic Publ., Lancaster, PA, 1999.)

Primarysettling

Tricklingfilter

Secondarysettling

Recirculating

Organics Organisms

More Organisms CO Solid Wastes

+ + =

+ +

O2

2



A. Hydraulic loading rate

B. Organic loading rate

C. Recirculation

18.9.2.6 Operator Observations, Process Problems, and Troubleshooting

Trickling filter operation requires routine observation,meter readings, process control sampling and testing, andprocess control calculations. Comparison of daily resultswith expected normal ranges is the key to identifyingproblems and appropriate corrective actions.

18.9.2.6.1 Operator Observations

1. Slime — The operator checks the thickness ofslime to ensure that it is thin and uniform(normal) or thick and heavy (indicates organicoverload). The operator is concerned withensuring that excessive recirculation is not tak-ing place and checks slime toxicity (if any). Theoperator is also concerned about the color ofthe slime. Green slime is normal, dark green orblack slime indicates organic overload. Othercolors may indicate industrial waste or chemi-cal additive contamination. The operator shouldcheck the subsurface growth of the slime toensure that it is normal (thin and translucent).If growth is thick and dark, organic overloadconditions are indicated. Distribution arm oper-ation is a system function important to slimeformation. It must be checked regularly forproper operation. For example, the distributionof slime should be even and uniform. Stripedconditions indicate clogged orifices or nozzles.

2. Flow — Flow distribution must be checked toensure uniformity. If nonuniform, the arms arenot level or the orifices are plugged. Flow drain-age is also important. Drainage should beuniform and rapid. If not, ponding may occurfrom media breakdown or debris on surface.

3. Distributor — Movement of the distributor iscritical to proper operation of the trickling filter.Movement should be uniform and smooth.Chattering, noisy operation may indicate bear-ing failure. The distributor seal must be checkedto ensure there is no leakage.

4. Recirculation — The operator must check therate of recirculation to ensure that it is withindesign specifications. Rates above design spec-ifications indicate hydraulic overloading, whilerates under design specifications indicatehydraulic underloading.

5. Media — The operator should check to ensurethat media are uniform.

18.9.2.6.2 Process Control Sampling and TestingTo ensure proper operation of the trickling filter, samplingand scheduling are important. For samples and the testsderived from the samples to be beneficial, operators mustperform a variety of daily or variable tests. Individual testsand sampling may be needed daily, weekly, or monthly,depending on seasonal change. Frequency may be lowerduring normal operations and higher during abnormalconditions.

The information gathered through collection and anal-ysis of samples from various points in the trickling filterprocess is helpful in determining the current status of theprocess as well as identifying and correcting operationalproblems.

The following routine sampling points and types oftests will permit the operator to identify normal and abnor-mal operating conditions.

1. Filter influent — Tests include DO, pH, tem-perature, settleable solids, BOD, suspended sol-ids, and metals.

2. Recirculated flow — Tests include DO, pH,flow rate, and temperature.

3. Filter effluent — Tests include DO, pH, and jartests.

4. Process effluent — Tests include DO, pH,settleable solids, BOD, and suspended solids.

18.9.2.6.3 Troubleshooting Operational Problems(Note: Much of the information in this section is basedon the Environmental Protection Agency’s (EPA) Perfor-mance Evaluation and Troubleshooting at Municipal

Hydraulic Loading Rate gal d ft

Q gal d including recirculation

Media Top Surface ft

2

2

( ) =

( ) ( )( )

Organic Loading Rate lb d 1000 ft

BOD in Filter lb d

edia Volume 1000 ft

3

3

( ) =

( )( )M

Recirculation ratio

Recirculation Flow MGD

Average Influent Flow MGD

( ) =

( )( )



559

Wastewater Treatment Facilities, Washington, D.C., cur-rent editions.)

The following sections are not all-inclusive; they donot cover all of the operational problems associated withthe trickling filter process. They do provide informationon the most common operational problems.

18.9.2.6.3.1 Ponding

1. SymptomsA. Small pools or puddles of water on the sur-

face of the media.B. Decreased performance in the removal of

BOD and TSS.C. Possible odors due to anaerobic conditions

in the media.D. Poor air flow through the media.

2. Causal factorsA. Inadequate hydraulic loading to keep the

media voids flushed clear.B. Application of high strength wastes without

sufficient recirculation to provide dilution.C. Nonuniform media.D. Degradation of the media due to aging or

weatheringE. Medium is uniform, but is too small.F. Debris (moss, leaves, sticks) or living organ-

isms (snails) clog the void spaces.3. Corrective actions

Corrective actions are listed in increasingimpact on the quality of the plant effluent:A. Remove all leaves, sticks, and other debris

from the media.B. Increase recirculation of dilute, high-

strength wastes to improve sloughing tokeep voids open.

C. Use high-pressure stream of water to agitateand flush the ponded area.

D. Rake or fork the ponded area.E. Dose the filter with chlorine solution for 2 to

4 h. The specific dose of chlorine requiredwill depend on the severity of the pondingproblem. When using elemental chlorine,the dose must be sufficient to provide aresidual at the orifices of 1–50 mg/L. If thefilter is severely clogged, the higher residu-als may be needed to unload the majority ofthe biomass. If the filter cannot be dosed byelemental chlorine, chlorinated lime or hightest hypochlorite powder may be used. Dos-ing should be in the range of 8 to 10 lb ofchlorine/1000 ft2 of media.

F. If the filter design permits, the filter mediacan be flooded for a period of 4 h. Remem-ber, if the filter is flooded, care must be taken

to prevent hydraulic overloads of the finalsettling tank. The trickling filter should bedrained slowly at low flow periods.

G. Dry the media. By stopping the flow to thefilter, the slime will dry and loosen. Whenthe flow is restarted, the loosened slime willflow out of the filter. The amount of dryingtime will be dependent on the thickness ofthe slime and the amount of removaldesired. Time may range from a few hoursto several days.

Note: Portions of the media can be dried without tak-ing the filter out of service by plugging theorifices that normally service the area.

Note: If these corrective actions do not provide thedesired improvement, the media must be care-fully inspected. Remove a sample of the mediafrom the affected area. Carefully clean it,inspect for its solidity, and determine its sizeuniformity (3 to 5 in.). If it is acceptable, themedia must be carefully replaced. If the mediaappear to be decomposing or are not uniform,then they should be replaced.

18.9.2.6.3.2 Odors

Frequent offensive odors usually indicate an operationalproblem. These foul odors occur within the filter periodi-cally and are normally associated with anaerobic condi-tions. Under normal circumstances, a slight anaerobicslime layer forms due to the inability of oxygen to penetrateall the way to the media. Under normal operation, the outerslime layers will remain aerobic, and no offensive odorsare produced.

1. Causal factorsA. Excessive organic loading due to poor filter

effluent quality (recirculation), poor primarytreatment operation, and poor control ofsludge treatment process that results in highBOD recycle flows.

B. Poor ventilation because of submerged orobstructed underdrains, clogged vent pipes,or clogged void spaces.

C. Filter is overloaded hydraulically or organ-ically.

D. Poor housekeeping.2. Corrective actions

A. Evaluate the operation of the primary treat-ment process. Eliminate any short-circuiting.Determine any other actions that can betaken to improve the performance of theprimary process.



B. Evaluate and adjust control of sludge treat-ment processes to reduce the BOD or recycleflows.

C. Increase recirculation rate to add additionalDO to filter influent. Do not increase recir-culation rate if the flow rate through theunderdrains would cause less than 50%open space.

D. Maintain aerobic conditions in filter influent.E. Remove debris from media surface.F. Flush underdrains and vent pipes.G. Add one of the commercially available

masking agents to reduce odors and preventcomplaints.

H. Add chlorine at a 1 to 2 mg/L residual forseveral hours at low flow. This will reduceactivity and cut down on the oxygendemand. Chlorination only treats symptoms;a permanent solution must be determinedand instituted.

18.9.2.6.3.3 High Clarifier Effluent Suspended Solids and BOD

1. SymptomA. The effluent from the trickling filter process-

settling unit contains a high concentrationof suspended solids.

2. Causal factorsA. Recirculated flows are too high, causing

hydraulic overloading of the settling tank.In multiple unit operations, the flow is notevenly distributed.

B. Settling tank baffles or skirts have corrodedor broken.

C. Sludge collection mechanism is broken ormalfunctioning.

D. Effluent weirs are not level.E. Short-circuiting occurs because of tempera-

ture variations.F. Improper sludge withdrawal rate or frequency.G. Excessive solids loading from excessive

sloughing.3. Corrective actions

A. Check hydraulic loading and adjust recircu-lated flow if hydraulic loading is too high.

B. Adjust flow to ensure equal distribution.C. Inspect sludge removal equipment. Repair

broken equipment.D. Monitor sludge blanket depth and sludge

solids concentration; adjust withdrawal rateand/or frequency to maintain aerobic condi-tions in settling tank.

E. Adjust effluent weir to obtain equal flowover all parts of the weir length.

F. Determine temperature in the clarifier atvarious points and depths throughout theclarifier. If depth temperatures are consis-tently 1 to 2°F lower than surface readings,a temperature problem exists. Baffles may beinstalled to help to break up these currents.

G. High sloughing rates because of the biolog-ical activity or temperature changes maycreate excessive solids loading. An additionof 1 to 2 mg/L of cationic polymer may behelpful in improving solids capture. Remem-ber, if polymer addition is used, solids with-drawal must be increased.

H. High sloughings because of organic over-loading, toxic wastes, or wide variations ininfluent flow are best controlled at theirsource.

18.9.2.6.3.4 Filter Flies

1. SymptomsA. The trickling filter and surrounding area

become populated with large numbers ofvery small flying insects (psychoda moths).

2. Causal factorsA. Poor housekeeping.B. Insufficient recirculation.C. Intermittent wet and dry conditions.D. Warm weather.

3. Corrective actionsCorrective actions for filter fly problemsrevolve around the need to disrupt the fly’s lifecycle (7 to 10 d in warm weather):A. Increase recirculation rate to obtain a

hydraulic loading of at least 200 gal/d/ft2.At this rate, filter fly larvae are normallyflushed out of the filter.

B. Clean filter walls and remove weeds, brush,and shrubbery around the filter. Thisremoves some of the area for fly breeding.

C. Dose the filter periodically with low chlo-rine concentrations (less than 1 mg/L). Thisnormally destroys larvae.

D. Dry the filter media for several hours.F. Flood the filter for 24 h.G. Spray area around the filter with insecticide.

Do not use insecticide directly on the media,because of the chance of carryover andunknown effects on the slime populations.

18.9.2.6.3.5 Freezing

1. SymptomsA. Decreased air temperature results in visible

ice formation and decreased performance.B. Distributed wastes are in a thin film or spray.

This is more likely to cause ice formation.



561

2. Causal factorsA. Recirculation causes increased temperature

drops and losses.B. Strong prevailing winds cause heat losses.C. Intermittent dosing allows water to stand too

long, causing freezing.3. Corrective actions

All corrective actions are based upon a need toreduce heat loss as the wastes move through thefilter.A. Reduce recirculation as much as possible to

minimize cooling effects.B. Operate two stage filters in parallel to reduce

heat loss.C. Adjust splash plates and orifices to obtain a

coarse spray.D. Construct a windbreak or plant evergreens

or shrubs in the direction of the prevailingwind.

E. If intermittent dosing is used, leave dumpgates open.

F. Cover pump wet wells and dose tanks toreduce heat losses.

G. Cover filter media to reduce heat loss.H. Remove ice before it becomes large enough

to cause stoppage of arms.

Note: During periods of cold weather, the filter willshow decreased performance. However, the fil-ter should not be shut off for extended periods.Freezing of the moisture trapped within themedia causes expansion and may cause struc-tural damage.

18.9.2.7 Process Calculations

Several calculations are useful in the operation of a trick-ling filter, these include: total flow, hydraulic loading, andorganic loading.

18.9.2.7.1 Total Flow

If the recirculated flow rate is given, total flow is:

Note: The total flow to the tricking filter includes theinfluent flow and the recirculated flow. This canbe determined using the recirculation ratio:

EXAMPLE 18.31

Problem:

The trickling filter is currently operating with a recircu-lation rate of 1.5. What is the total flow applied to thefilter when the influent flow rate is 3.65 MGD?

Solution:

18.9.2.7.2 Hydraulic Loading

Calculating the hydraulic loading rate is important inaccounting for both the primary effluent as well as therecirculated trickling filter effluent. Both of these are com-bined before being applied to the surface of the filter. Thehydraulic loading rate is calculated based on the surfacearea of the filter.

EXAMPLE 18.32

Problem:

A trickling filter 90-ft in diameter is operated with aprimary effluent of 0.488 MGD and a recirculated effluentflow rate of 0.566 MGD. Calculate the hydraulic loadingrate on the filter in units gallons per day per square foot.

Solution:

The primary effluent and recirculated trickling filter efflu-ent are applied together across the surface of the filter,therefore:

18.9.2.7.3 Organic Loading Rate

As mentioned earlier, trickling filters are sometimes clas-sified by the organic loading rate applied. The organic

Total Flow MGD Influent Flow MGD

Recirculation Flow MGD

Total Flow gal d Total Flow MGD

gal MG

( ) = ( ) +

( )

( ) = ( ) ¥

, ,1 000 000

(18.24)

Total Flow MGD Influent Flow

Recirculation Rate

( ) = ¥

+( )1 0.

Total Flow MGD MGD 1.5

MGD

( ) ( )= ¥ +

=

3 65 1 0

9 13

. .

.

0 488 0 566

0 785 90

6359

1 054 000

6359165 72

. .

.

, ,.

MGD 1.054 MGD

1,054,000 gal d

Circular Surface Area 0.785 Diameter

ft

ft

gal d

gal d ft

2

2

2

2

MGD

ft

+ =

=

= ¥

= ¥

=

=

( )

( )


562


loading rate is expressed as a certain amount of BODapplied to a certain volume of media.

EXAMPLE 18.33

Problem:

A trickling filter, 50 ft in diameter, receives a primaryeffluent flow rate of 0.445 MGD. Calculate the organicloading rate in units of pounds of BOD applied per dayper 900 ft3 of media volume. The primary effluent BODconcentration is 85 mg/L. The media depth is 9 ft.

Solution:

To determine the pounds of BOD/1000 ft3 in a volume ofthousands of cubic feet, we must set up the equation asshown below:

Regrouping the numbers and the units together:

18.9.2.7.4 Settling TanksIn the operation of settling tanks that follow tricklingfilters, various calculations are routinely made to determinedetention time, surface settling rate, hydraulic loading andsludge pumping.

18.9.3 ROTATING BIOLOGICAL CONTACTORS

The RBC is a biological treatment system (see Figure 18.8)and is a variation of the attached growth idea provided bythe trickling filter. Still relying on microorganisms thatgrow on the surface of a medium, the RBC is a fixed filmbiological treatment device; the basic biological process

is similar to that occurring in the trickling filter. An RBCconsists of a series of closely spaced (mounted side byside), circular, plastic (synthetic) disks that are typicallyabout 3.5 m in diameter and attached to a rotating hori-zontal shaft (see Figure 18.8). Approximately 40% of eachdisk is submersed in a tank containing the wastewater tobe treated. As the RBC rotates, the attached biomass film(zoogleal slime) that grows on the surface of the diskmoves into and out of the wastewater. While submergedin the wastewater, the microorganisms absorb organics;while they are rotated out of the wastewater, they aresupplied with needed oxygen for aerobic decomposition.As the zoogleal slime reenters the wastewater, excess sol-ids and waste products are stripped off the media assloughings. These sloughings are transported with thewastewater flow to a settling tank for removal.

Modular RBC units are placed in series (seeFigure 18.9) simply because a single contactor is not suffi-cient to achieve the desired level of treatment; the resultingtreatment achieved exceeds conventional secondary treat-ment. Each individual contactor is called a stage and thegroup is known as a train. Most RBC systems consist oftwo or more trains with three or more stages in each. Thekey advantage in using RBCs instead of trickling filtersis that RBCs are easier to operate under varying loadconditions, since it is easier to keep the solid medium wetat all times. The level of nitrification, which can beachieved by a RBC system, is also significant. This isespecially the case when multiple stages are employed.

18.9.3.1 RBC Equipment

The equipment that makes up a RBC includes the rotatingbiological contactor (the media: either standard or high

0 445 85

34 15 5

0 785 50

1962 5

1962 5 9 17 662 5

.

. .

.

.

. , .

mg L

8 lb gal 3 BOD applied d

Surface Area 0.785 Diameter

ft

ft

ft ft

Trickling filter volume

2

2

2

2 2

MGD

A D v

¥ ¥

=

= ¥

= ¥

=

¥ =

¥ =

( )

( )

( )

315.5 BOD d

17 662 5

1000

1000, .¥

=¥

¥

=

315.5 lb BOD d

ft

lb BOD d 1000 ft

3

3

1000

17 662 5 1000

17 9

, .

.

lb

FIGURE 18.8 Cross-section of a rotating biological contactor(RBC) treatment system. (From Spellman, F.R., Spellman’sStandard Handbook for Wastewater Operators, Vol. 1, Tech-nomic Publ., Lancaster, PA, 1999.)

Organicmatter

Sloughings

Wastewater holding tank

Oxygen

Media

Zoogleal slime



563

density), a center shaft, drive system, tank, baffles, hous-ing or cover, and a settling tank.

The rotating biological contactor consists of circularsheets of synthetic material (usually plastic) that aremounted side by side on a shaft. The sheets (media) containlarge amounts of surface area for growth of the biomass.

The center shaft provides the support for the disks ofmedia and must be strong enough to support the weightof the media and the biomass. Experience has indicated amajor problem has been the collapse of the support shaft.

The drive system provides the motive force to rotatethe disks and shaft. The drive system may be mechanical,air driven, or a combination of each. When the drivesystem does not provide uniform movement of the RBC,major operational problems can arise.

The tank holds the wastewater where the RBC rotates.It should be large enough to permit variation of the liquiddepth and detention time.

Baffles are required to permit proper adjustment ofthe loading applied to each stage of the RBC process.Adjustment can be made to increase or decrease the sub-mergence of the RBC.

RBC stages are normally enclosed in some type ofprotective structure (cover) to prevent loss of biomass dueto severe weather changes (snow, rain, temperature, wind,sunlight, etc.). In many instances this housing greatlyrestricts access to the RBC.

The settling tank is provided to remove the sloughingmaterial created by the biological activity and is similarin design to the primary settling tank. The settling tankprovides 2- to 4-h detention times to permit settling oflighter biological solids.

18.9.3.2 RBC Operation

During normal operation, operator vigilance is requiredto observe the RBC movement, slime color, and appear-ance. If the unit is covered, observations may be limitedto that portion of the media, which can be viewed throughthe access door.

Sampling and testing should be conducted daily for DOcontent and pH. BOD and suspended solids testing shouldalso be accomplished to aid in assessing performance.

18.9.3.3 RBC: Expected Performance

The RBC normally produces a high quality effluent withBOD at 85 to 95% and suspended solids removal at 85 to95%. The RBC treatment process may also significantlyreduce (if designed for this purpose) the levels of organicnitrogen and ammonia nitrogen.

18.9.3.4 Operator Observations, Process Problems, and Troubleshooting

Rotating biological filter operation requires routine observa-tion, process control sampling and testing, troubleshooting,and process control calculations. Comparison of dailyresults with expected normal ranges is the key to identi-fying problems and appropriate corrective actions.


Note: If the RBC is covered, observations may belimited to the portion of the media that can beviewed through the access door.

1. Rotation — The operator routinely checks theoperation of the RBC to ensure that smooth,uniform rotation is occurring (normal opera-tion). Erratic, nonuniform rotation indicates amechanical problem or uneven slime growth. Ifno movement is observed, mechanical prob-lems or extreme excess of slime growth areindicated.

2. Slime color and appearance — Slime color andappearance can indicate process condition.Gray, shaggy slime growth on the RBC indi-cates normal operation. Reddish brown orgolden brown shaggy growth indicates normalduring nitrification. A very dark brown, shaggygrowth (with worms present) indicates a veryold slime. White chalky growth indicates highinfluent sulfur or sulfide levels. No visible slimegrowth to the RBC indicates a severe pH ortemperature change.

FIGURE 18.9 Rotating biological contactor (RBC) treatment system in series. (From Spellman, F.R., Spellman’s Standard Handbookfor Wastewater Operators, Vol. 1, Technomic Publ., Lancaster, PA, 1999.)

Rotating biological contactors Cl2

Effluent

Solids disposal

Secondarysettlingtanks

Primarysettling

tank

Influent


564


18.9.3.4.2 Process Control Sampling and TestingFor process control, the RBC process does not requirelarge amounts of sampling and testing to provide the infor-mation required. The frequency for performing suggestedtesting depends on available resources and variability ofprocess. Frequency may be lower during normal operationand higher during abnormal conditions.

The following routine sampling points and types oftests will permit the operator to identify normal and abnor-mal operating conditions:

1. RBC train influent — Tests include pH, tem-perature, settleable solids, BOD, suspended sol-ids, and metals.

2. RBC — Test includes speed of rotation.3. RBC train effluent — Tests include DO, pH, jar

tests.4. Process effluent — Tests include DO, pH, settle-

able solids, BOD, and suspended solids

18.9.3.4.3 Troubleshooting Operational Problems(Note: Much of the information in this section is basedon material provided by EPA in Performance Evaluationand Troubleshooting at Municipal Wastewater TreatmentFacilities, Washington, D.C., current edition.)

The following sections are not all-inclusive; they donot cover all of the operational problems associated withthe rotating biological contactor process. They do provideinformation on the most common operational problems.

18.9.3.4.3.1 White Slime

1. SymptomA. White slime on most of the disk area.

2. Causal factorsA. High hydrogen sulfide in influentB. Septic influentC. First stage overloaded

3. Corrective actionsA. Aerate RBC or plant influent.B. Add sodium nitrate or hydrogen peroxide to

influent.C. Adjust baffles between stages 1 and 2 to

increase fraction of total surface area in firststage.

18.9.3.4.3.2 Excessive Sloughing

1. SymptomA. Loss of slime.

2. Causal factorsA. Excessive pH variance.B. Toxic influent.

3. Corrective actionsA. Implement and enforce pretreatment program.B. Install pH control equipment.C. Equalize flow to acclimate organisms.

18.9.3.4.3.3 RBC Rotation

1. SymptomA. RBC rotation is uneven.

2. Causal factorsA. Mechanical growth.B. Uneven growth.

3. Corrective actionsA. Repair mechanical problem.B. Increase rotational speed.C. Adjust baffles to decrease loading.D. Increase sloughing.

18.9.3.4.3.4 Solids

1. SymptomA. Solids accumulating in reactors.

2. Causal factorsA. Inadequate pretreatment.

3. Corrective actionsA. Identify and correct grit removal problem.B. Identify and correct primary settling problem.

18.9.3.4.3.5 Shaft Bearings

1. SymptomA. Shaft bearings running hot or failing.

2. Causal factorA. Inadequate maintenance.

3. Corrective actionA. Follow manufacturer’s recommendations.

18.9.3.4.3.6 Drive Motor

1. SymptomA. Drive motor running hot.

2. Causal factorsA. Inadequate maintenance.B. Improper chain drive alignment.

3. Corrective actionsA. Follow manufacturer’s recommendations.B. Adjust alignment.

18.9.3.5 RBC: Process Control Calculations

Several process control calculations may be useful in theoperation of a RBC. These include soluble BOD, totalmedia area, organic loading rate, and hydraulic loadingrate. Settling tank calculations and sludge pumping cal-culations may be helpful for evaluation and control of thesettling tank following the RBC.

18.9.3.5.1 RBC: Soluble BODThe soluble BOD concentration of the RBC influent canbe determined experimentally in the laboratory or it canbe estimated using the suspended solids concentration andthe K factor. The K factor is used to approximate the BOD(particulate BOD) contributed by the suspended matter.The K factor must be provided or determined experimentally



565

in the laboratory. The K factor for domestic wastes nor-mally ranges from 0.5 to 0.7.

EXAMPLE 18.34

Problem:

The suspended solids concentration of a wastewater is250 mg/L. If the normal K value at the plant is 0.6, whatis the estimated particulate BOD concentration of thewastewater?

Solution:

The K value of 0.6 indicates that about 60% of the sus-pended solids are organic suspended solids (particulateBOD):

EXAMPLE 18.35

Problem:

An RBC receives a flow of 2.2 MGD with a BOD contentof 170 mg/L and suspended solids concentration of 140mg/L. If the K value is 0.7, how many pounds of solubleBOD enter the RBC daily?

Solution:

Now the pounds per day of soluble BOD may be deter-mined:

18.9.3.5.2 RBC: Total Media AreaSeveral process control calculations for the RBC use thetotal surface area of all the stages within the train. As wasthe case with the soluble BOD calculation, plant designinformation or information supplied by the unit manufac-

turer must provide the individual stage areas (or the totaltrain area) because physical determination of this wouldbe extremely difficult.

(18.26)

18.9.3.5.3 RBC: Organic Loading RateIf the soluble BOD concentration is known, the organicloading on a RBC can be determined. Organic loading ona RBC based on soluble BOD concentration can rangefrom 3 to 4 lb/d/1000 ft2.

EXAMPLE 18.36

Problem:

An RBC has a total media surface area of 102,500 ft2 andreceives a primary effluent flow rate of 0.269 MGD. Ifthe soluble BOD concentration of the RBC influent is159 mg/L, what is the organic loading rate in pounds per1000 ft2?

Solution:

18.9.3.5.4 RBC: Hydraulic Loading RateThe manufacturer normally specifies the RBC media sur-face area and the hydraulic loading rate is based on themedia surface area (usually in square feet). Hydraulicloading on a RBC can range from 1 to 3 gal/d/ft2.

EXAMPLE 18.37

Problem:

An RBC treats a primary effluent flow rate of 0.233 MGD.What is the hydraulic loading rate in gal/d/ft2 if the mediasurface area is 96,600 ft2?

Solution:

18.10 ACTIVATED SLUDGE

The biological treatment systems discussed to this point(ponds, trickling filters, and RBCs) have been around foryears. The trickling filter, for example, has been aroundand successfully used since the late 1800s. The problem

Soluble BOD Total BOD

K Factor Total Suspended Solids

5 5= -

¥( )

(18.25)

250 0 6 150 mg L mg L particulate BOD¥ =.

Total BOD Particulate BOD Soluble BOD

mg L mg L mg L

mg L mg L mg L

mg L mg L

mg L soluble BOD

= +

= ¥ +

= +

- =

=

170 140 0 7

170 98

170 98

72

. x

x

x

x

mg L soluble BOD MGD Flow 8.34 lb gal lb d

mg L MGD 8.34 lb gal lb d

soluble BOD

¥ ¥ =

¥ ¥ =72 2 2 1321.

Total Area st Sage Area 2nd Stage Area

Stage Area

= + +

º +

1

nth

0 269 356 7

102 500

1000

10003 482

. .

,.

MGD 159 mg L8.34 lb

1 gal lb d

356.7 lb d

number

unitlb d 1000 ft2

¥ ¥ =

¥ =( )( )ft

233,000 gal d

96,600 ft gal d ft2

2= 2 41.



with ponds, trickling filters, and RBCs is that they aretemperature sensitive and remove less BOD. In addition,trickling filters cost more to build than the activated sludgesystems that were later developed.

Note: Although trickling filters and other systems costmore to build than activated sludge systems, itis important to point out that activated sludgesystems cost more to operate because of theneed for energy to run pumps and blowers.

As shown in Figure 18.10, the activated sludge pro-cess follows primary settling. The basic components of anactivated sludge sewage treatment system include an aer-ation tank and a secondary basin, settling basin, or clarifier(see Figure 18.10). Primary effluent is mixed with settledsolids recycled from the secondary clarifier and is thenintroduced into the aeration tank. Compressed air isinjected continuously into the mixture through porous dif-fusers located at the bottom of the tank, usually along oneside.

Wastewater is fed continuously into an aerated tank,where the microorganisms metabolize and biologicallyflocculate the organics. Microorganisms (activated sludge)are settled from the aerated mixed liquor under quiescentconditions in the final clarifier and are returned to theaeration tank. Left uncontrolled, the number of organismswould eventually become too great; therefore, some mustperiodically be removed (wasted). A portion of the con-centrated solids from the bottom of the settling tank mustbe removed from the process (waste activated sludge).Clear supernatant from the final settling tank is the planteffluent.

18.10.1 ACTIVATED SLUDGE TERMINOLOGY

To better understand the discussion of the activated sludgeprocess presented in the following sections, you mustunderstand the terms associated with the process. Someof these terms have been used and defined earlier in thetext, but we list them here again to refresh your memory.

Review these terms and remember them. They are usedthroughout the discussion.

Absorption taking in or reception of one substanceinto the body of another by molecular or chem-ical actions and distribution throughout theabsorber.

Activated to speed up reaction. When applied tosludge, it means that many aerobic bacteria andother microorganisms are in the sludge particles.

Activated sludge a floc or solid formed by the micro-organisms. It includes organisms, accumulatedfood materials, and waste products from theaerobic decomposition process.

Activated sludge process a biological wastewatertreatment process in which a mixture or influentand activated sludge is agitated and aerated. Theactivated sludge is subsequently separated fromthe treated mixed liquor by sedimentation andis returned to the process as needed. The treatedwastewater overflows the weir of the settlingtank in which separation from the sludge takesplace.

Adsorption the adherence of dissolved, colloidal, orfinely divided solids to the surface of solid bod-ies when they are brought into contact.

Aeration mixing air and a liquid by one of the follow-ing methods: spraying the liquid in the air, dif-fusing air into the liquid, or agitating the liquidto promote surface adsorption of air.

Aerobic a condition in which free or dissolved oxygenis present in the aquatic environment. Aerobicorganisms must be in the presence of DO to beactive.

Bacteria single-cell plants that play a vital role instabilization of organic waste.

Biochemical oxygen demand (BOD) a measure ofthe amount of food available to the microorgan-isms in a particular waste. It is measured by theamount of dissolved oxygen used up during a

FIGURE 18.10 The activated sludge process. (From Spellman, F.R., Spellman’s Standard Handbook for Wastewater Operators, Vol.1, Technomic Publ., Lancaster, PA, 1999.)

Air

Aeration tank Settling tank

Activated sludge



specific time period (usually 5 d, expressed asBOD5).

Biodegradable from “degrade” (to wear away orbreak down chemically) and “bio” (by livingorganisms). Put it all together, and you have asubstance, usually organic, that can be decom-posed by biological action.

Bulking a problem in activated sludge plants thatresults in poor settleability of sludge particles.

Coning a condition that may be established in a sludgehopper during sludge withdrawal, when part ofthe sludge moves toward the outlet while theremainder tends to stay in place. Developmentof a cone or channel of moving liquids sur-rounded by relatively stationary sludge.

Decomposition generally, in waste treatment, decom-position refers to the changing of waste matterinto simpler, more stable forms that will notharm the receiving stream.

Diffuser a porous plate or tube through which air isforced and divided into tiny bubbles for distribu-tion in liquids. Commonly made of carborundum,aluminum, or silica sand.

Diffused air aeration a diffused air activated sludgeplant takes air, compresses it, then dischargesthe air below the water surface to the aeratorthrough some type of air diffusion device.

Dissolved oxygen (DO) atmospheric oxygen dissolvedin water or wastewater.

Note: The typical required DO for a well-operated acti-vated sludge plant is between 2.0 and 2.5 mg/L.

Facultative facultative bacteria can use either molec-ular (dissolved) oxygen or oxygen obtainedfrom food materials. In other words, facultativebacteria can live under aerobic or anaerobicconditions.

Filamentous bacteria organisms that grow in threador filamentous form.

Food-to-microorganisms ratio (F:M ratio) a processcontrol calculation used to evaluate the amountof food (BOD or COD) available per pound ofmixed liquor volatile suspended solids.

Fungi multicellular aerobic organisms.Gould sludge age a process control calculation used

to evaluate the amount of influent suspended

solids available per pound of mixed liquor sus-pended solids.

Mean cell residence time (MCRT) the average lengthof time mixed liquor suspended solids particleremains in the activated sludge process. This isusually written as MCRT and may also bereferred to as sludge retention rate.

Mixed liquor the contribution of return activatedsludge and wastewater (either influent or efflu-ent) that flows into the aeration tank.

Mixed liquor suspended solids (MLSS) t h e su s -pended solids concentration of the mixedliquor. Many references use this concentrationto represent the amount of organisms in theliquor or the amount of organisms in the acti-vated sludge process.

Mixed liquor volatile suspended solids (MLVSS) theorganic matter in the mixed liquor suspendedsolids. This can also be used to represent theamount of organisms in the process.

Nematodes microscopic worms that may appear inbiological waste treatment systems.

Nutrients substances required to support plant organ-isms. Major nutrients are carbon, hydrogen,oxygen, sulfur, nitrogen, and phosphorus.

Protozoa single-cell animals that are easily observedunder the microscope at a magnification of100¥. Bacteria and algae are prime sources offood for advanced forms of protozoa.

Return activated sludge (RAS) the solids returnedform the settling tank to the head of the aerationtank.

Rising sludge rising sludge occurs in the secondaryclarifiers or activated sludge plant when thesludge settles to the bottom of the clarifier, iscompacted, and then rises to the surface in rel-atively short time.

Rotifiers multicellular animals with flexible bodiesand cilia near their mouths used to attract food.Bacteria and algae are their major source offood.

Secondary treatment a wastewater treatment processused to convert dissolved or suspended materi-als into a form that can be removed.

Settleability a process control test used to evaluate thesettling characteristics of the activated sludge.Readings taken at 30 to 60 min are used tocalculate the settled sludge volume and thesludge volume index.

Food lb d

MLVSS lb

Q MGD BOD mg 8.34 lb gal

v MG MLVSS mg 8.34 lb gal

L

L

Microorganism

BOD=

=¥ ¥

¥ ¥

( )

( ) ( )( ) ( )

MCRT dSolids in Activated Sludge Process lb

Solids Removed from Process lb d( ) ( )

( )=



Settled sludge volume (SSV) the volume of milligramsper liter (or percent) occupied by an activatedsludge sample after 30 or 60 minutes of settling.Normally written as SSV with a subscript toindicate the time of the reading used for calcu-lation (SSV30 or SSV60).

Shock load the arrival at a plant of a waste toxic toorganisms in sufficient quantity or strength tocause operating problems, such as odor orsloughing off of the growth of slime on the trick-ling filter media. Organic overloads also cancause a shock load.

Sludge volume index (SVI) a process control calcu-lation used to evaluate the settling quality of theactivated sludge. Requires the SSV30 and mixedliquor suspended solids test results to calculate.

Solids material in the solid state. The different typesof solids include:Dissolved solids present in solution. Solids that

will pass through a glass fiber filter.Fixed also known as the inorganic solids. The

solids that are left after a sample is ignited at550°C for 15 min.

Floatable solids that will float to the surface ofstill water, sewage, or other liquid. Usuallycomposed of grease particles, oils, light plas-tic material, etc. Also called scum.

Nonsettleable finely divided suspended solidsthat will not sink to the bottom in still water,sewage, or other liquid in a reasonable peri-od, usually 2 h. Non-settleable solids are alsoknown as colloidal solids.

Suspended solids that will not pass through aglass fiber filter.

Total solids in water, sewage, or other liquids.It includes the suspended solids and dis-solved solids.

Volatile organic solids. Measured as the solidsthat are lost on ignition of the dry solids at550°C.

Waste activated sludge (WAS) the so l ids be ingremoved from the activated sludge process.

18.10.2 ACTIVATED SLUDGE PROCESS: EQUIPMENT

The equipment requirements for the activated sludge pro-cess are more complex than other processes discussed.Equipment includes an aeration tank, aeration, system-settling tank, return sludge, and waste sludge.

18.10.2.1 Aeration Tank

The aeration tank is designed to provide the requireddetention time (depends on the specific modification) andensure that the activated sludge and the influent waste-water are thoroughly mixed. Tank design normallyattempts to ensure no dead spots are created.

18.10.2.2 Aeration

Aeration can be mechanical or diffused. Mechanical aer-ation systems use agitators or mixers to mix air and mixedliquor. Some systems use a sparge ring to release airdirectly into the mixer. Diffused aeration systems use pres-surized air released through diffusers near the bottom ofthe tank. Efficiency is directly related to the size of theair bubbles produced. Fine bubble systems have a higherefficiency. The diffused air system has a blower to producelarge volumes of low pressure air (5 to 10 psi), air linesto carry the air to the aeration tank, and headers to dis-tribute the air to the diffusers that release the air into thewastewater.

18.10.2.3 Settling Tank

Activated sludge systems are equipped with plain settlingtanks designed to provide 2 to 4 h HDT.

18.10.2.4 Return Sludge

The return sludge system include pumps, a timer or vari-able speed drive to regulate pump delivery and a flowmeasurement device to determine actual flow rates.

18.10.2.5 Waste Sludge

In some cases, the WAS withdrawal is accomplished byadjusting valves on the return system. When a separatesystem is used it includes pumps, a timer or variable speeddrive, and a flow measurement device.

18.10.3 OVERVIEW OF ACTIVATED SLUDGE PROCESS

The activated sludge process is a treatment technique inwhich wastewater and reused biological sludge full ofliving microorganisms are mixed and aerated. The biolog-ical solids are then separated from the treated wastewaterin a clarifier and are returned to the aeration process orwasted.

The microorganisms are mixed thoroughly with theincoming organic material, and they grow and reproduceby using the organic material as food. As they grow andare mixed with air, the individual organisms cling together(flocculate). Once flocculated, they more readily settle inthe secondary clarifiers.

The wastewater being treated flows continuously intoan aeration tank where air is injected to mix the wastewater

Sludge Vol. Index SVI mL g

30 min settled vol., mL L mg g

Mixed Liquor Suspended Solids, mg L

( ) =

( )( )

,

1000



with the returned activated sludge and to supply the oxygenneeded by the microbes to live and feed on the organics.Aeration can be supplied by injection through air diffusersin the bottom of tank or by mechanical aerators locatedat the surface.

The mixture of activated sludge and wastewater in theaeration tank is called the mixed liquor. The mixed liquorflows to a secondary clarifier where the activated sludgeis allowed to settle.

The activated sludge is constantly growing, and moreis produced than can be returned for use in the aerationbasin. Some of this sludge must be wasted to a sludgehandling system for treatment and disposal. The volumeof sludge returned to the aeration basins is normally 40 to60% of the wastewater flow. The rest is wasted.

18.10.4 ACTIVATED SLUDGE PROCESS: FACTORS AFFECTING OPERATION

A number of factors affect the performance of an activatedsludge system. These include the following:

1. Temperature2. Return rates3. Amount of oxygen available4. Amount of organic matter available5. pH6. Waste rates7. Aeration time8. Wastewater toxicity

To obtain the desired level of performance in an acti-vated sludge system, a proper balance must be maintainedbetween the amount of food (organic matter), organisms(activated sludge), and oxygen (DO). The majority ofproblems with the activated sludge process result from animbalance between these three items.

To fully appreciate and understand the biological pro-cess taking place in a normally functioning activatedsludge process, the operator must have knowledge of thekey players in the process: the organisms. This makes acertain amount of sense when you consider that the heartof the activated sludge process is the mass of settleablesolids formed by aerating wastewater containing biologicaldegradable compounds in the presence of microorganisms.Activated sludge consists of organic solids plus bacteria,fungi, protozoa, rotifers, and nematodes.

18.10.4.1 Growth Curve

To understand the microbiological population and its func-tion in an activated sludge process, the operator must befamiliar with the microorganism growth curve (see Sec-tion 11.12.5, Figure 11.15).

In the presence of excess organic matter, the micro-organisms multiply at a fast rate. The demand for foodand oxygen is at its peak. Most of this is used for theproduction new cells. This condition is known as the loggrowth phase (see Figure 11.15).

As time continues, the amount of food available fororganisms declines. Floc begins to form, while the growthrate of bacteria and protozoa begins to decline. This isreferred to as the declining growth phase (see Figure 11.15).

The endogenous respiration phase occurs as the foodavailable becomes extremely limited and the organismmass begins to decline (see Figure 11.15). Some of themicroorganisms may die and break apart, releasingorganic matter that can be consumed by the remainingpopulation.

The actual operation of an activated-sludge system isregulated by three factors: (1) the quantity of air suppliedto the aeration tank, (2) the rate of activated-sludge recir-culation, and (3) the amount of excess sludge withdrawnform the system. Sludge wasting is an important opera-tional practice because it allows the operator to establishthe desired concentration of MLSS, F:M ratio, and sludgeage.

Note: Air requirements in an activated sludge basinare governed by (1) BOD loading and thedesired removal effluent, (2) volatile suspendedsolids concentration in the aerator, and (3) sus-pended solids concentration of the primaryeffluent.

18.10.5 ACTIVATED SLUDGE FORMATION

The formation of activated sludge is dependent on threesteps. The first step is the transfer of food from wastewaterto organism. Second is the conversion of wastes to a usableform. Third is the flocculation step.

1. Transfer — Organic matter (food) is transferredfrom the water to the organisms. Soluble mate-rial is absorbed directly through the cell wall.Particulate and colloidal matter is adsorbed tothe cell wall, where it is broken down into sim-pler soluble forms and absorbed through thecell wall.

2. Conversion — Food matter is converted to cellmatter by synthesis and oxidation into endproducts such as CO2, H2O, NH3, stable organicwaste, and new cells.

3. Flocculation — Flocculation is the gathering offine particles into larger particles. This processbegins in the aeration tank and is the basicmechanism for removal of suspended matter inthe final clarifier. The concentrated bio-floc thatsettles and forms the sludge blanket in the sec-ondary clarifier is known as activated sludge.



18.10.6 ACTIVATED SLUDGE: PERFORMANCE-CONTROLLING FACTORS

To maintain the working organisms in the activated sludgeprocess, the operator must ensure that a suitable environ-ment is maintained by being aware of the many factorsinfluencing the process and by monitoring them repeat-edly. Control is defined as maintaining the proper solids(floc mass) concentration in the aerator for the incomingwater (food) flow by adjusting the return and waste sludgepumping rate and regulating the oxygen supply to main-tain a satisfactory level of DO in the process.

18.10.6.1 Aeration

The activated sludge process must receive sufficient aer-ation to keep the activated sludge in suspension and tosatisfy the organism oxygen requirements. Insufficientmixing results in dead spots, septic conditions, and lossof activated sludge.

18.10.6.2 Alkalinity

The activated sludge process requires sufficient alkalinityto ensure that pH remains in the acceptable range of 6.5 to9.0. If organic nitrogen and ammonia are being convertedto nitrate (nitrification), sufficient alkalinity must be avail-able to support this process as well.

18.10.6.3 Nutrients

The microorganisms of the activated sludge processrequire nutrients (nitrogen, phosphorus, iron, and othertrace metals) to function. If sufficient nutrients are notavailable, the process will not perform as expected. Theaccepted minimum ratio of carbon to nitrogen, phospho-rus, and iron is 100 parts carbon to 5 parts nitrogen, 1 partphosphorus, and 0.5 parts iron.

18.10.6.4 pH

The pH of the mixed liquor should be maintained withinthe range of 6.5 to 9.0 (ideally 6.0 to 8.0). Gradual fluc-tuations within this range will normally not upset theprocess. Rapid fluctuations or fluctuations outside thisrange can reduce organism activity.

18.10.6.5 Temperature

As temperature decreases, activity of the organisms willalso decrease. Cold temperatures also require longerrecovery time for systems that have been upset. Warmtemperatures tend to favor denitrification and filamentousgrowth.

Note: The activity level of bacteria within the acti-vated sludge process increases with rise in tem-perature.

18.10.6.6 Toxicity

Sufficient concentrations of elements or compounds thatenter a treatment plant that have the ability to kill themicroorganisms (the activated sludge) are known as toxicwaste (shock level). Common to this group are cyanidesand heavy metals.

Note: A typical example of a toxic substance added byoperators is the uninhabited use of chlorine forodor control or control of filamentous organisms(prechlorination). Chlorination is for disinfec-tion. Chlorine is a toxicant and should not beallowed to enter the activated sludge process; itis not selective with respect to type of organismsdamaged or killed. It may kill the organismsthat should be retained in the process as workers.However, chlorine is very effective in disinfectingthe plant effluent after treatment by the acti-vated sludge process.

18.10.6.7 Hydraulic Loading

Hydraulic loading is the amount of flow entering the treat-ment process. When compared with the design capacityof the system, it can be used to determine if the processis hydraulically overloaded or underloaded. If more flowis entering the system than it was designed to handle, thesystem is hydraulically overloaded. If less flow is enteringthe system than it was designed to handle, the system ishydraulically underloaded. Generally, the system is moreaffected by overloading than by underloading.

Overloading can be caused by stormwater, infiltrationof groundwater, excessive return rates, or many othercauses. Underloading normally occurs during periods ofdrought or in the period following initial start-up whenthe plant has not reached its design capacity.

Excess hydraulic flow rates through the treatmentplant will reduce the efficiency of the clarifier by allowingactivated sludge solids to rise in the clarifier and pass overthe effluent weir. This loss of solids in the effluentdegrades effluent quality and reduces the amount of acti-vated sludge in the system, reducing process performance.

18.10.6.8 Organic Loading

Organic loading is the amount of organic matter enteringthe treatment plant. It is usually measured as BOD. Anorganic overload occurs when the amount of BOD enter-ing the system exceeds the design capacity of the system.An organic underload occurs when the amount of BOD



entering the system is significantly less than the designcapacity of the plant.

Organic overloading may occur when the systemreceives more waste than it was designed to handle. It canalso occur when an industry or other contributor dis-charges more wastes to the system than originally planned.Wastewater treatment plant processes can also causeorganic overloads returning high-strength wastes from thesludge treatment processes.

Regardless of the source, an organic overloading ofthe plant results in increased demand for oxygen. Thisdemand may exceed the air supply available from theblowers. When this occurs, the activated sludge processmay become septic.

Excessive wasting can also result in a type of organicoverload. The food available exceeds the number of acti-vated sludge organisms, resulting in increased oxygendemand and very rapid growth.

Organic underloading may occur when a new treat-ment plant is initially put into service. The facility maynot receive enough waste to allow the plant to operate atits design level. Underloading can also occur when exces-sive amounts of activated sludge are allowed to remain inthe system. When this occurs, the plant will have difficultyin developing and maintaining a good activated sludge.

18.10.7 ACTIVATED SLUDGE MODIFICATIONS

First developed in 1913, the original activated sludge pro-cess has been modified over the years to provide betterperformance for specific operating conditions or with dif-ferent influent waste characteristics.

18.10.7.1 Conventional Activated Sludge

1. Employing the conventional activated sludgemodification requires primary treatment.

2. Conventional activated sludge provides excel-lent treatment, but large aeration tank capacityis required, and construction costs are high.

3. In operation, initial oxygen demand is high. Theprocess is also very sensitive to operationalproblems (e.g., bulking).

18.10.7.2 Step Aeration

1. Step aeration requires primary treatment.2. It provides excellent treatment.3. Operation characteristics are similar to conven-

tional.4. It distributes organic loading by splitting influ-

ent flow.5. It reduces oxygen demand at the head of the

system.6. It reduces solids loading on settling tank.

18.10.7.3 Complete Mix

1. May or may not include primary treatment.2. Distributes waste, return, and oxygen evenly

throughout tank.3. Aeration may be more efficient.4. Maximizes tank use.5. Permits a higher organic loading.

Note: During the complete mix, activated sludge pro-cess organisms are in declining phase on growthcurve.

18.10.7.4 Pure Oxygen

1. Requires primary treatment.2. Permits higher organic loading.3. Uses higher solids levels.4. Operates at higher F/M ratios.5. Uses covered tanks.6. Potential safety hazards (pure oxygen).7. Oxygen production is expensive.

18.10.7.5 Contact Stabilization

1. Contact stabilization does not require primarytreatment.

2. During operation, organisms collect organicmatter (during contact).

3. Solids and activated sludge are separated fromflow via settling.

4. Activated sludge and solids are aerated for 3 to6 h (stabilization).

Note: Return sludge is aerated before it is mixed withinfluent flow.

5. The activated sludge oxidizes available organicmatter.

6. While the process is complicated to control, itrequires less tank volume than other modifica-tions and can be prefabricated as a package unitfor flows of 0.05 to 1.0 MGD.

7. A disadvantage is that common process controlcalculations do not provide usable information.

18.10.7.6 Extended Aeration

1. Does not require primary treatment.2. Used frequently for small flows such as schools

and subdivisions.3. Uses 24-h aeration.4. Produces low BOD effluent.5. Produces the least amount of WAS.



6. Process is capable of achieving 95% or moreremovals of BOD.

7. Can produce effluent low in organic and ammo-nia nitrogen.

18.10.7.7 Oxidation Ditch

1. Does not require primary treatment.2. The oxidation ditch process is similar to the

extended aeration process.

Table 18.5 lists the process parameters for each of thefour most commonly used activated sludge modifications.

18.10.8 ACTIVATED SLUDGE: PROCESS CONTROL PARAMETERS

In operating an activated sludge process, the operator mustbe familiar with the many important process controlparameters that must be monitored frequently and adjustedoccasionally to maintain optimal performance.

18.10.8.1 Alkalinity

Monitoring alkalinity in the aeration tank is essential tocontrol of the process. Insufficient alkalinity will reduceorganism activity and may result in low effluent pH and,in some cases, extremely high chlorine demand in thedisinfection process.

18.10.8.2 Dissolved Oxygen

The activated sludge process is an aerobic process thatrequires some DO be present at all times. The amount of

oxygen required is dependent on the influent food (BOD),the activity of the activated sludge, and the degree oftreatment desired.

18.10.8.3 pH

Activated sludge microorganisms can be injured ordestroyed by wide variations in pH. The pH of the aerationbasin will normally be in the range of 6.5 to 9.0. Gradualvariations within this range will not cause any major prob-lems; rapid changes of one or more pH units can have asignificant impact on performance. Industrial waste dis-charges, septic wastes, or significant amounts of stormwaterflows may produce wide variations in pH.

pH should be monitored as part of the routine processcontrol-testing schedule. Sudden changes or abnormal pHvalues may indicate an industrial discharge of stronglyacidic or alkaline wastes. Because these wastes can upsetthe environmental balance of the activated sludge, the pres-ence of wide pH variations can result in poor performance.

Processes undergoing nitrification may show a signif-icant decrease in effluent pH.

18.10.8.4 Mixed Liquor Suspended Solids, Mixed Liquor Volatile Suspended Solids, and Mixed Liquor Total Suspended Solids

The MLSS or MLVSS can be used to represent the acti-vated sludge or microorganisms present in the process.Process control calculations, such as sludge age and SVI,cannot be calculated unless the MLSS is determined.

Adjust the MLSS and MLVSS by increasing ordecreasing the waste sludge rates.

TABLE 18.5Activated Sludge Modifications

Parameter Conventional Contact Stabilization Extended Aeration Oxidation Ditch

Aeration time (h) 4–8 0.5–1.5 (contact) 3–6 (reaeration)

24 24

Settling time (h) 2–4 2–4 2–4 2–4Return rate (% of influent flow) 25–100 25–100 25–100 25–100MLSS (mg/L) 1500–4000 1000–3000

3000–80002000–6000 2000–6000

DO (mg/L) 1–3 1–3 1–3 1–3SSV30 (mL/L) 400–700 400–700 (contact) 400–700 400–700F:M ratio (lb BOD5/lb MLVSS) 02–0.5 0.2–0.6 (contact) 0.05–0.15 0.05–0.15MCRT (whole system [d]]) 5–15 N/A 20–30 20–30% Removal BOD5 85–95% 85–95% 85–95% 85–95%% Removal TSS 85–95% 85–95% 85–95% 85–95%Primary treatment Yes No No No

N/A = not available.

Source: Spellman, F.R., Spellman’s Standard Handbook for Wastewater Operators, Vol. 1, Technomic Publ., Lancaster,PA, 1999.



The mixed liquor total suspended solids (MLTSS) isan important activated sludge control parameter. Toincrease the MLTSS, for example, the operator mustdecrease the waste rate or increase the MCRT. The MCRTmust be decreased to prevent the MLTSS from changingwhen the number of aeration tanks in service is reduced.

Note: In performing the Gould sludge age test,assume that the source of the MLTSS in theaeration tank is influent solids.

18.10.8.5 Return Activated Sludge Rate and Concentration

The sludge rate is a critical control variable. The operatormust maintain a continuous return of activated sludge tothe aeration tank or the process will show a drasticdecrease in performance. If the rate is too low, solidsremain in the settling tank, resulting in solids loss and aseptic return. If the rate is too high, the aeration tank canbecome hydraulically overloaded, causing reduced aera-tion time and poor performance.

The return concentration is also important because itmay be used to determine the return rate required to main-tain the desired MLSS.

18.10.8.6 Waste Activated Sludge Flow Rate

Because the activated sludge contains living organismsthat grow, reproduce, and produce waste matter, theamount of activated sludge is continuously increasing. Ifthe activated sludge is allowed to remain in the systemtoo long, the performance of the process will decrease. Iftoo much activated sludge is removed from the system,

the solids become very light and will not settle quicklyenough to be removed in the secondary clarifier.

18.10.8.7 Temperature

Because temperature directly affects the activity of themicroorganisms, accurate monitoring of temperature canbe helpful in identifying the causes of significant changesin organization populations or process performance.

18.10.8.8 Sludge Blanket Depth

The separation of solids and liquid in the secondary clar-ifier results in a blanket of solids. If solids are not removedfrom the clarifier at the same rate they enter, the blanketwill increase in depth. If this occurs, the solids may carryover into the process effluent. The sludge blanket depthmay be affected by other conditions, such as temperaturevariation, toxic wastes, or sludge bulking.

The best sludge blanket depth is dependent upon suchfactors as hydraulic load, clarifier design, and sludge char-acteristics (see Figure 18.11). The best blanket depth mustbe determined on an individual basis by experimentation.

Note: In measuring sludge blanket depth, it is generalpractice to use a 15 to 20 ft long clear plasticpipe marked at 6-in. intervals, the pipe isequipped with a ball valve at the bottom.

18.10.9 OPERATIONAL CONTROL LEVELS

(Note: Much of the information in this section is basedon Activated Sludge Process Control, Part II, 2nd ed.,Virginia Water Control Board, 1990.)

The operator has two methods available to operate anactivated sludge system. The operator can wait until the

FIGURE 18.11 Settling tank mass balance. (From Spellman, F.R., Spellman’s Standard Handbook for Wastewater Operators, Vol.1, Technomic Publ., Lancaster, PA, 1999.)

Suspended solids inSettling tank

Suspended solids out

Sludge solids out

Sludge solids outSuspended solids out

Suspendedsolids in



process performance deteriorates and make drasticchanges, or the operator can establish normal operationallevels and make minor adjustments to keep the processwithin the established operational levels.

Note: Control levels can be defined as the upper andlower values for a process control variable thatcan be expected to produce the desired effluentquality.

While the method will guarantee that plant performancewill always be maintained within effluent limitations, thesecond method has a much higher probability of achievingthis objective. This section discusses methods used toestablish normal control levels for the activated sludgeprocess.

Several major factors should be considered whenestablishing control levels for the activated sludge system.These include the following:

1. Influent characteristics

2. Industrial contributions

3. Process sidestreams

4. Seasonal variations

5. Required effluent quality

18.10.9.1 Influent Characteristics

Influent characteristics were discussed earlier. A majorarea to consider when evaluating influent characteristicsis the nature and volume of industrial contributions to thesystem. Waste characteristics (BOD, solids, pH, metals,toxicity, and temperature), volume, and discharge pattern(continuous, slug, daily, weekly, etc.) should be evaluatedwhen determining if a waste will require pretreatment bythe industry or adjustments to operational control levels.

18.10.9.2 Industrial Contributions

One or more industrial contributors produce a significantportion of the plant loading (in many systems). Identifyingand characterizing all industrial contributors is important.Remember that the volume of waste generated may notbe as important as the characteristics of the waste.Extremely high-strength wastes can result in organic over-loading and poor performance because of insufficientnutrient availability.

A second consideration is the presence of materialsthat even in small quantities are toxic to the process micro-organisms or create a toxic condition in the plant effluentor plant sludge.

Industrial to a biological treatment system should bethoroughly characterized prior to acceptance, monitored

frequently, and controlled by either local ordinance or byimplementation of a pretreatment program.

18.10.9.3 Process Sidestreams

Process sidestreams are flows produced in other treatmentprocesses that must be returned to the wastewater systemfor treatment prior to disposal. Examples of process side-streams include the following:

1. Thickener supernatant2. Aerobic and anaerobic digester supernatant3. Liquids removed by sludge dewatering pro-

cesses (filtrate, centrate, and subnate)4. Supernatant from heat treatment and chlorine

oxidation sludge treatment processes

Testing these flows periodically to determine boththeir quantity and strength is important. In many treatmentsystems, a significant part of the organic and/or hydraulicloading for the plant is generated by sidestream flows. Thecontribution of the plant sidestream flows can significantlychange the operational control levels of the activatedsludge system.

18.10.9.4 Seasonal Variations

Seasonal variations in temperature, oxygen solubility,organism activity, and waste characteristics may requireseveral normal control levels for the activated sludge pro-cess. For example, during cold months of the year, aerationtank solids levels may have to be maintained at signifi-cantly higher level than are required during warm weather.Likewise, the aeration rate may be controlled by the mixingrequirements of the system during the colder months andby the oxygen demand of the system during the warmmonths.

18.10.9.5 Control Levels at Start-Up

Control levels for an activated sludge system during start-up are usually based upon design engineer recommendationsor information available from recognized referencesources. Although these levels provide a starting point,you should recognize that both the process control param-eter sensitivity and control levels should be established ona plant-by-plant basis.

During the first 12 months of operation, you shouldevaluate all potential process control options, to determinethe following:

1. Sensitivity to effluent quality changes2. Seasonal variability3. Potential problems



18.10.10 OPERATOR OBSERVATIONS: INFLUENT AND AERATION TANK

Wastewater operators are required to monitor or makecertain observations of treatment unit processes to ensureoptimum performance and make adjustments whenrequired. In monitoring the operation of an aeration tank,the operator should look for three physical parameters(turbulence, surface foam and scum, and sludge color andodor), that aid in determining how the process is operatingand indicate if any operational adjustments should bemade.

This information should be recorded each time oper-ational tests are performed. We summarize aeration tankand secondary settling tank observations in the followingsections. Remember that many of these observations arevery subjective and must be based upon experience. Plantpersonnel must be properly trained on the importance ofensuring that recorded information is consistent through-out the operating period.

18.10.10.1 Visual Indicators: Influent and Aeration Tank

18.10.10.1.1 Turbulence

Normal operation of an aeration basin includes a certainamount of turbulence. This turbulent action is required toensure a consistent mixing pattern. Whenever excessive,deficient or nonuniform mixing occurs, adjustments maybe necessary to airflow, or diffusers may need cleaning orreplacement.

18.10.10.1.2 Surface Foam and Scum

The type, color and amount of foam or scum present mayindicate the required wasting strategy to be employed.Types of foam include the following:

1. Fresh, crisp, white foam — Moderate amountsof crisp, white foam are usually associated withactivated sludge processes producing an excel-lent final effluent. Adjustment: None, normaloperation.

2. Thick, greasy, dark tan foam — A thick, greasydark tan or brown foam or scum normally indi-cates an old sludge that is overoxidized, has ahigh mixed liquor concentration, and has awaste rate that is too high. Adjustment: Indi-cates old sludge, more wasting required.

3. White billowing foam — Large amounts of awhite, soap suds-like foam indicate a veryyoung, underoxidized sludge. Adjustment:Young sludge, less wasting required.

18.10.10.1.3 Sludge Color and Odor

Though not as reliable an indicator of process operationsas foam, sludge colors and odor are also useful indicators.Colors and odors that are important include the following:

1. Chocolate brown or earthy odor — Indicatesnormal operation.

2. Light tan or brown or no odor — Indicates sandand clay from infiltration or inflow. Adjustment:Extremely young sludge, decrease wasting.

3. Dark brown or earthy odor — Indicates oldsludge and high solids. Adjustment: Increasewasting.

4. Black color or rotten egg odor — Indicatesseptic conditions, low DO concentration, andan airflow rate that is too low. Adjustment:Increase aeration.

18.10.10.1.4 Mixed Liquor Color

A light chocolate brown mixed liquor color indicates awell-operated activated sludge process.

18.10.10.2 Final Settling Tank (Clarifier) Observations

Settling tank observations include flow pattern (normallyuniform distribution), settling, amount and type of solidsleaving with the process effluent (normally very low) andthe clarity or turbidity of the process effluent (normallyvery clear).

Observations should include the following conditions:

1. Sludge bulking — Occurs when solids areevenly distributed throughout the tank and leav-ing over the weir in large quantities.

2. Sludge Solids Washout — Sludge blanket isdown, but solids are flowing over the effluentweir in large quantities. Control tests indicategood quality sludge.

3. Clumping — Large clumps or masses of sludge(several inches or more) rise to the top of thesettling tank.

4. Ashing — Fine particles of gray to white materialflowing over the effluent weir in large quantities.

5. Straggler floc — Small, almost transparent,very fluffy, buoyant solids particles (1/8 to1/4 in. diameter rising to the surface). Usuallyis accompanied by a very clean effluent. Newgrowth is usually most noted in the early morn-ing hours. Sludge age is slightly below optimum.

6. Pin floc — Very fine solids particles (usuallyless than 1/32 in. diameter) suspended through-out lightly turbid liquid. Usually the result ofan overoxidized sludge.



18.10.11 PROCESS CONTROL TESTING AND SAMPLING

The activated sludge process generally requires more sam-pling and testing to maintain adequate process control thanany of the other unit processes in the wastewater treatmentsystem. During periods of operational problems, both theparameters tested and the frequency of testing mayincrease substantially.

Process control testing may include the following:

1. Settleability testing to determine the settledsludge volume

2. Suspended solids testing to determine influentand MLSS

3. RAS solids and WAS concentrations4. Determination of the volatile content of the

mixed liquor suspended solids5. DO and pH of the aeration tank6. BOD and COD of the aeration tank influent and

process effluent7. Microscopic evaluation of the activated sludge

to determine the predominant organism.

The following sections describe most of the commonprocess control tests.

18.10.11.1 Aeration Influent Sampling

18.10.11.1.1 pHpH is tested daily with a sample taken from the aerationtank influent and process effluent. pH is normally close to7.0 (normal) with the best pH range from 6.5 to 8.5 (6.5 to9.0 is satisfactory). A pH of less than 9.0 may indicatetoxicity from an industrial waste contributor. A pH ofgreater than 6.5 may indicate loss of flocculating organ-isms, potential toxicity, industrial waste contributor, oracid storm flow. Keep in mind that the effluent pH maybe lower because of nitrification.

18.10.11.1.2 TemperatureTemperature is important because it forecasts the following:

1. Temperature increases:A. Organism activity increasesB. Aeration efficiency decreasesC. Oxygen solubility decreases

2. Temperature decreases:A. Organism activity decreasesB. Aeration efficiency increasesC. Oxygen solubility increases

18.10.11.1.3 Dissolved OxygenThe content of DO in the aeration process is critical toperformance. DO should be tested at least daily (peakdemand). Optimum is determined for individual plants,

but normal is from 1 to 3 mg/L. If the system containstoo little DO, the process will become septic. If it containstoo much DO, energy and money is wasted.

18.10.11.1.4 Settled Sludge Volume (Settleability)SSV is determined at specified times during sample test-ing. Both 30- and 60-min observations are used for con-trol. Subscript numbers indicates settling time (e.g., SSV30

and SSV60). The test is performed on aeration tank effluentsample.

(18.28)

Under normal conditions, sludge settles as a mass,producing clear supernatant with SSV60 in the range of400 to 700 mL/L. When higher values are indicated, thismay indicate excessive solids (old sludge) and bulkingconditions. Rising solids (if sludge is well oxidized) mayrise after 2 or more hours. However, rising solids in lessthan 1 h indicates a problem.

Note: Running the settleability test with a dilutedsample can assist in determining if the activatedsludge is old (too many solids) or bulking (notsettling). Old sludge will settle to a more com-pact level when diluted.

18.10.11.1.4.1 Centrifuge TestingThe centrifuge test provides a quick, relatively easy con-trol test for the solids level in the aerator, but does notusually correlate with MLSS results. Results are directlyaffected by variations in sludge quality.

18.10.11.1.5 AlkalinityAlkalinity is essential to biological activity. Nitrificationneeds 7.3-mg/L alkalinity per milligrams per liter or TKN.

18.10.11.1.6 Biochemical Oxygen DemandTesting showing an increase in BOD indicates increasedorganic loading; a decrease in BOD indicates decreasedorganic loading.

18.10.11.1.7 Total Suspended Solids An increase in TSS indicates an increase in organic loading;a decrease TSS indicates a decrease in organic loading.

18.10.11.1.8 Total Kjeldahl NitrogenTKN determination is required to monitor process nitrifi-cation status and to determine alkalinity requirements.

18.10.11.1.9 Ammonia NitrogenDetermination of ammonia nitrogen is required to monitorprocess nitrification status.

SSVMilliliters of Sample

= ( )Milliliters of Settled Sludge 1000 mL L

(18.27)

%SSVMilliliters of Sample

= ¥Milliliters of Settled Sludge 100



18.10.11.1.10 Metals

Metal contents are measured to determine toxicity levels.

18.10.11.2 Aeration Tank

18.10.11.2.1 pH

Normal pH range in the aeration tank is 6.5 to 9.0 pHdecreases indicate process sidestreams or insufficient alka-linity available.

18.10.11.2.2 Dissolved Oxygen

Normal DO range in an aeration tank is 1 to 3 mg/L. DOlevel decreases may indicate increased activity, increasedtemperature, increased organic loading, or decreased MLSSor MLVSS. An increase in DO could be indicative ofdecreased activity, decreased temperature, decreased organicloading, increased MLSS or MLVSS, or influent toxicity.

18.10.11.2.3 Dissolved Oxygen Profile

All DO profile readings should be less than 0.5 mg/L.Readings of greater than 0.5 mg/L indicate inadequateaeration or poor mixing.

18.10.11.2.4 Mixed Liquor Suspended Solids

The range of MLSS is determined by the process modifi-cation used. When MLSS levels increase, more solids,organisms, and an older, more oxidized sludge are typical.

18.10.11.2.5 Microscopic Examination

The activated sludge process can not operate as designedwithout the presence microorganisms. Microscopic exami-nation of an aeration basin sample, which determines thepresence and the type of microorganisms, is important.Different species prefer different conditions; therefore, thepresence of different species can indicate process conditions.

Note: It is important to point out that during micro-scopic examination, identifying of all organ-isms present is not required, but identificationof the predominant species is required.

Table 18.6 lists process conditions indicated by thepresence and population of certain microorganisms.

18.10.11.2.5.1 Interpretation

Routine process control identification can be limited tothe general category of organisms present. For trouble-shooting more difficult problems, a more detailed studyof organism distribution may be required (the knowledgerequired to perform this type of detailed study is beyondthe scope of this text). The major categories of organismsfound in the activated sludge are:

TABLE 18.6Process Condition vs. Organisms Present and Population

Process Condition Organism Population

Poor BOD and TSS removal Predominance of amoeba and flagellatesNo floc formation Mainly dispersed bacteriaVery cloudy effluent A few ciliates present

Poor quality effluent Predominance of amoeba and flagellatesDispersed bacteria Some free-swimming ciliatesSome floc formationCloudy effluent

Satisfactory effluent Predominance of free-swimming ciliatesGood floc formation Few amoeba and flagellatesGood settleabilityGood Clarity

High-quality effluent Predominance of stalked ciliatesExcellent floc formation Some free-swimming ciliatesExcellent Settleability A few rotifersHigh effluent clarity A few flagellates

Effluent High TSS and Low BOD Predominance of rotifersHigh settled sludge volume Large numbers of stalked ciliatesCloudy effluent A few free-swimming ciliates

No flagellates

Source: Spellman, F.R., Spellman’s Standard Handbook for Wastewater Operators,Vol. 1, Technomic Publ., Lancaster, PA, 1999.



1. Protozoa

2. Rotifers

3. Filamentous organisms

Note: Bacteria are the most important microorganismsin the activated sludge. They perform most of thestabilization or oxidation of the organic matterand are normally present in extremely large num-bers. They are not, however, normally visiblewith a conventional microscope operating at therecommended magnification and are not includedin the Table 18.6 list of indicator organisms.

Note: The presence of free-swimming and stalked cil-iates, some flagellates, and rotifers in mixedliquor indicate a balanced, properly settlingenvironment.

Protozoa

Protozoa are secondary feeders in the activated sludgeprocess (secondary as feeders, but nonetheless definitelyimportant to the activated sludge process). Their principalfunction is to remove (eat or crop) dispersed bacteria andhelp to produce a clear process effluent.

To help gain an appreciation for the role of protozoain the activated sludge process consider the followingexplanation.

The activated sludge process is typified by the succes-sive development of protozoa and mature floc particles.This succession can be indicated by the presence of thetype of dominant protozoa present. At the start of theactivated process (or recovery from an upset condition),the amoebas dominate.

Note: Amoebas have very flexible cell walls and moveby shifting fluids within the cell wall. Theypredominate during process start-up or duringrecovery from severe plant upsets.

As the process continues uninterrupted or without upset,small populations of bacteria begin to grow in logarithmicfashion; as the population increases, they develop into mixedliquor. When this occurs, the flagellates dominate.

Note: Flagellated protozoa typically have a singlehair-like flagella or tail that they use for move-ment. The flagellate predominates when theMLSS and bacterial populations are low andorganic load is high. As the activated sludgegets older and denser, the flagellates decreaseuntil they are seldom used.

When the sludge attains an age of about 3 d, lightlydispersed floc particles (flocculation grows fine solids intolarger, more settleable solids) begin to form and bacteriaincrease. At this point, free-swimming ciliates dominate.

Note: The free-swimming ciliated protozoa have hair-like projections (cilia) that cover all or part ofthe cell. The cilia are used for motion and createcurrents that carry food to the organism. Thefree-swimming ciliates are sometimes dividedinto two subcategories: free swimmers andcrawlers. The free swimmers are usually seenmoving through the fluid portion of the acti-vated sludge, while the crawlers appear to bewalking or grazing on the activated sludge solids.The free-swimming ciliated protozoa usuallypredominate when a large number of dispersedbacteria are present that can be used as food.Their predominance indicates a process nearingoptimum conditions and effluent quality.

The process continues with floc particles beginning tostabilize, taking on irregular shapes, and starting to showfilamentous growth. At this stage, the crawling ciliatesdominate. Eventually, mature floc particles develop andincrease in size, and large numbers of crawling and stalkedciliates are present. When this occurs, the succession pro-cess has reached its terminal point.

The succession of protozoan and mature floc particledevelopment just described details the occurrence ofphases of development in a step-by-step progression. Pro-tozoan succession is also based on other factors, includingDO and food availability.

Probably the best way to understand protozoan suc-cession based on DO and food availability is to view thewastewater treatment plant’s aeration basin as a streamwithin a container. The saprobity system classifies thevarious phases of the activated sludge process in relation tothe self-purification process that takes place in a stream.With this system, a clear relationship between the two pro-cesses based on available DO and food supply is evident.

Any change in the relative numbers of bacteria in theactivated sludge process has a corresponding change tomicroorganism’s population. Decreases in bacteriaincrease competition between protozoa and result in seces-sion of dominant groups of protozoa.

The degree of success or failure of protozoa to capturebacteria depends on several factors. Those with moreadvanced locomotion capability are able to capture morebacteria. Individual protozoan feeding mechanisms arealso important in the competition for bacteria. At thebeginning of the activated sludge process, amoebas andflagellates are the first protozoan groups to appear in largenumbers. They can survive on smaller quantities of bac-teria because their energy requirements are lower thanother protozoan types. Because few bacteria are present,competition for dissolved substrates is low. As the bacteriapopulation increases, these protozoa are not able to com-pete for available food. This is when the next group ofprotozoa (the free-swimming protozoa) enters the scene.



Free-swimming protozoa take advantage of the largepopulations of bacteria because they are better equippedwith food-gathering mechanisms than the amoebas andflagellates. The free swimmers are important for theirinsatiable appetites for bacteria and are also important infloc formation. Secreting polysaccharides and mucoproteinsthat are absorbed by bacteria — which make the bacteriasticky through biological agglutination (biological gluingtogether) — allows them to stick together and, moreimportantly, to stick to floc. Large quantities of floc areprepared for removal form secondary effluent and areeither returned to aeration basins or wasted.

The crawlers and stalked ciliates succeed the freeswimmers.

Note: Stalked ciliated protozoa are attached directlyto the activated sludge solids by a stalk. In somecases, the stalk is rigid and fixed in place, whilein others, the organism can more (contract orexpand the stalk) to change its position. Thestalked ciliated protozoa normally have severalcilia that are used to create currents that carrybacteria and organic matter to them. The stalkedciliated protozoa predominate when the dis-persed bacteria population decreases and doesnot provide sufficient food for the free swim-mers. Their predominance indicates a stableprocess, operating at optimum conditions.

The free swimmers are replaced in part because theincreasing level of mature floc retards their movement.Additionally, the type of environment that is provided bythe presence of mature floc is more suited to the needs ofthe crawlers and stalked ciliates. The crawlers and stalkedciliates also aid in floc formation by adding weight to flocparticles, enabling removal.

Rotifers

Rotifers are a higher life form normally associated withclean, unpolluted waters. Significantly larger than most ofthe other organisms observed in activated sludge, rotiferscan use other organisms, as well as organic matter, as theirfood source. Rotifers are usually the predominant organ-ism; the effluent will usually be cloudy (pin of ash floc)and will have very low BOD.

Filamentous Organisms

Filamentous organisms (bacteria, fungi, etc.) occur when-ever the environment of the activated sludge favors theirpredominance. They are normally present in smallamounts and provide the basic framework for floc forma-tion. When the environmental conditions (i.e., pH, nutrientlevels, DO, etc.) favor their development, they become thepredominant organisms. When this occurs, they restrictsettling, and the condition known as bulking occurs.

Note: Microorganism examination of activated sludgeis a useful control tool. In attempting to identifythe microscopic contents of a sample, the oper-ator should try to identify the predominantgroups of organisms.

Note: During microscopic examination of the activatedsludge, a predominance of amoebas indicatesthat the activated sludge is very young.

18.10.11.3 Settling Tank Influent

18.10.11.3.1 Dissolved Oxygen

The DO level of the activated sludge-settling tank shouldbe 1 to 3 mg/L; lower levels may result in rising sludge.

18.10.11.3.2 pH

Normal pH range in an activated sludge-settling tankshould be maintained between 6.5 to 9.0. Decreases in pHmay indicate alkalinity deficiency.

18.10.11.3.3 Alkalinity

A lack of alkalinity in an activated sludge-settling tankwill prevent nitrification.

18.10.11.3.4 Total Suspended Solids

MLSS sampling and testing is required for determiningsolids loading, mass balance, and return rates.

18.10.11.3.5 Settled Sludge Volume (Settleability)

SSV is determined at specified times during sample test-ing. Thirty- and 60-minute observations:

1. Normal operation — When the process is oper-ating property, the solids will settle as a “blanket”(a mass), with a crisp or sharp edge betweenthe solids and the liquor above. The liquid overthe solids will be clear, with little or no visiblesolids remaining in suspension. Settled sludgevolume at the end of 30 to 60 min will be inthe range of 400 to 700 mL.

2. Old or overoxidized activated sludge — Whenthe activated sludge is overoxidized, the solidswill settle as discrete particles. The edge betweenthe solids and liquid will be fuzzy, with a largenumber of visible solids (pin floc, ash floc, etc.)in the liquid. The settled sludge volume at theend of 30 or 60 min will be greater than 700 mL.

3. Young or under-oxidized activated sludge —When the activated sludge is under-oxidized,the solids settle as discrete particles, and theboundary between the solids and the liquid ispoorly defined. Large amounts of small visiblesolids are suspended in the liquid. The settled



sludge volume after 30 to 60 minutes will usu-ally be less than 400 mL.

4. Bulking activated sludge — When the activatedsludge is experiencing a bulking condition, verylittle or no settling is observed.

(18.29)

(18.30)

Note: Running the settleability test with a dilutedsample can assist in determining if the activatedsludge is old (too many solids) or bulking (notsettling). Old sludge will settle to a more com-pact level when diluted.

18.10.11.3.6 Flow

Monitoring flow in settling tank influent is important fordetermination of mass balance.

18.10.11.3.7 Jar Tests

Jar tests are performed as required on settling tank influentand are beneficial in determining the best flocculant aidand appropriate doses to improve solids capture duringperiods of poor settling.

18.10.11.4 Settling Tank

18.10.11.4.1 Sludge Blanket Depth

As mentioned, sludge blanket depth refers to the distancefrom the surface of the liquid to the solids-liquid interface.It can also refer to the thickness of the sludge blanket asmeasured from the bottom of the tank to the solids-liquidinterface. Part of the operator’s sampling routine, this mea-surement is taken directly in the final clarifier. Sludgeblanket depth is dependent upon hydraulic load, returnrate, clarifier design, waste rate, sludge characteristics, andtemperature. If all other factors remain constant, the blan-ket depth will vary with the amount of solids in the systemand the return rate; it will vary throughout the day.

Note: Depth of sludge blanket provides an indicationof sludge quality; it is used as a trend indicator.Many factors affect test result.

18.10.11.4.2 Suspended Solids and Volatile Suspended Solids

Suspended solids and volatile suspended solids concen-trations of the mixed liquor (MLSS), RAS, and WAS areroutinely sampled and tested because they are critical toprocess control.

18.10.11.5 Settling Tank Effluent

18.10.11.5.1 Biochemical Oxygen Demand and Total Suspended Solids

BOD and TSS testing is conducted variably (daily, weekly,and monthly). Increases indicate treatment performanceis decreasing; decreases indicate treatment performance isincreasing.

18.10.11.5.2 Total Kjeldahl NitrogenTKN sampling and testing is variable. An increase in TKNindicates nitrification is decreasing; a decrease in TKNindicates nitrification is increasing.

18.10.11.5.3 Nitrate NitrogenNitrate nitrogen sampling and testing are variable.Increases in nitrate nitrogen indicate nitrification isincreasing or industrial contribution of nitrates. A decreaseindicates reduced nitrification.

18.10.11.5.4 FlowSettling tank effluent flow is sampled and tested daily.Results are required for several process control calcula-tions.

18.10.11.6 Return Activated Sludge and Waste Activated Sludge

18.10.11.6.1 Total Suspended Solids and Volatile Suspended Solids

TSS and total volatile suspended solids concentrations ofthe mixed liquor (MLSS), RAS, and WAS are routinelysampled (using either grab or composite samples) andtested, because they are critical to process control.

The results of the suspended and volatile suspendedtests can be used directly or to calculate such processcontrol figures as MCRT or ratio F/M ratio. In most situ-ations, increasing the MLSS produces an older, densersludge, while decreasing MLSS produces a younger, lessdense sludge.

Note: Control of the sludge wasting rate by constantMLVSS concentration involves maintaining acertain concentration of volatile suspended sol-ids in the aeration tank.

Note: The activated sludge aeration tank should beobserved daily. Included in this daily observa-tion should be a determination of the type andamount of foam, mixing uniformity, and color.

18.10.11.6.2 FlowTest the flow of RAS daily. Test results are required todetermine mass balance and for control of sludge blanket,MLSS, and MLVSS. For WAS, flow is sampled and testedwhenever sludge is wasted. Results are required to deter-mine mass balance and to control solids level in process.

SSVMilliliters of Settled Sludge 1000 mL

Milliliters of Sample=

%SSVMilliliters of Sample

= ¥Milliliters of Settled Sludge 100



581

18.10.12 PROCESS CONTROL ADJUSTMENTS

In the routine performance of their duties, wastewateroperators make process control adjustments to various unitprocesses, including the activated sludge process. In thefollowing a summary is provided of the process controlsavailable for the activated sludge process and the resultthat will occur from adjustment of each.

1. Process control: Return rateA. Condition: Return rate is too high.

Results:1. Hydraulic overloading of aeration and

settling tanks.2. Reduced aeration time.3. Reduced settling time.4. Loss of solids over time.

B. Condition: Return rate is too low.Results:1. Septic return.2. Solids buildup in settling tank.3. Reduced MLSS in aeration tank.4. Loss of solids over weir.

2. Process control: Waste rateA. Condition: Waste rate is too high.

Results:1. Reduced MLSS.2. Decreased sludge density.3. Increased SVI.4. Decreased MCRT.5. Increased F/M Ratio.

B. Condition: Waste rate is too low.Results:1. Increased MLSS.2. Increased sludge density.3. Decreased SVI.4. Increased MCRT.5. Decreased F/M ratio.

3. Process control: Aeration rateA. Condition: Aeration rate is too high.

Results:1. Wasted energy.2. Increased operating cost.3. Rising solids.4. Breakup of activated sludge.

B. Condition: Aeration rate is too low.Results:1. Septic aeration tank.2. Poor performance.3. Loss of nitrification.

18.10.13 TROUBLESHOOTING OPERATIONAL PROBLEMS

The most important dual function performed by the waste-water operator is the identification of process control prob-lems and implementing the appropriate actions to correctthe problems. In this section, typical aeration system oper-ational problems are listed with their symptoms, causes,and the appropriate corrective actions required to restorethe unit process to a normal or optimal performance level.

1. Symptom 1: The solids blanket is flowing overthe effluent weir (classic bulking). Settleabilitytest shows no settling.A. Cause: Organic overloading.

Corrective action: Reduce organic loading.B. Cause: Low pH.

Corrective action: Add alkalinity.C. Cause: Filamentous growth.

Corrective action: Add nutrients. Add chlo-rine or peroxide to return.

D. Cause: Nutrient deficiency.Corrective action: Add nutrients.

E. Cause: Toxicity.Corrective action: Identify source. Imple-ment pretreatment.

F. Cause: Overaeration.Corrective action: Reduce aeration duringlow flow periods.

2. Symptom 2: Solids settled properly in settle-ability test, but large amounts of solids are lostover effluent weir.A. Cause: Billowing solids due to short-circuiting.

Corrective action: Identify short circuitingcause and eliminate if possible.

3. Symptom 3: Large amounts of small pinheadsized solids are leaving the settling tank.A. Cause: Old sludge.

Corrective action: Reduce sludge age (grad-ual change is best). Increase waste rate.

B. Cause: Excessive turbulence.Corrective Action: Decrease turbulence(adjust aeration during low flows).

4. Symptom 4: Large amount of light floc (lowBOD and high solids) leaving settling tank.A. Cause: Extremely old sludge.

Corrective action: Reduce age. Increase waste.5. Symptom 5: Large amounts of small translucent

particles (1/16 to 1/8 in.) are leaving the settlingtank.A. Cause: Rapid solids growth.

Corrective action: Increase sludge age.B. Cause: Slightly young activated sludge.

Corrective action: Decrease waste.


582


6. Symptom 6: Solids are settling properly, butrise to surface within a short time. Many small(1/4 in.) to large (several feet) clumps of solidson surface of settling tank.A. Cause: Denitrification.

Corrective action: Increase rate of return.Adjust sludge age to eliminate nitrification.

B. Cause: Overaeration.Corrective action: Reduce aeration.

7. Symptom 7: RAS has a rotten egg odor.A. Cause: Return is septic.

Corrective action: Increase aeration rateB. Cause: Return rate is too low.

Corrective action: Increase rate of return.8. Symptom 8: Activated sludge organisms die

during a short time.A. Cause: Influent contained toxic material.

Corrective action: Isolate activated sludge (ifpossible). Return all available solids. Stopwasting. Increase return rate. Implement pre-treatment program.

9. Symptom 9: Surface of aeration tank coveredwith thick, greasy foam.A. Cause: Extremely old activated sludge.

Corrective action: Reduce activated sludgeage. Increase wasting. Use foam controlsprays.

B. Cause: Excessive grease and oil in system.Corrective action: Improve grease removal.Use foam control sprays. Implement pre-treatment program.

C. Cause: Froth forming bacteria.Corrective action: Remove froth formingbacteria.

10. Symptom 10: Large clouds of billowing whitefoam on the surface of the aeration tank.A. Cause: Young activated sludge.

Corrective action: Increase sludge age.Decrease wasting. Use foam control sprays.

B. Cause: Low solids in aeration tank.Corrective action: Increase sludge age.Decrease wasting. Use foam control sprays.

C. Cause: Surfactants (detergents).Corrective action: Eliminate surfactants. Usefoam control sprays. Add antifoam.

18.10.14 PROCESS CONTROL CALCULATIONS

As with other wastewater treatment unit processes, pro-cess control calculations are important tools used by theoperator to optimize and control process operations. Inthis section we review the most frequently used activatedsludge calculations.

18.10.14.1 Settled Sludge Volume

SSV is the volume that a settled activated sludge occupiesafter a specified time. The settling time may be shown asa subscript (i.e., SSV60 indicates the reported value wasdetermined at 60 min). SSV can be determined for anytime interval; the most common values are the 30-minreading (SSV30) and 60-min reading (SSV60). The settledsludge volume can be reported as milliliters of sludge perliter of sample or as a percent of SSV.

(18.31)

Note: 1,000 mL = 1 L

(18.32)

(18.33)

EXAMPLE 18.38

Problem:

Using the information provided in the table, calculate theSSV30 and the % SSV60.

Solution:

18.10.14.2 Estimated Return Rate

There are many different methods available for estimationof the proper return sludge rate. A simple methoddescribed in the Operation of Wastewater TreatmentPlants, Field Study Program (1986) — developed by theCalifornia State University, Sacramento — uses the%SSV60. This value can provide an approximation of theappropriate RAS rate. The results of this calculation canthen be adjusted based upon sampling and visual obser-vations to develop the optimum return sludge rate:

Time Milliliters

Start 250015 min 225030 min 180045 min 170060 min 1600

SVISample

mL LSSV mL L

Volume L( ) = ( )

( )

Sample Volume lSample Volume mL

ml L( ) = ( )

1000

% Settled Sludge Volume

Settled Sludge Volum, MI 100Sample Volume, MI

=

¥

SSV30 2 5720= =

1800 mL

L mL L

.

% % 1600 mL 100

mLSSV60 2500

64=¥

=



Note: The %SSV60 must be converted to a decimalpercent and total flow rate (wastewater flow andcurrent return rate in million gallons per daymust be used).

This equation:

1. Assumes %SSV60 is representative.2. Assumes return rate, in per cent equals %SSV60.3. Actual return rate is normally set slightly higher

to ensure organisms are returned to the aerationtank as quickly as possible. The rate of returnmust be adequately controlled to prevent thefollowing:A. Aeration and settling hydraulic overloads.B. Low MLSS levels in the aerator.C. Organic overloading of aeration.D. Solids loss due to excessive sludge blanket

depth.

EXAMPLE 18.39

Problem:

The influent flow rate is 4.2 MGD and the current returnactivated sludge flow rate is 1.5 MGD. The SSV60 is 38%.Based upon this information what should be the returnsludge rate in million gallons per day?

Solution:

18.10.14.3 Sludge Volume Index

SVI is a measure of the settling quality (a quality indica-tor) of the activated sludge. As the SVI increases thesludge settles slower, does not compact as well, and islikely to result in an increase in effluent suspended solids.As the SVI decreases the sludge becomes denser, settlingis more rapid, and the sludge becomes older. SVI is thevolume in milliliters occupied by 1 g of activated sludge.SSV, (milliliters per liter) and the MLSS (milligrams perliter) are required for this calculation:

(18.34)

EXAMPLE 18.40

Problem:

The SSV30 is 365 mL/L and the MLSS is 2365 mg/L.What is the SVI?

Solution:

In this example, SVI equals 154.3. What does thismean? It means is that the system is operating normallywith good settling and low effluent turbidity. We knowthis because we compare the result with the parameterslisted below to obtain the expected condition (the result).

The SVI is best used as a trend indicator to evaluatewhat is occurring compared to previous SVI values. Basedupon this evaluation, the operator may determine if the SVItrend is increasing or decreasing (refer to the chart below).

18.10.14.4 Waste Activated Sludge

The quantity of solids removed from the process as WAS,is an important process control parameter that operatorsneed to be familiar with. More importantly, operators mustalso know how to calculate it and can do so with thefollowing equation:

Estimated Return Rate MGD Influent Flow MGD

Curent Return Flow MGD

( ) ( )[( )]

= +

¥ %SSV60

Estimated Return Rate MGD( )

[ ]= + ¥

=

4 2 1 5 0 38

2 2

. . .

.

MGD MGD

MGD

Sludge Volume Index SVI

SSV, MI LMLSS mg L

( ) =

¥

1000

SVI ValueExpected Condition

(Indications in Parentheses)

Less than 100 Old sludge — possible pin floc (effluent turbidity increasing)

100–200 Normal operation — good Settling (low effluent turbidity)

Greater than 250 Bulking sludge — poor settling (high effluent turbidity)

SVI Value Result Adjustment

Increasing Sludge is becoming less dense Decrease wasteSludge is either younger or bulking

Increase return rate

Sludge will settle more slowlySludge will compact less

Decreasing Sludge is becoming denser Increase waste rateSludge is becoming olderSludge will settle more rapidly Decrease return rateSludge will compact more with no other process changes

Holding constant

No changes indicatedSludge should continue to haveIts current characteristics

Sludge Volume Index =¥

=365 1000

2365154 3

MI L

mg L.



(18.35)

EXAMPLE 18.41

Problem:

The operator wastes 0.44 MGD of activated sludge. TheWAS has a solids concentration of 5540 mg/L. How manypounds of WAS are removed from the process?

Solution:

18.10.14.5 Food to Microorganism Ratio (F:M Ratio)

The F:M ratio is a process control calculation used inmany activated sludge facilities to control the balancebetween available food materials (BOD or COD) andavailable organisms (MLVSS). The COD test is sometimesused, because the results are available in a relatively shortperiod of time.

To calculate the F:M ratio, the following informationis required and Equation 18.46 is used:

1. Aeration tank influent flow rate (MGD)2. Aeration tank influent BOD or COD (mg/L)3. Aeration tank MLVSS (mg/L)4. Aeration tank volume (MG)

Typical F/M ratio for activated sludge processes isshown in the chart below:

EXAMPLE 18.42

Problem:

Given the following data, what is the F:M ratio?

Solution:

Note: If the MLVSS concentration is not available, itcan be calculated if %VM of the MLSS isknown (see Equation 18.37):

(18.37)

Note: The F value in the F:M ratio for computingloading to an activated sludge process can beeither BOD or COD. Remember that the reasonfor sludge production in the activated sludgeprocess is to convert BOD to bacteria. Oneadvantage of using COD over BOD for analysisof organic load is that COD is more accurate.

EXAMPLE 18.43

Problem:

The aeration tank contains 2985 mg/L of MLSS. Labora-tory tests indicate the MLSS is 66% volatile matter. Whatis the MLVSS concentration in the aeration tank?

Solution:

18.10.14.5.1 F:M Ratio ControlMaintaining the F:M ratio within a specified range can bean excellent control method. Although the F:M ratio isaffected by adjustment of the return rates, the most prac-tical method for adjusting the ratio is through waste rateadjustments. Increasing the rate will decrease the MLVSSand increase the F:M ratio. Decreasing the waste rate willincrease the MLVSS and decrease the F:M ratio

18.10.14.5.2 Establishing Desired F:M LevelsThe desired F:M ratio must be established on a plant-by-plant basis. Comparison of F:M ratios with plant effluentquality is the primary means to identify the most effectiverange for individual plants, when the range of F:M valuesthat produce the desired effluent quality is established.

Process lb BOD/lb MLVSS lb COD/lb MLVSS

Conventional 0.2–0.4 0.5–1.0Contact stabilization 0.2–0.6 0.5–1.0Extended aeration 0.05–0.15 0.2–0.5Oxidation ditch 0.05–0.15 0.2–0.5Pure oxygen 0.25–1.0 0.5–2.0

Waste lb d WAS Concentration mg L

WAS Flow MGD lb MG mg L

( ) = ( )¥ ( ) ¥ .8 34

Waste lb d

mg L MGD 8.34 lb MG mg L

lb d

( )= ¥ ¥

=

5540 0 44

20 329 6

.

, .

F M Ratio

Prim. Eff. COD BOD mg Flow MGD 8.34 lb mg MG

MLVSS mg Aerator Volume, MG 8.34 lb mg MG

L L

L L

=

¥ ¥

¥ ¥

(18.36)

Primary effluent flow 2.5 MGD Aeration volume 0.65 MGPrimary effluent BOD 145 mg/L Settling volume 0.30 MGPrimary effluent TSS 165 mg/L MLSS 3,650 mg/LEffluent flow 2.2 MGD MLVSS 2,550 mg/LEffluent BOD 22 mg/L % Waste Volatile 71%Effluent TSS 16 mg/L Desired F:M 0.3

F M Ratio 145 mg L 2.2 MGD 8.34 lb mg L MG

2550 mg L 0 MG 8.34 lb mg L MG

lb BOD lb MLVSS

=¥ ¥

¥ ¥

=

.

.

65

0 19

MLVSS mg L MLSS VM( ) = ¥ ( )% decimal

MLVSS mg L mg L mg L( ) = ¥ =2985 0 66 1970.



18.10.14.5.3 Required MLVSS Quantity The pounds of MLVSS required in the aeration tank toachieve optimum F:M ratio can be determined from theaverage influent food (BOD or COD) and the desired F:Mratio:

The required pounds of MLVSS determined by thiscalculation can then be converted to a concentration valuewith the following equation:

EXAMPLE 18.44

Problem:

The aeration tank influent flow is 4.0 MGD, and the influ-ent COD is 145 mg/L. The aeration tank volume is0.65 MG. The desired F:M ratio is 0.3 lb COD/lb MLVSS.

1. How many pounds of MLVSS must be main-tained in the aeration tank to achieve the desiredF:M ratio?

2. What is the required concentration of MLVSSin the aeration tank?

Solution:

1.

2.

18.10.14.5.4 Calculating Waste Rates Using F:M Ratio

Maintaining the desired F:M ratio is accomplished by con-trolling the MLVSS level in the aeration tank. This maybe accomplished by adjustment of return rates, but the mostpractical method is by proper control of the waste rate:

(18.40)

If the desired MLVSS is greater than the actualMLVSS, wasting is stopped until the desired level isachieved.

Practical considerations require that the requiredwaste quantity be converted to a required volume to wasteper day. This is accomplished by converting the wastepounds to flow rate in million gallons per day or gallonsper minute.

Note: When F:M ratio is used for process control, thevolatile content of the waste activated sludgeshould be determined.

EXAMPLE 18.45

Problem:

Given the following information, determine the requiredwaste rate in gallons per minute to maintain an F:M ratioof 0.17 lb COD/lb MLVSS:

Solution:

M

rimary Effluent of BOD or COD

D MLVSS

LVSS lb

P mg L Q MGD 8.34 lb gal

esired F M Ratio lb BOD or COD lb

( )

( ) ( )( )

=

¥ ¥

(18.38)

MLVSS mg L

Desired MLVSS lb Aeration Volume MG .34 lb mg L MG

( ) =

( )( ) ¥

8

(18.39)

MLVSS lb45 mg L 4 MGD 8.34 lb gal

.3 lb COD lb MLVSS

lb MLVSS

( ) =¥ ¥

=

1 0

0

16 124

.

,

MLVSS mg LMLVSS

MG lb gal

mg L MLVSS

,,

. .

.

=¥

=

[ ]16 124

0 65 8 34

2 974

Waste Volume of Solids lb d

Actual MLVSS lb Desired MLVSS lb

( ) =

( ) - ( )

Primary effluent COD 140 mg/LPrimary effluent flow 2.2 MGDMLVSS 3549 mg/LAeration tank volume 0.75 MGWaste volatile concentration 4440 mg/L (volatile solids)

Waste, MGD

Waste Volatile, lb dWaste Volatile Concentration, mg L

=

¥[ ] .8 34 lb gal

(18.41)

Actual MLVSS lb

mg L MG 8.34 lb gal

( )

= ¥ ¥

=

3 549 0 75

22 199

. .

, lb

Required MLVSS, lb

mg L MGD

lb COD lb MLVSS

lb MLVSS

=¥ ¥

=

140 2 2 8 34

0 17

15 110

. .

.

.

lb gal

Waste, lb d lb lb= - =22 199 15 110 7 089, , , lb

Waste, MGD lb d

mg L MGD=

¥=[ ]

7 089

4440 8 340 19

,

..

lb gal

Waste, gpm MGD gpd MGD

min d

gpm

=¥

=

0 19 1 000 000

1440

132

. , ,



18.10.14.6 Mean Cell Residence Time (MCRT)

MCRT (sometimes called SRT) is a process control cal-culation used for activated sludge systems. The MCRTcalculation illustrated in Example 18.46 uses the entirevolume of the activated sludge system (aeration and set-tling). Equation 18.52 is used to calculate the MCRT:

Note: MCRT can be calculated using only the aerationtank solids inventory. When comparing plantoperational levels to reference materials, it isimportant to determine which calculation thereference manual uses to obtain its examplevalues. Other methods are available to determinethe clarifier solids concentration. The simplestmethod assumes that the average suspendedsolids concentration is equal to the aerationtank’s solids concentration.

EXAMPLE 18.46

Problem:

Given the following data, what is the MCRT?

18.10.14.6.1 Mean Cell Residence Time Control

Because it provides an accurate evaluation of the processcondition and takes all aspects of the solids inventory intoaccount, the MCRT is an excellent process control tool.Increases in the waste rate will decrease the MCRT, aswill large losses of solids over the effluent weir. Reduc-tions in waste rate will result in increased MCRT values.

Note: You should remember the following importantprocess control parameters:

1. To increase F:M, decrease MCRT.

2. To increase MCRT, decrease waste rate.

3. MCRT is increased, MLTSS and 30-min set-ting increases.

4. Return sludge rate has no impact on MCRT.

5. MCRT has no impact on F:M change whenthe number of aeration tanks in service isreduced.

18.10.14.6.2 Typical MCRT Values

The following chart lists the various aeration process mod-ifications and associated MCRT values.

18.10.14.6.3 Control Values for MCRT

Control values for the MCRT are normally establishedbased on effluent quality. Once the MCRT range requiredto produce the desired effluent quality is established, it canbe used to determine the waste rate required to maintain it.

18.10.14.6.4 Waste Quantities and Requirements

MCRT for process control requires the determination ofthe optimum range for MCRT values. This is accom-plished by comparison of the effluent quality with MCRTvalues. When the optimum MCRT is established, thequantity of solids to be removed (wasted) is determined by

(18.43)

EXAMPLE 18.47

Problem:

Given the following data, determine the waste rate tomaintain an MCRT of 8.6 d:

Influent flow 4.2 MGD Aeration volume 1.20 MGInfluent BOD 135 mg/L Settling volume 0.60 MGInfluent TSS 150 mg/L MLSS 3,350 mg/LEffluent flow 4.2 MGD Waste rate 0.080 MGDEffluent BOD 22 mg/L Waste concentration 6100 mg/LEffluent TSS 10 mg/L Desired MCRT 8.5 d

MCRT

WAS

Q

d

MLSS mg L Aeration Volume MG Clarifier Volume MG

8.34 lb mg L MG

WAS mg L Flow MGD 8.34 lb mg L MG

TSS mg L MGD 8.34 lb mg L MGout

( )

( ) ( ) ( )[ ]

( ) ( )[ ]

( ) ( )[ ]

=

¥ + ¥

¥ ¥ +

¥ ¥

(18.42)

MCRT

=¥ + ¥

¥ ¥ +¥ ¥

=

[ ][ ][ ]

3350 mg L MGD 0.6 MG 8.34 lb mg L MG

6100 mg L 0.08 8.34 lb mg L MG

1 mg L 4.2 MGD 8.34 lb mg L MG

d

1 2

0

11 4

.

.

Process MCRT (d)

Conventional 5–15Step aeration 5–15Contact stabilization (contact) 5–15Extended aeration 20–30Oxidation ditch 20–30Pure oxygen 8–20

W

D

Q

aste Quantity lb d

MLSS mg L Aeration Volume MG Clarifier Volume MG

8.34 lb mg L MG

esired MCRT d

TSSout

mg L MGD 8.34 lb mg L MG

( )

( ) ( ) ( )][

( )

( ) ( )[ ]

=

¥ + ¥

- ¥ ¥



Solution:

18.10.14.6.4.1 Waste Rate in Million Gallons/Day

When the quantity of solids to be removed from the systemis known, the desired waste rate in million gallons per daycan be determined. The unit used to express the rate(MGD, gal/d, and gal/min) is a function of the volume ofwaste to be removed and the design of the equipment.

(18.44)

(18.45)

EXAMPLE 18.48

Problem:

Given the following data, determine the required wasterate to maintain an MCRT of 8.8 d:

Solution:

18.10.14.7 Mass Balance

Mass balance is based upon the fact that solids and BODare not lost in the treatment system. In simple terms, themass balance concept states that what comes in must equalwaste that goes out. The concept can be used to verifyoperational control levels and determine if potential prob-lems exist within the plant’s process control monitoringprogram.

Note: If influent values and effluent values do notcorrelate within 10 to 15%, it usually indicateseither a sampling or testing error or a processcontrol discrepancy.

Mass balance procedures for evaluating the operationof a settling tank and a biological process are describedin this section. Operators should recognize that althoughthe procedures are discussed in reference to the activatedsludge process, the concepts can be applied to any settlingor biological process.

18.10.14.7.1 Mass Balance: Settling Tank Suspended Solids

The settling tank mass balance calculation assumes thatno suspended solids are produced in the settling tank. Anysettling tank operation can be evaluated by comparing thesolids entering the unit with the solids leaving the tank aseffluent suspended solids or as sludge solids (seeFigure 18.11). If sampling and testing are accurate andrepresentative, and process control and operation areappropriate, the quantity of suspended solids entering thesettling tank should equal (±10%) the quantity of sus-pended solids leaving the settling tanks as sludge, scum,and effluent total suspended solids.

Note: In most instances, the amount of suspendedsolids leaving the process as scum is so smallthat it is ignored in the calculation.

18.10.14.7.1.1 Mass Balance Calculation

MLSS 3400 mg/LAeration volume 1.4 MGClarifier volume 0.5 MGEffluent TSS 10 mg/LEffluent flow 5.0 MGD

MLSS 2500 mg/LAeration volume 1.2 MGClarifier volume 0.2 MGEffluent TSS 11 mg/LEffluent flow 5.0 MGDWaste concentrations 6000 mg/L

W lb

MGD

aste Quantity lb d

3400 mg L 1 MG 0.5 MG 8.34 lb mg L MG

.6 d

10 mg L 8.34 lb mg L MG

( )

( ) [ ]

[ ]

=

¥ + ¥-

¥ ¥

5848

4

8

5 0

.

.

Waste MGD

Waste Pounds dWAS Concentrations, mg L

( ) =

¥

.8 34

Waste gal min

Waste MGD gal d MGD

1440 min d

( ) =

( ) ¥

, ,1 000 000

W

MGD

aste Quantity lb d

2500 mg L 1 MG 0.2 MG 8.34 lb mg L MG

.8 d

10 mg L 8.34 lb mg L MG

lb d lb d

lb d

( )[ ]

[ ]

=¥ + ¥

-

¥ ¥

= -

=

.

.

2

8

5 0

3317 459

2858

W MGDaste MGD2858 lb d

6000 mg l 8.34 lb gal ( ) =

¥= 0 057.

Waste gal min0.57 MGD gal d MGD

1440 min d

gal min

( ) =¥

=

1 000 000

40

, ,

TSS

TSS mg L Q MGD

in

in

TSS

TSS mg L Q MGD

out

out

lb d

lb mg L MG

lb d

lb mg L MG

( )

( )

( ) =

( ) ¥ ¥

=

¥ ¥( ) .

( ) .

8 34

8 34

(18.46)



18.10.14.7.1.2 Explanation of Results

1. The mass balance is ±15% or less — The pro-cess is considered to be in balance. Sludgeremoval should be adequate with the sludgeblanket depth remaining stable. Sampling isconsidered to be producing representative sam-ples that are being tested accurately.

2. The mass balance is greater than ±15% — Indi-cates that more solids are entering the settlingtank than are being removed. Sludge blanketdepth should be increasing, effluent solids mayalso be increasing, and effluent quality in decreas-ing. If changes described are not occurring, themass balance may indicate that sample type,location, times, or procedures and/or testing pro-cedures are not producing representative results.

3. If the mass balance is greater than 15%, it indi-cates that fewer solids are entering the settlingtank than are being removed. Sludge blanketdepth should be decreasing; sludge solids con-centration may also be decreasing. This couldadversely impact sludge treatment processes. Ifchanges described are not occurring, the massmay indicate that sample type, location, times,or procedures and/or testing procedures are notproducing representative results.

EXAMPLE 18.49

Problem:

Given the following data, determine the solids mass bal-

ance for the settling tank:

Solution:

The mass balance indicates:

1. The sampling point or collection procedure orlaboratory procedure is producing inaccurate dataupon which to make process control decisions.

2. More solids are entering the settling tank eachday than are being removed. This should resultin either:A. A solids buildup in the settling tank.B. A loss of solids over the effluent weir.

Investigate further to determine the specific cause of theimbalance.

18.10.14.7.2 Mass Balance: Biological ProcessSolids are produced whenever biological processes areused to remove organic matter from wastewater (seeFigure 18.11). Mass balance for an aerobic biological pro-cess must take into account both the solids removed byphysical settling processes and the solids produced bybiological conversion of soluble organic matter to insolu-ble suspended matter or organisms. Research has shownthat the amount of solids produced per pound of BODremoved can be predicted based upon the type of processbeing used. Although the exact amount of solids producedcan vary from plant to plant, research has developed aseries of K factors that can be used to estimate the solidsproduction for plants using a particular treatment process.These average factors provide a simple method to evaluatethe effectiveness of a facility’s process control program.

Process Extended Aeration (No Primary)

Influent Flow 2.6 MGDTSS 2445 mg/L

Effluent Flow 2.6 MGDTSS 17 mg/L

Return Flow 0.5 MGDTSS 8470 mg/L

Sludge Solids

Sludge Pumped Solids

TSS TSS

TSSin out

in

lb d

gal lb mg L MG

Mass Balance

lb lb Sludge Solids lb 100

lb

( )

( )

( ) ( ) ( )( )[ ]( )

=

¥ ¥

=

- + ¥

% .

%

8 34 S

MGD

S

MGD

olids in lb d

mg L lb mg L MG

53,017 lb d

olids out lb d

mg L lb mg L MG

369 lb d

( )

( )

= ¥ ¥

=

= ¥ ¥

=

2445 2 6 8 34

17 2 6 8 34

. .

. .

Sludge Solids out lb d

mg L MGD lb mg L MG

35,320 lb d

Mass Balance

lb d 396 lb d 35,320 lb d)] 100

lb d

( )= ¥ ¥

=

=- + ¥

=

8470 0 5 8 34

53 017

53 017

32 7

. .

%

[ , (

,

. %



The mass balance also provides an excellent mechanismto evaluate the validity of process control and effluentmonitoring data generated. Table 18.7 lists average K fac-tors in pounds of solids produced per pound of BODremoved for selected processes.

18.10.14.7.2.1 Conversion Factor

Conversion factors depend on the activated sludge modifi-cation involved. Factors generally range from 0.5 to 1.0 lbof solids/lb BOD removed (see Table 18.7).

18.10.14.7.2.2 Mass Balance Calculation

18.10.14.7.2.3 Explanation of Results

If the mass balance is ±15%, the process sampling andtesting and process control are within acceptable levels.If the balance is greater than15%, investigate further todetermine if the discrepancy represents a process controlproblem or is the result of nonrepresentative sampling andinaccurate testing.

18.10.14.7.2.4 Sludge Waste Based upon Mass Balance

The mass balance calculation predicts the amount orsludge that will be produced by a treatment process. Thisinformation can then be used to determine what, under

current operating conditions, that waste rate must be tomaintain the current solids level.

EXAMPLE 18.50

Problem:

Given the following data, determine the mass balance ofthe biological process and the appropriate waste rate tomaintain current operating conditions:

TABLE 18.7 Conversion Factors K

Process lb Solids/lb BOD Removed

Primary 1.7Activated sludge with primary 0.7Activated sludge without primary Conventional 0.85 Step feed 0.85 Extended aeration 0.65Oxidation ditch 0.65Contact stabilization 1.0Trickling filter 1.0Rotating Biological contactor 1.0

Source: Spellman, F.R., Spellman’s Standard Handbook for Waste-water Operators, Vol. 1, Technomic Publ., Lancaster, PA, 1999.

BOD lb BOD Q

BOD lb BOD Q

BOD lb BOD lb

Q

in in

out out

in out

( ) ( ) ( )

( ) ( ) ( )

( ) ( ) ( )[ ]( ) ( ) ( )

( )

= ¥ ¥

= ¥ ¥

= - ¥

= ¥ ¥

=

mg L MGD lb mg L MG

mg L MGD lb mg L MG

Solids Produced lb d K

TSS lb d TSS mg L MGD lb mg L MG

Waste lb d

out out

8 34

8 34

8 34

.

.

.

WW mg L MGD lb mg L MG

Solids Removed lb d TSS lb d Waste lb d

%Mass Balance Produced Removed 100

olids Produced

out

aste Q

Solids Solids

S

( ) ( )

( ) ( ) ( )

[ ]

¥ ¥

= +

=- ¥

8 34.

(18.47)

Process Extended Aeration (No Primary)

Influent Flow 1.1 MGDBOD 220 mg/LTSS 240 mg/L

Effluent Flow 1.5 MGDBOD 18 mg/LTSS 22 mg/L

Waste Flow 24,000 gal/dTSS 8710 mg/L

Waste Rate, MGDSolids Produced, lb d

Waste Concentration=

¥( )8 34. lb gal

(18.44)

BOD lb d MGD

lb d

BOD lb d MGD

lb d

B R lb d lb d lb d

lb d

Solids lb d lb d

lb d

olids lb d

in

out

( )

( )

( )

( )

( )

= ¥ ¥

=

= ¥ ¥

=

= -

=

= ¥

=

220 1 1

8 34

18 1 1

8 34

2018 165

1204

mg L

lb mg L MG

2018

mg L

lb mg L MG

165

OD emoved

1853

Produced 1853 0.65 lb lb BOD

S Out

.

.

.

.

== ¥ ¥

=

= ¥ ¥

=

= +

=

=- ¥

=

( )

( )

[ ]

22 1 1

8 34

202

710

8 34

1204 1945

1

62

mg L

lb mg L MG

Sludge Out 8 mg L 0.024 MGD

lb mg L MG

1743

Solids Removed 292 1743

1945

Mass Balance 100

204 lb d

.

.

.

%

%

MGD

lb d

lb d

lb d

lb d lb d lb d

lb d

lb d lb d



The mass balance indicates:

1. The sampling points, collection methods, and labo-ratory testing procedures are producing nonrepresen-tative results.

2. The process is removing significantly more solids thanis required. Additional testing should be performedto isolate the specific cause of the imbalance.

To assist in the evaluation, the waste rate based upon themass balance information can be calculated:

Using this equation results in the following:

18.10.15 SOLIDS CONCENTRATION: SECONDARY CLARIFIER

The solids concentration in the secondary clarifier can beassumed to be equal to the solids concentration in theaeration tank effluent. It may also be determined in thelaboratory using a core sample taken from the secondaryclarifier. The secondary clarifier solids concentration canbe calculated as an average of the secondary effluent sus-pended solids and the RAS suspended solids concentration.

18.10.16 ACTIVATED SLUDGE PROCESS RECORD KEEPING REQUIREMENTS

Wastewater operators soon learn that record keeping is amajor requirement and responsibility of their jobs.Records are essential for process control, providing infor-mation on the cause of problems, providing informationfor making seasonal changes, and compliance with regula-tory agencies. Records should include sampling and testingdata, process control calculations, meter readings, processadjustments, operational problems and corrective actionstaken, and process observations.

18.11 DISINFECTION OF WASTEWATER

Like drinking water, liquid wastewater effluent is disin-fected. Unlike drinking water, wastewater effluent isdisinfected not to directly (direct end-of-pipe connection)protect a drinking water supply, but instead is treated toprotect public health in general. This is particularly impor-tant when the secondary effluent is discharged into a bodyof water used for swimming or for a downstream watersupply.

In the treatment of water for human consumption,treated water is typically chlorinated (although ozonationis also currently being applied in many cases). Chlorina-tion is the preferred disinfection in potable water suppliesbecause of chlorine’s unique ability to provide a residual.This chlorine residual is important because when treatedwater leaves the waterworks facility and enters the distri-bution system, the possibility of contamination isincreased. The residual works to continuously disinfectwater right up to the consumer’s tap.

In this section, we discuss basic chlorination anddechlorination. In addition, we describe UV irradiation,ozonation, bromine chlorine, and no disinfection. Keep inmind that much of the chlorination material presented hereis similar to the information presented in Chapter 17,Water Treatment Operations and Unit Processes.

18.11.1 CHLORINE DISINFECTION

Chlorination for disinfection, as shown in Figure 18.1,follows all other steps in conventional wastewater treatment.The purpose of chlorination is to reduce the population oforganisms in the wastewater to levels low enough toensure that pathogenic organisms will not be present insufficient quantities to cause disease when discharged.

Note: Chlorine gas is heavier than air (vapor densityof 2.5). Exhaust from a chlorinator room shouldbe taken from floor level.

Note: The safest action to take in the event of a majorchlorine container leak is to call the fire depart-ment.

Note: You might wonder why it is that chlorinationof critical waters such as natural trout streamsis not normal practice. This practice is strictlyprohibited because chlorine and its by-products(i.e., chloramines) are extremely toxic toaquatic organisms.

18.11.1.1 Chlorination Terminology

Remember that there are several terms used in discussionof disinfection by chlorination. Because it is important forthe operator to be familiar with these terms, we repeat keyterms again.

Chlorine a strong oxidizing agent that has strong dis-infecting capability. A yellow-green gas that isextremely corrosive and is toxic to humans inextremely low concentrations in air.

Contact time the length of time the time the disinfect-ing agent and the wastewater remain in contact.

Demand the chemical reactions that must be satisfiedbefore a residual or excess chemical willappear.

WasteWaste

gal dSolids Produced lb d

TSS mg L 8.34 ( ) ( )

( )=¥

Waste gal d1204 lb d 1,000,000

710 mg L 8.34

gal d

( ) ( )=

¥¥

=

8

16 575,



Disinfection the selective destruction of disease-caus-ing organisms. All the organisms are notdestroyed during the process. This differentiatesdisinfection from sterilization, which is thedestruction of all organisms.

Dose the amount of chemical being added in milligramsper liter.

Feed rate the amount of chemical being added inpounds per day.

Residual the amount of disinfecting chemical remain-ing after the demand has been satisfied.

Sterilization the removal of all living organisms.

18.11.1.2 Wastewater Chlorination: Facts and Process Description

18.11.1.2.1 Chlorine Facts

1. Elemental chlorine (Cl2 — gaseous) is a yellow-green gas, 2.5 times heavier than air.

2. The most common use of chlorine in waste-water treatment is for disinfection. Other usesinclude odor control and activated sludge bulkingcontrol. Chlorination takes place prior to thedischarge of the final effluent to the receivingwaters (see Figure 18.1).

3. Chlorine may also be used for nitrogen removalthrough a process called breakpoint chlorina-tion. For nitrogen removal, enough chlorine isadded to the wastewater to convert all the ammo-nium nitrogen gas. To do this, approximately10 mg/L of chlorine must be added for every1 mg/L of ammonium nitrogen in the wastewater.

4. For disinfection, chlorine is fed manually orautomatically into a chlorine contact tank orbasin, where it contacts flowing wastewater forat least 30 min to destroy disease-causingmicroorganisms (pathogens) found in treatedwastewater.

5. Chorine may be applied as a gas, a solid, orliquid hypochlorite form.

6. Chorine is a very reactive substance. It has thepotential to react with many different chemicals(including ammonia), as well as with organicmatter. When chlorine is added to wastewater,several reactions occur:A. Chlorine will react with any reducing agent

(i.e., sulfide, nitrite, iron, and thiosulfate)present in wastewater. These reactions areknown as chlorine demand. The chlorineused for these reactions is not available fordisinfection.

B. Chlorine also reacts with organic compoundsand ammonia compounds to form chlor-organics and chloramines. Chloramines are

part of the group of chlorine compounds thathave disinfecting properties and show up aspart of the chlorine residual test.

C. After all of the chlorine demands are met,the addition of more chlorine will producefree residual chlorine. Producing free resid-ual chlorine in wastewater requires verylarge additions of chlorine.

18.11.1.2.2 Hypochlorite Facts

Although there are some minor hazards associated withits use (e.g., skin irritation, nose irritation, and burningeyes), hypochlorite is relatively safe to work with. It isnormally available in dry form as a white powder, pelletor tablet, or liquid form. It can be added directly using adry chemical feeder or it can be dissolved and fed as asolution.

Note: In most wastewater treatment systems, disinfec-tion is accomplished by means of combinedresidual.

18.11.1.2.3 Process Description

Chlorine is a very reactive substance. Chlorine is addedto wastewater to satisfy all chemical demands (i.e., to reactwith certain chemicals such as sulfide, sulfite, ferrous iron,etc.). When these initial chemical demands have beensatisfied, chlorine will react with substances, such asammonia, to produce chloramines and other substancesthat, although not as effective as chlorine, have disinfect-ing capability. This produces a combined residual, whichcan be measured using residual chlorine test methods. Ifadditional chlorine is added, free residual chlorine can beproduced. Due to the chemicals normally found in waste-water, chlorine residuals are normally combined ratherthan free residuals. Control of the disinfection process isnormally based upon maintaining total residual chlorine(TRC) of at least 1.0 mg/L for a contact time of at least30 min at design flow.

Note: Residual level, contact time, and effluent qual-ity affect disinfection. Failure to maintain thedesired residual levels for the required contacttime will result in lower efficiency andincreased probability that disease organismswill be discharged.

Based on water quality standards, total residual lim-itations on chlorine are:

1. Fresh water — Less than 11 ppb total residualchlorine.

2. Estuaries — Less than 7.5 ppb for halogen pro-duced oxidants.

3. Endangered species — Use of chlorine is pro-hibited.



18.11.1.3 Chlorination Equipment

18.11.1.3.1 Hypochlorite SystemsDepending on the form of hypochlorite selected for use,special equipment that controls the addition of hypochlo-rite to the wastewater is required. Liquid forms requirethe use of metering pumps, which can deliver varyingflows of hypochlorite solution. Dry chemicals require theuse of a feed system designed to provide variable doses ofthe form used. The tablet form of hypochlorite requires theuse of a tablet chlorinator designed specifically to providethe desired dose of chlorine. The hypochlorite solution ordry feed systems dispenses the hypochlorite, which is thenmixed with the flow. The treated wastewater then entersthe contact tank to provide the required contact time.

18.11.1.3.2 Chlorine SystemsBecause of the potential hazards associated with the useof chlorine, the equipment requirements are significantlygreater than those associated with hypochlorite use. Thesystem most widely used is a solution feed system. In thissystem, chlorine is removed from the container at a flowrate controlled by a variable orifice. Water moving throughthe chlorine injector creates a vacuum that draws the chlo-rine gas to the injector and mixes it with the water. Thechlorine gas reacts with the water to form hypochlorous andhydrochloric acid. The solution is then piped to the chlorinecontact tank and dispersed into the wastewater through adiffuser. Larger facilities may withdraw the liquid form ofchlorine and use evaporators (heaters) to convert to thegas form. Small facilities will normally draw the gas formof chlorine from the cylinder. As gas is withdrawn liquidwill be converted to the gas form. This requires heatenergy and may result in chlorine line freeze-up if thewithdrawal rate exceeds the available energy levels.

18.11.1.4 Chlorination: Operation

In both the hypochlorite and chlorine systems normaloperation requires adjustment of feed rates to ensure therequired residual levels are maintained. This normallyrequires chlorine residual testing and adjustment basedupon the results of the test. Other activities include theremoval of accumulated solids from the contact tank,collection of bacteriological samples to evaluate processperformance, and maintenance of safety equipment (res-pirator air pack, safety lines, etc.).

Hypochlorite operation may also include makeup solu-tion (solution feed systems), adding powder or pellets tothe dry chemical feeder or tablets to the tablet chlorinator.

Chlorine operations include the adjustment of chlori-nator feed rates, inspection of mechanical equipment, test-ing for leaks using an ammonia swab (white smoke indi-cates leaks), changing containers (requires more than oneperson for safety), and adjusting the injector water feedrate when required.

Chlorination requires routine testing of plant effluentfor TRC and may also require the collection and analysisof samples to determine the fecal coliform concentrationin the effluent.

18.11.1.5 Troubleshooting Operational Problems

Operational problems with the plant’s disinfection processoccasionally develop. The wastewater operator must notonly be able to recognize these problems, but also correctthem. For proper operation, the chlorination processrequires routine observation, meter readings, process con-trol and testing, and various process control calculations.Comparison of daily results with expected normal rangesis the key to identifying problems during the troubleshoot-ing process and taking appropriate corrective actions (ifrequired).

In this section, we review normal operational and per-formance factors. We point out the various problems thatcan occur with the plant’s disinfection process, the causes,and the corrective actions that should be taken.


The operator should consider the following items:

1. Flow distribution — The operator monitors theflow to ensure that it is evenly distributedbetween all units in service, and that the flowthrough each individual unit is uniform, withno indication of short-circuiting.

2. Contact tank — The contact tanks or basinsmust be checked to ensure that no excessiveaccumulation of scum is on the surface, no indi-cation of solids accumulation is on the bottom,and mixing appears to be adequate.

3. Chlorinator — The operator should check toensure that there is no evidence of leakage,operating pressure or vacuum is within speci-fied levels, current chlorine feed settling iswithin expected levels, in-line cylinders havesufficient chlorine to ensure continuous feed,and the exhaust system is operating as designed.

18.11.1.5.1.1 Factors Affecting Performance

Operators must be familiar with those factors that affectchlorination performance. Any item that interferes withthe chlorine reactions or increases the demand for chlorinecan affect performance and may produce nondisinfectantproducts. We discuss the main factors affecting chlorina-tion performance below:

1. Effluent quality — Poor quality effluents havehigher chlorine demands. In addition, high con-centrations of solids prevent chlorine-organism



contact, and incomplete nitrification can causeextremely high chlorine demand.

2. Mixing — In order to be effective, chlorinemust be in contact with the organisms. Poormixing results in poor chlorine distribution.Installing of baffles and using a high length-to-width ratio will improve mixing and contact.

3. Contact time — The chlorine disinfection pro-cess is time dependent. As the contact timedecreases, process effectiveness decreases. Aminimum of 30 min of contact must be avail-able at design flow.

4. Residual levels — The chlorine disinfectionprocess is TRC dependent. The concentrationof residual must be sufficient to ensure thedesired reactions occur. At the design contacttime, the required minimum TRC concentrationis 1.0 mg/L.

18.11.1.5.1.2 Process Control Sampling and Testing

To ensure proper operation of the chlorination process,the operator must perform process control testing for thechlorination process. (Note: The process performanceevaluation is based on the bacterial content (fecalcoliform) of the final effluent.) Process control testingconsists of performing a total chlorine residual test onchlorine contact effluent. The frequency of the testing isspecified in the plant permit. The normal expected rangeof results is also specified in the plant permit.

18.11.1.5.1.3 Troubleshooting

The following sections present common operational prob-lems, symptoms, casual factors, and corrective actionsassociated with chlorination system use in wastewatertreatment.

1. Symptom 1: Coliform count fails to meetrequired standards for disinfection.A. Cause: Inadequate chlorination equipment

capacity.Corrective action: Replace equipment asnecessary to provide treatment based onmaximum flow through the pipe.

B. Cause: Inadequate chlorine residual control.Corrective action: Use chlorine residualanalyzer to monitor and control chlorinedosage automatically.

C. Cause: Short circuiting in chlorine contactchamber.Corrective action: Install baffling in thechlorine contact chamber. Install mixingdevice in chlorine contact chamber.

D. Cause: Solids buildup in contact chamber.Corrective action: Clean contact chamber.

E. Cause: Chlorine residual is too low.Corrective action: Increase contact time orincrease chlorine feed rate.

2. Symptom 2: Low chlorine gas pressure at thechlorinator.A. Cause: Insufficient number of cylinders con-

nected to the system.Corrective action: Connect enough cylin-ders to system so that feed rate does notexceed recommended withdrawal rate forcylinders.

B. Cause: Stoppage or restriction of flowbetween cylinders and chlorinator.Corrective action: Disassemble chlorineheader system at point where cooling begins,locate stoppage, and clean with solvent.

3. Symptom 3: No chlorine gas pressure at thechlorinator.A. Cause: Chlorine cylinders empty or not con-

nected to the system. Corrective action: Connect cylinders orreplace empty cylinders.

B. Cause: Plugged or damaged pressure reduc-ing valve.Corrective action: Repair reducing valveafter shutting cylinder valves and decreasinggas in the header system.

4. Symptom 4: Chlorinator will not feed anychlorine.A. Cause: Pressure reducing valve in chlorina-

tor is dirty.Corrective action: Disassemble chlorinatorand clean valve stem and seat. Precede valvewith filter or sediment trap.

B. Cause: Chlorine cylinder is hotter than chlo-rine control apparatus (chlorinator).Corrective action: Reduce temperature incylinder area; do not connect a new cylinder,which has been sitting in the sun.

5. Symptom 5: Chlorine gas escaping from thechlorine pressure reducing valve (CPRV).A. Cause: Main diaphragm of CPRV has

ruptured.Corrective action: Disassemble valve anddiaphragm. Inspect chlorine supply systemfor moisture intrusion.

6. Symptom 6: Inability to maintain chlorine feedrate without icing of chlorine system.A. Cause: Insufficient evaporator capacity.

Corrective action: Reduce feed rate to 75%of evaporator capacity. If this eliminatesproblem, then main diaphragm of CPRV isruptured.

B. Cause: External CPRV cartridge is clogged.Corrective action: Flush and clean cartridge.



7. Symptom 7: Chlorinator system is unable tomaintain sufficient water bath temperature tokeep external CPRV open.A. Cause: Heating element malfunction.

Corrective action: Remove and replace heat-ing element.

8. Symptom 8: Inability to obtain maximum feedrate from chlorinator.A. Cause: Inadequate chlorine gas pressure.

Corrective action: Increase pressure andreplace empty or low cylinders.

B. Cause: Water pump injector clogged withdeposits.Corrective action: Clean injector parts usingmuriatic acid. Rinse parts with fresh waterand place back in service.

C. Cause: Leak in vacuum relief valve.Corrective action: Disassemble vacuumrelief valve and replace all springs.

D. Cause: Vacuum leak in joints, gaskets, tub-ing, etc. in chlorinator system.Corrective action: Repair all vacuum leaksby tightening joints, replacing gaskets, andreplacing tubing and compression nuts.

9. Symptom 9: Inability to maintain adequatechlorine feed rate.A. Cause: Malfunction or deterioration of chlo-

rine water supply pump.Corrective action: Overhaul pump (if turbinepump is used, try closing valve to maintainproper discharge pressure).

10. Symptom 10: Chlorine residual too high inplant effluent to meet requirements.A. Cause: Chlorine residual too high

Corrective action: Install dechlorinationfacilities.

11. Symptom 11: Wide variation in chlorine resid-ual produced in the effluent.A. Cause: Chlorine flow proportion meter

capacity inadequate to meet plant flow rates.Corrective action: Replace with highercapacity chlorinator meter.

B. Cause: Malfunctioning controls.Corrective action: Call manufacturer techni-cal representative.

C. Cause: Solids settled in chlorine contactchamber.Corrective action: Clean chlorine contacttank.

D. Cause: Flow proportioning control devicenot zeroed or spanned correctlyCorrective action: Rezero and span thedevice in accordance with manufacturer’sinstructions.

12. Symptom 12: Unable to obtain chlorine residual.A. Cause: High chemical demand.

Corrective action: Locate and correct thesource of the high demand.

B. Cause: Test interference.Corrective action: Add sulfuric acid to sam-ples to reduce interference.

13. Symptom 13: Chlorine residual analyzer,recorder, and controller does not control chlo-rine residual properly.A. Cause: Electrodes fouled.

Corrective action: Clean electrodes.B. Cause: Loop time is too long.

Corrective actions: Reduce control looptime by:1. Moving the injector closer to the point of

application.2. Increasing the velocity in the sample line

to the analyzer.3. Moving the cell closer to the sample

point.4. Moving the sample point closer to the

point of application.C. Cause: Insufficient potassium iodide being

added for the amount of residual beingmeasured.Corrective action: Adjust potassium iodidefeed to correspond with the chlorine residualbeing measured.

D. Cause: Buffer additive system is malfunc-tioning.Corrective action: Repair buffer additivesystem.

E. Cause: Malfunctioning of analyzer cell.Corrective action: Call authorized servicepersonnel to repair electrical components.

F. Cause: Poor mixing of chlorine at point ofapplication.Corrective action: Install mixing device tocause turbulence at point of application.

G. Cause: Rotameter tube range is improperlyset.Corrective action: Replace rotameter with aproper range of feed rate.

18.11.1.6 Dechlorination

The purpose of dechlorination is to remove chlorine andreaction products (chloramines) before the treated waste-stream is discharged into its receiving waters. Dechlorinationfollows chlorination, usually at the end of the contact tankto the final effluent. Sulfur dioxide gas, sodium sulfate,sodium metabisulfate, or sodium bisulfates are the chemi-cals used to dechlorinate. No matter which chemical is usedto dechlorinate, its reaction with chlorine is instantaneous.



18.11.1.7 Chlorination Environmental Hazards and Safety

Chlorine is an extremely toxic substance that can causesevere damage when released to the environment. For thisreason, most state regulatory agencies have established achlorine water quality standard (e.g., in Virginia, 0.011 mg/Lin fresh waters for TRC and 0.0075 mg/L for chlorineproduced oxidants in saline waters). Studies have indi-cated that quantities above these levels chlorine can reduceshellfish growth and destroy sensitive aquatic organisms.This standard has resulted in many treatment facilitiesbeing required to add an additional process to remove thechlorine prior to discharge. As mentioned, the process,known as dechlorination, uses chemicals that react quicklywith chlorine to convert it to a less harmful form.

Elemental chlorine is a chemical with potentially fatalhazards associated with it. For this reason many differentstate and federal agencies regulate the transport, storage,and use of chlorine. All operators required to work withchlorine should be trained in proper handling techniques.They should also be trained to ensure that all proceduresfor storage transport, handling and use of chlorine are incompliance with appropriate state and federal regulations.

18.11.1.8 Chlorine: Safe Work Practice

Because of the inherent dangers involved with handlingchlorine, each facility using chlorine (for any reason)should ensure that a written safe work practice is in placeand is followed by plant operators. A sample safe workpractice for handling chlorine is provided below.

WORK: CHEMICAL HANDLING: CHLORINE

Practice

1. Plant personnel must be trained and instructedon the use and handling of chlorine, chlorineequipment, chlorine emergency repair kits, andother chlorine emergency procedures.

2. Use extreme care and caution when handlingchlorine.

3. Lift chlorine cylinders only with an approvedand load-tested device.

4. Secure chlorine cylinders into position imme-diately. Never leave a cylinder suspended.

5. Avoid dropping chlorine cylinders.6. Avoid banging chlorine cylinders into other

objects.7. Store chlorine 1-ton cylinders in a cool dry

place away from direct sunlight or heatingunits. Railroad tank cars are direct sunlightcompensated.

8. Store chlorine 1-ton cylinders on their sidesonly (horizontally).

9. Do not stack unused or used chlorine cylinders.10. Provide positive ventilation to the chlorine stor-

age area and chlorinator room.11. Always keep chlorine cylinders at ambient tem-

perature. Never apply direct flame to a chlorinecylinder.

12. Use the oldest chlorine cylinder in stock first.13. Always keep valve protection hoods in place until

the chlorine cylinders are ready for connection.14. Except to repair a leak, do not tamper with the

fusible plugs on chlorine cylinders.15. Wear a self-contained breathing apparatus

(SCBA) whenever changing a chlorine cylinderand have at least one other person with astandby SCBA unit outside the immediate area.

16. Inspect all threads and surfaces of a chlorinecylinder, and have at least one other person witha standby SCBA unit outside the immediatearea.

17. Use new lead gaskets each time a chlorine cyl-inder connection is made.

18. Use only the specified wrench to operate chlo-rine cylinder valves.

19. Open chlorine cylinder valves slowly (no morethan one full turn).

20. Do not hammer, bang, or force chlorine cylin-der valves under any circumstances.

21. Check for chlorine leaks as soon as the chlorinecylinder connection is made. Leaks are checkedfor by gently expelling ammonia mist from aplastic squeeze bottle filled with approximately2 oz of liquid ammonia solution. Do not putliquid ammonia on valves or equipment.

22. Correct all minor chlorine leaks at the chlorinecylinder connection immediately.

23. Except for automatic systems, draw chlorinefrom only one manifolded chlorine cylinder ata time. Never simultaneously open two or morechlorine cylinders connected to a commonmanifold pulling liquid chlorine. Two or morecylinders connected to a common manifoldpulling gaseous chlorine are acceptable.

24. Wear SCBA and chemical protective clothingcovering face, arms, and hands before enteringan enclosed chlorine area to investigate a chlo-rine odor or chlorine leak (two-person rulerequired).

25. Provide positive ventilation to a contaminatedchlorine atmosphere before entering wheneverpossible.

26. Have at least two personnel present before enter-ing a chlorine atmosphere. One person shouldenter the chlorine atmosphere, and the othershould observe in the event of an emergency.Never enter a chlorine atmosphere unattended.



Remember that the Occupational Safety andHealth Administration (OSHA) mandates thatonly fully qualified Level III hazardous material(HAZMAT) responders are authorized toaggressively attack a HAZMAT leak such aschlorine.

27. Use supplied-air-breathing equipment whenentering a chlorine atmosphere. Never use can-ister-type gas masks when entering a chlorineatmosphere.

28. Ensure that all supplied air-breathing apparatuseshave been properly maintained in accordancewith the plant’s SCBA inspection guidelines asspecified in the plant’s respiratory protectionprogram.

29. Stay upwind from all chlorine leak danger areasunless involved with making repairs. Look toplant windsocks for wind direction.

30. Contact trained plant personnel to repair chlo-rine leaks.

31. Roll uncontrollable leaking chlorine cylindersso that the chlorine escapes as a gas, not as aliquid.

32. Stop leaking chlorine cylinders or leaking chlo-rine equipment (by closing off valves if possible)prior to attempting repair.

33. Connect uncontrollable leaking chlorine cylin-ders to the chlorination equipment and feed themaximum chlorine feed rate possible.

34. Keep leaking chlorine cylinders at the plant site.Chlorine cylinders received at the plant sitemust be inspected for leaks prior to takingdelivery from the shipper. Never ship a leakingchlorine cylinder back to the supplier after ithas been accepted (bill of lading has beensigned by plant personnel) from the shipper.Instead, repair or stop the leak first.

35. Keep moisture away from a chlorine leak.Never put water onto a chlorine leak.

36. Call the fire department or rescue squad if aperson is incapacitated by chlorine.

37. Administer cardiopulmonary resuscitation (usebarrier mask if possible) immediately to personwho has been incapacitated by chlorine.

38. Breathe shallow rather than deep if exposed tochlorine without the appropriate respiratoryprotection.

39. Place a person who does not have difficultybreathing and is heavily contaminated withchlorine into a deluge shower. Remove theirclothing under the water and flush all body partsthat were exposed to chlorine.

40. Flush eyes contaminated with chlorine withcopious quantities of lukewarm running waterfor at least 15 min.

41. Drink milk if throat is irritated by chlorine.42. Never store other materials in chlorine cylinder

storage areas. Substances like acetylene andpropane are not compatible with chlorine.

18.11.1.9 Chlorination Process Calculations

There are several calculations that may be useful in oper-ating a chlorination system. Many of these calculationsare discussed and illustrated in this section.

18.11.1.9.1 Chlorine DemandChlorine demand is the amount of chlorine in milligramsper liter that must be added to the wastewater to completeall of the chemical reactions that must occur prior toproducing a residual:

(18.48)

EXAMPLE 18.51

Problem:

The plant effluent currently requires a chlorine dose of7.1 mg/L to produce the required 1.0 mg/L chlorine resid-ual in the chlorine contact tank. What is the chlorinedemand in milligrams per liter?

Solution:

18.11.1.9.2 Chlorine Feed RateChlorine feed rate is the amount of chlorine added to thewastewater in pounds per day:

EXAMPLE 18.52

Problem:

The current chlorine dose is 5.55 mg/L. What is the feedrate in pounds per day if the flow is 22.89 MGD?

Solution:

Chlorine Demand Chlorine Dose mg L

Chlorine Residual mg L

= ( ) -

( )

Chlorine Demand mg L( ) = -

=

7 1 1 0

6 1

. .

.

mg L mg L

mg L

Chlorine Feed Rate Dose mg L

lb mg L MG

= ( ) ¥ ( ) ¥Q MGD

.8 34 (18.49)

Chlorine Feed Rate lb d mg L MGD

lb mg L MG

lb d

( ) = ¥ ¥

=

5 55 22 89

8 34

1060

. .

.



18.11.1.9.3 Chlorine DoseChlorine dose is the concentration of chlorine being addedto the wastewater. It is expressed in milligrams per liter:

(18.50)

EXAMPLE 18.53

Problem:

Three hundred twenty pounds of chlorine are added perday to a wastewater flow of 5.6 MGD. What is the chlorinedoes in milligrams per liter?

Solution:

18.11.1.9.4 Available ChlorineWhen hypochlorite forms of chlorine are used, the avail-able chlorine is listed on the label. In these cases, theamount of chemical added must be converted to the actualamount of chlorine using the following calculation:

EXAMPLE 18.54

Problem:

The calcium hypochlorite used for chlorination contains62.5% available chlorine. How many pounds of chlorineare added to the plant effluent if the current feed rate is30 lb of calcium hypochlorite per day?

Solution:

18.11.1.9.5 Required Quantity of Dry Hypochlorite

Use Equation 18.64 to determine the amount of hypochlo-rite needed to achieve the desired dose of chlorine:

EXAMPLE 18.55

Problem:

The laboratory reports that the chlorine dose required tomaintain the desired residual level is 8.5 mg/L. Today’sflow rate is 3.25 MGD. The hypochlorite powder used fordisinfection is 70% available chlorine. How many poundsof hypochlorite must be used?

Solution:

18.11.1.9.6 Required Quantity of Liquid Hypochlorite

Use Equation 18.65 to calculate the required quantity ofliquid hypochlorite:

(18.53)

EXAMPLE 18.56

Problem:

The chlorine dose is 8.8 mg/L and the flow rate is 3.28MGD. The hypochlorite solution is 71% available chlorineand has a specific gravity of 1.25. How many pounds ofhypochlorite must be used?

18.11.1.9.7 Chlorine Ordering

Because disinfection must be continuous, the supply ofchlorine must never be allowed to run out. The followingcalculation provides a simple method for determiningwhen additional supplies must be ordered. The processconsists of three steps:

Dose mg LChlorine Feed Rate lb d

Q MGD lb mg L MG( ) = ( )

( ) ¥ 8 34.

Dose mg L320 lb d

5.6 MGD lb mg L MG

mg L

( ) =¥

=

8 34

6 9

.

.

Available Chlorine Amount of Hypochlorite

% Available Chlorine

= ¥

(18.51)

Available Chlorine lb

lb Chlorine

= ¥

=

30 0 625

18 75

.

.

Hypochlorite Quantity lb d

Required Chlorine Dose mg L Q MGD lg mg L MG

%Available Chlorine

( )

( ) ( )

=

¥ ¥

.8 34

(18.52)

Hypochlorite Quantity lb d

8.5 mg L 3.25 MGD lb mg L MG

0.70

lb d

( )

=¥ ¥

=

8 34

329

.

Hypochlorite Quantity gal d

Required Chlorine Dose mg L Q MGD

lb mg L MG %Available Chlorine lb gal

Hypochlorite Solution Specific Gravity

( ) =

( ) ¥ ( ) ¥

¥ ¥8 34

8 34.

.

Hypochlorite Quantity gal d

8.8 mg L 3.28 MGD lb mg L MG

0.71 lb gal

gal d

( )

=¥ ¥

¥ ¥

=

8 34

8 34 1 25

32 5

.

. .

.



1. Adjust the flow and use variations if projectedchanges are provided.

2. If an increase in flow or required dosage isprojected, the current flow rate or dose must beadjusted to reflect the projected change.

3. Use the following equation:

(18.54)

EXAMPLE 18.57

Problem:

Based on available information for the past 12 months,the operator projects that the effluent flow rate willincrease by 7.5% during the next year. If the average dailyflow has been 4.5 MGD, what will be the projected flowfor the next 12 months?

Solution:

EXAMPLE 18.58

Problem:

The plant currently uses 90 lb of chlorine/d. The townwishes to order enough chlorine to supply the plant for3 months (assume 31 d/month). How many pounds ofchlorine should be ordered to provide the needed supply?

Solution:

Determine the amount of chlorine required for a givenperiod:

Note: In some instances, projections for flow or dosechanges are not available, but the plant operatormay wish to include an extra amount of chlorineas a safety factor. This safety factor can bestated as a specific quantity or as a percentage

of the projected usage. Safety factor as a spe-cific quantity can be expressed as follows:

Note: Chlorine is only shipped in full containers.Unless you specifically ask for the amount ofchlorine actually required or used during aspecified period, all decimal parts of a cylinderare rounded up to the next highest number offull cylinders.

18.11.2 UV IRRADIATION

Although ultraviolet disinfection was recognized as amethod for achieving disinfection in the late nineteenthcentury, its application virtually disappeared with the evo-lution of chlorination technologies. However, in recentyears, there has been resurgence in its use in the waste-water field, largely as a consequence of concern fordischarge of toxic chlorine residual. Even more recently,UV has gained more attention because of the tough newregulations on chlorine use imposed by both OSHA andEPA. Because of this relatively recent increased regulatorypressure, many facilities are actively engaged in substitutingchlorine for other disinfection alternatives. UV technologyhas made many improvements, making UV attractive asa disinfection alternative.

UV light has very good germicidal qualities and isvery effective in destroying microorganisms. It is used inhospitals, biological testing facilities, and many other sim-ilar locations. In wastewater treatment, the plant effluentis exposed to ultraviolet light of a specified wavelengthand intensity for a specified contact period. The effective-ness of the process is dependent upon

1. UV light intensity2. Contact time3. Wastewater quality (turbidity)

The Achilles’ heel of UV for disinfecting wastewateris turbidity. If the wastewater quality is poor, the ultravi-olet light will be unable to penetrate the solids and theeffectiveness of the process decreases dramatically. Forthis reason, many states limit the use of UV disinfectionto facilities that can reasonably be expected to produce aneffluent containing less than or equal to 30 mg/L of BODand TSS.

In the operation of UV systems, UV lamps must bereadily available when replacements are required. The bestlamps are those with a stated operating life of at least7500 h that do not produce significant amounts of ozoneor hydrogen peroxide. The lamps must also meet technical

Projected Flow Current Flow MGD

Change

Projected Dose Current Dose mg L

Change

= ( ) ¥

+[ ]= ( ) ¥

+[ ]

. %

. %

1 0

1 0

Projected Flow MGD MGD

MGD

( ) [ ]= ¥ +

=

4 5 1 0 0 075

4 84

. . .

.

Chlorine Required Feed Rate lb d

Number of Days Required

= 90 lb d d

lb

= ¥

¥

=

( )

,

124

11 160

Total Required Cl Chlorine Required lb

Safety Factor

2 = +( )



specifications for intensity, output, and arc length. If theUV light tubes are submerged in the wastestream, theymust be protected inside quartz tubes. These tubes notonly protect the lights, but also make cleaning and replace-ment easier.

Contact tanks must be used with UV disinfection.They must be designed with the banks of UV lights in ahorizontal position that is either parallel or perpendicularto the flow or with banks of lights placed in a verticalposition perpendicular to the flow.

Note: The contact tank must provide a minimum of10 sec of exposure time.

We stated earlier that turbidity problems have beenthe main hinderance with using UV in wastewater treat-ment. If turbidity is UV’s Achilles’ heel, then the need forincreased maintenance (as compared to other disinfectionalternatives) is the toe of the same foot.

UV maintenance requires that the tubes be cleaned ona regular basis or as needed. In addition, periodic acidwashing is also required to remove chemical buildup.

In operating UV disinfection systems, routine moni-toring is required. Monitoring to check on bulb burnout,buildup of solids on quartz tubes, and UV light intensityis required.

Note: UV light is extremely hazardous to the eyes.Never enter an area where UV lights are inoperation without proper eye protection. Neverlook directly into the UV light.

18.11.3 OZONATION

Ozone is a strong oxidizing gas that reacts with mostorganic and many inorganic molecules. It is producedwhen oxygen molecules separate, collide with other oxy-gen atoms, and form a molecule consisting of three oxygenatoms. For high-quality effluents, ozone is a very effectivedisinfectant. Current regulations for domestic treatmentsystems limit use of ozonation to filtered effluents unlessthe system’s effectiveness can be demonstrated prior toinstallation.

Note: Effluent quality is the key performance factorfor ozonation.

For ozonation of wastewater, the facility must havethe capability to generate pure oxygen along with an ozonegenerator. A contact tank with greater than or equal to10-min contact time at design average daily flow isrequired. Off-gas monitoring for process control is alsorequired. In addition, safety equipment capable of moni-toring ozone in the atmosphere and a ventilation systemcapable of preventing ozone levels exceeding 0.1 ppm isrequired.

The actual operation of the ozonation process consistsof monitoring and adjusting the ozone generator and mon-

itoring the control system to maintain the required ozoneconcentration in the off-gas. The process must also beevaluated periodically using biological testing to assessits effectiveness.

Note: Ozone is an extremely toxic substance. Con-centrations in air should not exceed 0.1 ppm. Italso has the potential to create an explosiveatmosphere. Sufficient ventilation and purgingcapabilities should be provided.

Ozone has certain advantages over chlorine fordisinfection of wastewater: (1) it increases DOin the effluent, (2) it has a briefer contact time,(3) it has no undesirable effects on marine organ-isms, and (4) it decreases turbidity and odor.

18.11.4 BROMINE CHLORIDE

Bromine chloride is a mixture of bromine and chlorine. Itforms hydrocarbons and hydrochloric acid when mixedwith water. Bromine chloride is an excellent disinfectantthat reacts quickly and normally does not produce anylong-term residuals.

Note: Bromine chloride is an extremely corrosivecompound in the presence of low concentra-tions of moisture.

The reactions occurring when bromine chloride isadded to the wastewater are similar to those occurringwhen chlorine is added. The major difference is the pro-duction of bromamine compounds rather than chloramines.The bromamine compounds are excellent disinfectants,but are less stable and dissipate quickly. In most cases,the bromamines decay into other, less toxic compoundsrapidly and are undetectable in the plant effluent.

The factors that affect performance are similar to thoseaffecting the performance of the chlorine disinfection pro-cess. Such factors as effluent quality and contact time havea direct impact on the performance of the process.

18.11.5 NO DISINFECTION

In a very limited number of cases, treated wastewaterdischarges without disinfection is permitted. These areapproved on a case-by-case basis. Each request must beevaluated based upon the point of discharge, the qualityof the discharge, the potential for human contact, andmany other factors.

18.12 ADVANCED WASTEWATER TREATMENT

Advanced wastewater treatment is defined as the methodsand processes that remove more contaminants (suspendedand dissolved substances) from wastewater than are taken



out by conventional biological treatment. In other words,advanced wastewater treatment is the application of a pro-cess or system that follows secondary treatment or thatincludes phosphorus removal or nitrification in conven-tional secondary treatment.

Advanced wastewater treatment is used to augmentconventional secondary treatment because secondarytreatment typically removes only between 85 and 95% ofthe BOD and TSS in raw sanitary sewage. Generally, thisleaves 30 mg/L or less of BOD and TSS in the secondaryeffluent. To meet stringent water-quality standards, thislevel of BOD and TSS in secondary effluent may notprevent violation of water-quality standards — the plantmay not make permit. Thus, advanced wastewater treat-ment is often used to remove additional pollutants fromtreated wastewater.

In addition to meeting or exceeding the requirementsof water-quality standards, treatment facilities useadvanced wastewater treatment for other reasons as well.For example, conventional secondary wastewater treat-ment is sometimes not sufficient to protect the aquaticenvironment. This is the case when periodic flow eventsoccur in a stream; the stream may not provide the amountof dilution of effluent needed to maintain the necessaryDO levels for aquatic organism survival.

Secondary treatment has other limitations. It does notsignificantly reduce the effluent concentration of nitrogenand phosphorus (important plant nutrients) in sewage. Ifdischarged into lakes, these nutrients contribute to algalblooms and accelerated eutrophication (lake aging). Also,the nitrogen in the sewage effluent may be present mostlyin the form of ammonia compounds. If in high enoughconcentration, ammonia compounds are toxic to aquaticorganisms. Yet another problem with these compounds isthat they exert a nitrogenous oxygen demand in the receiv-ing water as they convert to nitrates. This process is callednitrification.

Note: The term tertiary treatment is commonly usedas a synonym for advanced wastewater treat-ment. These two terms do not have preciselythe same meaning. Tertiary suggests a third stepthat is applied after primary and secondarytreatment.

Advanced wastewater treatment can remove morethan 99% of the pollutants from raw sewage and canproduce an effluent of almost potable (drinking) waterquality. However, advanced treatment is not free. The costof advanced treatment for operation and maintenance aswell as for retrofit of present conventional processes isvery high (sometimes doubling the cost of secondary treat-ment). A plan to install advanced treatment technologycalls for careful study; the benefit-to-cost ratio is notalways big enough to justify the additional expense.

Even considering the expense, application of someform of advanced treatment is not uncommon. These treat-ment processes can be physical, chemical, or biological.The specific process used is based upon the purpose ofthe treatment and the quality of the effluent desired.

18.12.1 CHEMICAL TREATMENT

The purpose of chemical treatment is to remove:

1. BOD2. TSS3. Phosphorus4. Heavy metals5. Other substances that can be chemically con-

verted to a settleable solid

Chemical treatment is often accomplished as an add-on to existing treatment systems or by means of separatefacilities specifically designed for chemical addition. Ineach case, the basic process necessary to achieve thedesired results remains the same:

1. Chemicals are thoroughly mixed with thewastewater.

2. The chemical reactions that occur form solids(coagulation).

3. The solids are mixed to increase particle size(flocculation).

4. Settling and filtration (separation) remove thesolids.

The specific chemical used depends on the pollutantto be removed and the characteristics of the wastewater.Chemicals may include the following:

1. Lime2. Alum (aluminum sulfate)3. Aluminum salts4. Ferric or ferrous salts5. Polymers6. Bioadditives

18.12.1.1 Operation, Observation, and Troubleshooting Procedures

Operation and observation of performance of chemicaltreatment processes are dependent on the pollutant beingremoved and process design.

Operational problems associated with chemical treat-ment processes used in advanced treatment usuallyrevolve around problems with floc formation, settlingcharacteristics, removal in the settling tank, and sludge (insettling tank) turning anaerobic.



To correct these problems, the operator must be ableto recognize the applicable problem indicators throughproper observation. Below we list common indicators andobservations of operational problems, along with theapplicable causal factors and corrective actions:

1. Poor floc formation and settling characteristics.A. Causal factors:

1. Insufficient chemical dispersal duringrapid mix.

2. Excessive detention time in rapid mix.3. Improper coagulant dosage.4. Excessive flocculator speed.

B. Corrective actions (where applicable):1. Increase speed of rapid mixer.2. Reduce detention time to15–60 sec.3. Correct dosage (determine by jar testing).4. Reduce flocculator speed.

2. Good floc formation, poor removal in settlingtank.A. Causal factors:

1. Excessive velocity between flocculationand settling.

2. Settling tank operational problem.B. Corrective action:

1. Reduce velocity to acceptable range3. Settling tank sludge is turning anaerobic.

A. Causal factors:1. A sludge blanket has developed in set-

tling tank.2. Excessive organic carryover from sec-

ondary treatment.B. Corrective actions:

1. Increase sludge withdrawal to eliminateblanket.

2. Correct secondary treatment operationalproblems.

18.12.2 MICROSCREENING

Microscreening (also called microstraining) is anadvanced treatment process used to reduce suspended sol-ids. The microscreens are composed of specially wovensteel wire fabric mounted around the perimeter of a largerevolving drum. The steel wire cloth acts as a fine screen,with openings as small as 20 mm (or millionths of a meter)that are small enough to remove microscopic organismsand debris.

The rotating drum is partially submerged in the sec-ondary effluent, which must flow into the drum then out-ward through the microscreen. As the drum rotates, cap-tured solids are carried to the top where a high-velocitywater spray flushes them into a hopper or backwash traymounted on the hollow axle of the drum. Backwash solidsare recycled to plant influent for treatment. These units

have found greatest application in treatment of industrialwaters and final polishing filtration of wastewater efflu-ents. Expected performance for suspended solids removalis 95 to 99%, but the typical suspended solids removalachieved with these units is about 55%. The normal rangeis from 10 to 80%.

According to Metcalf & Eddy2, the functional design ofthe microscreen unit involves the following considerations:

1. The characterization of the suspended solidswith respect to the concentration and degree offlocculation

2. The selection of unit design parameter valuesthat will not only ensure capacity to meet max-imum hydraulic loadings with critical solidscharacteristics, but also provide desired designperformance over the expected range of hydrau-lic and solids loadings

3. The provision of backwash and cleaning facil-ities to maintain the capacity of the screen.


Microscreen operators typically perform sampling andtesting on influent and effluent TSS and monitor screenoperation to ensure proper operation. Operational prob-lems generally consist of gradual decrease in throughputrate, leakage at ends of the drum, reduced screen capacity,hot or noisy drive systems, erratic drum rotation, andsudden increases in effluent solids:

1. Decrease in throughput rate (from slime growth).A. Causal factors:

1. Inadequate cleaning.2. Spray nozzles plugged.

B. Corrective actions (where applicable):1. Increase backwash pressure (60 to 120 psi).2. Add hypochlorite upstream of the unit.3. Unclog nozzles.

2. Decreased performance from leakage at endsof the drum.A. Causal factor:

1. Defective or leaking units.B. Corrective actions:

1. Tighten tension on sealing bands.2. Replace sealing bands if excessive ten-

sion is required.3. Screen capacity is reduced after shutdown period.

A. Causal factor:1. Screen is fouled.

B. Corrective actions:1. Clean screen prior to shutdown.2. Clean screen with hypochlorite.



4. Drive System is running hot or noisy.A. Causal factor:

1. Inadequate lubrication.B. Corrective action:

1. Fill to specified level with recommendedoil.

5. Erratic drum rotation.A. Causal factors:

1. Improper drive belt adjustment.2. Drive belts are worn out.

B. Corrective actions:1. Adjust tension to specified level.2. Replace drive belts.

6. Sudden increase in effluent solids.A. Causal factors:

1. Hole in screen fabric.2. Screws that secure fabric are loose.3. Solids collection trough is overflowing.

B. Corrective actions (where applicable):1. Repair fabric.2. Tighten screws.3. Reduce microscreen influent flow rate.

7. Decreased screen capacity after high-pressurewashing.A. Causal factor:

1. Iron or manganese oxide film on fabric.B. Corrective action:

1. Clean screen with inhibited acid cleaner.Follow manufacturer’s instruction.

18.12.3 FILTRATION

The purpose of filtration processes used in advanced treat-ment is to remove suspended solids. The specific opera-tions associated with a filtration system are dependent onthe equipment used. A general description of the processfollows.

18.12.3.1 Filtration Process Description

Wastewater flows to a filter (gravity or pressurized). Thefilter contains single, dual, or multimedia. Wastewaterflows through the media, which removes solids. The solidsremain in the filter. Backwashing the filter as neededremoves trapped solids. Backwash solids are returned tothe plant for treatment. Processes typically remove 95 to99% of the suspended matter.


Operators routinely monitor filter operation to ensure opti-mum performance and to detect operational problemsbased on indication or observation of equipment malfunctionor process suboptimal performance. We discuss operationalproblems typically encountered in the list that follows:

1. High effluent turbidity.A. Causal factors:

1. Filter requires backwashing.2. Prior chemical treatment inadequate.

B. Corrective actions (where applicable):1. Backwash unit as soon as possible.2. Adjust/control chemical dosage properly.

2. High head loss through the filter.A. Causal factor:

1. Filter requires backwashing.B. Corrective action:

1. Backwash unit as soon as possible.3. High head loss through unit right after back-

washing.A. Causal factors:

1. Backwash cycle was insufficient.2. Surface scour or wash arm inoperative.

B. Corrective actions (where applicable):1. Increase backwash time.2. Repair air scour or surface scrubbing arm.

4. Backwash water requirement exceeds 5%.A. Causal factors:

1. Excessive solids in filter influent.2. Excessive filter aid dosage.3. Surface washing or air scour not operating.4. Surface washing or air scour not operated

long enough during backwash cycle.5. Excessive backwash cycle used.

B. Corrective actions (where applicable)1. Improve treatment prior to filtration.2. Reduce control or filter aid dose rates.3. Repair mechanical problem.4. Increase surface wash cycle time.5. Adjust backward cycle length.

5. Filter surface clogging.A. Causal factors:

1. Inadequate prior treatment (single mediafilters).

2. Excessive filter aid dosage (dual or mixedmedia filters).

3. Inadequate surface wash cycle.4. Inadequate backwash cycle.

B. Corrective actions (where applicable):1. Improve prior treatment.2. Replace single media with dual or mixed

media.3. Reduce or eliminate filter aid.4. Provide adequate surface wash cycle.5. Provide adequate backwash cycle.

6. Short filter runs.A. Causal factor:

1. High head loss.B. Corrective actions:



603

1. Improve prior treatment.2. Replace single media with dual or mixed

media.3. Reduce or eliminate filter aid.4. Provide adequate surface wash cycle.5. Provide adequate backwash cycle.

7. Filter effluent turbidity increases rapidly.A. Causal factors:

1. Inadequate filter aid dosage.2. Filter aid system mechanical failure.3. Filter aid requirement has changed.

B. Corrective actions (where applicable):1. Increase chemical dosage.2. Repair feed system.3. Adjust filter aid dose rate (do jar test).

8. Mud Ball Formation.A. Causal factors:

1. Inadequate backwash flow rate.2. Inadequate surface wash.

B. Corrective actions (where applicable):1. Increase backwash flow to specified levels.2. Increase surface wash cycle.

9. Gravel displacement.A. Causal factor:

1. Air is entering the underdrains duringbackwash cycle.

B. Corrective actions:1. Control backwash volume.2. Control backwash water head.3. Replace media (severe displacement).

10. Medium is lost during backwash cycle.A. Causal factors:

1. Excessive backwash flows.2. Excessive auxiliary scour.3. Air attached to filter media, causing it to

float.B. Corrective actions (where applicable):

1. Reduce backwash flow rate.2. Stop auxiliary scour several minutes before

end of backwash cycle.3. Increase backwash frequency to prevent

bubble displacement and maintain maxi-mum operating water depth above filtersurface.

11. Filter backwash cycle not effective duringwarm weather.A. Causal factor:

1. Decreased water viscosity due to highertemperatures.

B. Corrective action:1. Increase backwash rate until required bed

expansion is achieved.

12. Air binding causes premature head loss increase.A. Causal factors:

1. Air bubble produced by exposing aninfluent containing high dissolved oxygenlevels to less than atmospheric pressure.

2. Pressure drops occurring during change-over to backwash cycle.

B. Corrective actions (where applicable):1. Increase backwash frequency.2. Maintain maximum operating water depth.

18.12.4 BIOLOGICAL NITRIFICATION

Biological nitrification is the first basic step of biologicalnitrification-denitrification.

In nitrification, the secondary effluent is introducedinto another aeration tank, trickling filter, or biodisc.Because most of the carbonaceous BOD has already beenremoved, the microorganisms that drive in this advancedstep are the nitrifying bacteria nitrosomonas and nitro-bacter. In nitrification, the ammonia nitrogen is convertedto nitrate nitrogen, producing a nitrified effluent. At thispoint, the nitrogen has not actually been removed, onlyconverted to a form that is nontoxic to aquatic life andthat does not cause an additional oxygen demand.

The nitrification process can be limited (performanceaffected) by alkalinity (requires 7.3 parts alkalinity to1.0 part ammonia nitrogen), pH, DO availability, toxicity(ammonia or other toxic materials), and process MCRT(SRT). As a general rule, biological nitrification is moreeffective and achieves higher levels of removal during thewarmer times of the year.


Ensuring the nitrification process performs as per designrequires the operator to monitor the process and makeroutine adjustments. The loss of solids from settling tank,RBC, or from a trickling filter are common problems thatthe operator must be able to identify as well as to takeproper corrective actions. In these instances, the operatorneeds to be familiar with activated sludge system, RBC,or trickling filter operations.

The operator must also be familiar with other nitrifi-cation operational problems and must be able to take theproper corrective actions. We list typical nitrification oper-ational problems and recommended corrective actionsbelow:

1. pH decreases with loss of nitrification.A. Causal factors:

1. Insufficient alkalinity available for process.2. Acid wastes in process influent.



B. Corrective actions (where applicable):1. If process alkalinity is less than 30 mg/L,

add lime or sodium hydroxide to processinfluent.

2. Identify source and control of acid wastes.2. Incomplete nitrification.

A. Causal factors:1. Process is DO.2. Process is temperature limited.3. Influent nitorgen loading has increased.4. Low nitrifying bacteria population in

process.5. Peak hourly ammonium concentrations

exceed available oxygen supplies.B. Corrective actions (where applicable):

1. Increase process aeration rate.2. Decrease process nitrogen loading.3. Increase nitrifying bacteria population.4. Put additional units in service.5. Modify operation to increase nitrogen

removal.6. Decrease wasting or solids loss.7. Add settled raw sewage to nitrification

unit to increase biological solids.8. Increase oxygen supply.9. Install flow equalization to minimize

peaks.3. SVI of nitrification sludge is very high (>250).

A. Causal factor:1. Nitrification is occurring in the first stage

(BOD removal sludge).B. Corrective actions (where applicable):

1. Transfer sludge from first to second stages.2. Operate first stage at lower MCRT or SRT.

18.12.5 BIOLOGICAL DENITRIFCATION

Biological denitrification removes nitrogen from thewastewater. When bacteria come in contact with a nitrifiedelement in the absence of oxygen, they reduce the nitratesto nitrogen gas, which escapes the wastewater. The deni-trification process can be done in either an anoxic activatedsludge system (suspended growth) or in a column system(fixed growth). The denitrification process can remove upto 85% or more of nitrogen.

After effective biological treatment, little oxygendemanding material is left in the wastewater when itreaches the denitrification process.

The denitrification reaction will only occur if an oxygendemand source exists when no DO is present in the waste-water. An oxygen demand source is usually added to reducethe nitrates quickly. The most common demand sourceadded is soluble BOD or methanol. Approximately 3 mg/Lof methanol is added for every 1 mg/L of nitrate-nitrogen.

Suspended growth denitrification reactors are mixedmechanically, but only enough to keep the biomass fromsettling without adding unwanted oxygen.

Submerged filters of different types of media may alsobe used to provide denitrification. A fine media downflowfilter is sometimes used to provide both denitrification andeffluent filtration. A fluidized sand bed where wastewaterflows upward through a media of sand or activated carbonat a rate to fluidize the bed may also be used. Denitrifica-tion bacteria grow on the media.

18.12.5.1 Observation, Operation, and Troubleshooting Procedures

In operation of a denitrification process, operators monitorperformance by observing various parameters. Parametersor other indicators and observations that demonstrate pro-cess malfunction or suboptimal performance indicate theneed for various corrective actions. We discuss several ofthese indicators of poor process performance, their causalfactors, and corrective actions in the sections that follow.

1. Process effluent: sudden increase in BOD.A. Causal factor:

1. Excessive methanol or other organic mat-ter present.

B. Corrective actions (as required):1. Reduce methanol addition.2. Install automated methanol control system.3. Install aerated stabilization unit for

removal of excess methanol.2. Sudden increase in effluent nitrate concentration.

A. Causal factors:1. Inadequate methanol control.2. Denitrification pH is outside 7.0 to 7.5

range required for process.3. Loss of solids from denitrification pro-

cess due to pump failure.4. Excessive mixing introducing DO.

B. Corrective actions (where applicable):1. Identify and correct control problem.2. Correct pH problem in nitrification

process.3. Adjust pH at process influent.4. Correct denitrification sludge return.5. Increase denitrification sludge waste rate.6. Decrease denitrification sludge waste rate.7. Transfer sludge from carbonaceous units

to denitrification unit.8. Reduce mixer speed.9. Remove some mixers from service.

3. High head loss (packed bed nitrification).A. Causal factors:

1. Excessive solids in unit.2. Nitrogen gas accumulating in unit.



B. Corrective action:1. Backwash unit 1 to 2 min and return to

service.4. Out of service packed bed unit binds on start-up.

A. Causal factor:1. Solids have floated to top during shut

down.B. Corrective action:

1. Backwash units before removing fromservice and immediately before placingin service.

18.12.6 CARBON ADSORPTION

The main purpose of carbon adsorption used in advancedtreatment processes is the removal of refractory organiccompounds (non-BOD) and soluble organic material thatare difficult to eliminate by biological or physical or chem-ical treatment.

In the carbon adsorption process, wastewater passesthrough a container filled either with carbon powder orcarbon slurry. Organics adsorb onto the carbon (i.e.,organic molecules are attracted to the activated carbonsurface and are held there) with sufficient contact time.

A carbon system usually has several columns or basinsused as contactors. Most contact chambers are either openconcrete gravity-type systems or steel pressure containersapplicable to either upflow or downflow operation.

With use, carbon loses its adsorptive capacity. Thecarbon must then be regenerated or replaced with freshcarbon. As head loss develops in carbon contactors, theyare backwashed with clean effluent in much the same waythe effluent filters are backwashed. Carbon used foradsorption may be in granular or powdered form.

Note: Powdered carbon is too fine for use in columns.It is usually added to the wastewater and laterremoved by coagulation and settling.


In operation of a carbon adsorption system for advancedwastewater treatment, operators are primarily interested inmonitoring the system to prevent excessive head loss, reducelevels of hydrogen sulfide in the carbon contactor, ensurethat the carbon is not fouled, and ensure corrosion of metalparts and damage to concrete in contactors is minimal:

1. Excessive head loss.A. Casual factors:

1. Highly turbid influent.2. Growth and accumulation of biological

solids in unit.3. Excessive carbon fines due to deteriora-

tion during handling.4. Inlet or outlet screens plugged.

B. Corrective actions (where applicable):1. Backwash unit vigorously.2. Correct problem in prior treatment steps.3. Operate as an expanded upflow bed to

remove solids continuously.4. Increase frequency of backwashing for

downflow beds.5. Improve soluble BOD removal in prior

treatment steps.6. Remove carbon from unit and wash out

fines.7. Replace carbon with harder carbon.8. Backflush screens.

2. Hydrogen sulfide is in carbon contactor.A. Causal factors:

1. Low or no DO and nitrate in contactorinfluent.

2. High influent BOD concentrations3. Excessive detention time in carbon

contactorB. Corrective actions (where applicable):

1. Add air, oxygen, or sodium nitrate to unitinfluent.

2. Improve soluble BOD removal in priortreatment steps.

3. Precipitate sulfides already formed withiron on chlorine.

4. Reduce detention time by removing con-tactors from service.

5. Backwash units more frequently andmore violently, using air scour or surfacewash.

3. Large decrease in COD removed or pounds ofcarbon regenerated.A. Causal factor:

1. Carbon is fouled and losing efficiency.B. Corrective action:

1. Improve regeneration process performance.4. Corrosion of metal parts or damage to concrete

in contactors.A. Causal factors:

1. Hydrogen sulfide in carbon contactors.2. Holes in protective coatings exposed to

dewatered carbon.B. Corrective actions:

1. Add air, oxygen, or sodium nitrate to unitinfluent.

2. Improve soluble BOD removal in priortreatment steps.

3. Precipitate sulfides already formed withiron on chlorine.

4. Reduce detention time by removing con-tactors from service.



5. Backwash units more frequently andmore violently, using air scour or surfacewash.

6. Repair protective coatings.

18.12.7 LAND APPLICATION

The application of secondary effluent onto a land surfacecan provide an effective alternative to the expensive andcomplicated advanced treatment methods discussed pre-viously and the biological nutrient removal (BNR) systemdiscussed briefly in Section 18.12.8. A high-quality pol-ished effluent (i.e., effluent with high levels of TSS, BOD,phosphorus, and nitrogen compounds as well as refractoryorganics are reduced) can be obtained by the natural pro-cesses that occur as the effluent flows over the vegetatedground surface and percolates through the soil.

Limitations are involved with land application ofwastewater effluent. For example, the process needs largeland areas. Soil type and climate are also critical factorsin controlling the design and feasibility of a land treatmentprocess.

18.12.7.1 Types or Modes of Land Application

Three basic types or modes of land application or treat-ment are commonly used: irrigation (slow rate), overlandflow, and infiltration-percolation (rapid rate). The basicobjectives of these types of land applications and the con-ditions under which they can function vary.

In irrigation (also called slow rate), wastewater issprayed or applied (usually by ridge-and-furrow surfacespreading or by sprinkler systems) to the surface of theland. Wastewater enters the soil. Crops growing on theirrigation area utilize available nutrients. Soil organismsstabilize organic content of the flow. Water returns to thehydrologic (water) cycle through evaporation or by enter-ing the surface water or groundwater (see Figure 18.12A).

The irrigation land application method provides thebest results (compared with the other two types of landapplication systems) with respect to advanced treatmentlevels of pollutant removal. Not only are suspended solidsand BOD significantly reduced by filtration of the waste-water, but also biological oxidation of the organics in thetop few inches of soil occurs. Nitrogen is removed primarilyby crop uptake, and phosphorus is removed by adsorptionwithin the soil.

FIGURE 18.12 Land application. (From Spellman, F.R., Spellman’s Standard Handbook for Wastewater Operators, Vol. 1, Tech-nomic Publ., Lancaster, PA, 1999.)

Percolation

Appliedwastewater Evaporation

C.

A.

B.

Slope 2to 8%

Appliedwastewater

Grass andvegetative litter

Sheet flow

Runoffcollection

Percolation

Appliedwastewater

Evapotranspiration

Evapotranspiration



607

Irrigation expected performance levels are:

1. BOD — 98%2. Suspended solids — 98%3. Nitrogen — 85%4. Phosphorus — 95%5. Metals — 95%

The overland flow mode of land application used forwater purification is accomplished by physical, chemical,and biological processes as the wastewater flows in a thinfilm down the relatively impermeable surface. In the pro-cess, wastewater sprayed over sloped terraces flows slowlyover the surface. Soil and vegetation remove suspendedsolids, nutrients, and organics. A small portion of thewastewater evaporates. The remainder flows to collectionchannels. Collected effluent is discharged to surfacewaters (see Figure 18.2B).

Overflow flow expected performance levels are:

1. BOD — 92%2. Suspended solids — 92%3. Nitrogen — 70 to 90%4. Phosphorus — 40 to 80%5. Metals — 50%

In the infiltration-percolation (rapid rate) land applica-tion process, wastewater is sprayed/pumped to spreadingbasins (a.k.a. recharge basins or large ponds). Some waste-water evaporates. The remainder percolates/infiltrates intosoil. Solids are removed by filtration. Water recharges thegroundwater system. Most of the effluent percolates to thegroundwater; very little of it is absorbed by vegetation(see Figure 18.2C). The filtering and adsorption action ofthe soil removes most of the BOD, TSS, and phosphorousfrom the effluent; however, nitrogen removal is relativelypoor.

Infiltration-percolation expected performance levelsare:

1. BOD — 85 to 99%2. Suspended solids — 98%3. Nitrogen — 0 to 50%4. Phosphorus — 60 to 95%5. Metals — 50 to 95%

18.12.7.1.1 Operation, Observation, and Troubleshooting Procedures

Performance levels are dependent on the land applicationprocess used. To be effective, operators must monitor theoperation of the land application process employed. Expe-rience has shown that these processes can be very effective,but problems exist when the flow contains potentiallytoxic materials that may become concentrated in the cropsbeing grown on land. Along with this problem, other prob-

lems are common, including ponding, deterioration ofdistribution piping systems, malfunctioning sprinklerheads, waste runoff, irrigated crop die-off, poor cropgrowth, and too much flow rate:

1. In irrigated areas, water is ponding.A. Causal factors:

1. Excessive application rate.2. Inadequate drainage because of ground-

water levels.3. Damaged drainage wells.4. Inadequate well withdrawal rates.5. Damaged drain tiles.6. Broken pipe in distribution system.

B. Corrective actions (where applicable):1. Reduce application rate to acceptable

level.2. Irrigate in portions of site where ground-

water is not a problem.3. Store wastewater until condition is

corrected.4. Repair drainage wells.5. Increase drainage well pumping rates.6. Repair damaged drain tiles.7. Repair pipe.

2. Deterioration of distribution piping.A. Causal factors:

1. Effluent remains in pipe for long periods.2. Different metals used in same line.

B. Corrective actions (where applicable):1. Drain pipe after each use.2. Coat steel valves.3. Install cathodic or anodic protection.

3. No flow from source sprinkler nozzles.A. Casual factor:

1. Nozzles clogged.B. Corrective action:

1. Repair or replace screen on irrigationpump inlet.

4. Wastes are running off irrigation area.A. Causal factors:

1. High sodium adsorption ratio has causedclay soil to become impermeable.

2. Solids seal soil surface.3. Application rate is greater than soil infil-

tration rate.4. Break in distribution piping.5. Soil permeability has decreased because

of continuous application of wastewater.6. Rain has saturated the soil.

B. Corrective actions (where applicable):1. Feed calcium and magnesium to maintain

a sodium adsorption ratio of less than 9.2. Strip crop area.



3. Reduce application rate to acceptablelevel.

4. Repair system.5. Allow 2- to 3-d rest period between each

application.6. Store wastewater until soil has drained.

5. Irrigated crop is dead.A. Causal factors:

1. Too much or not enough water has beenapplied.

2. Wastewater contains toxic materials intoxic concentrations.

3. Excessive insecticide or herbicide applied.4. Inadequate drainage has flooded root

zone of crop.B. Corrective actions (where applicable):

1. Adjust application rate to appropriate level.2. Eliminate source of toxicity.3. Apply only as permitted or directed.

6. Poor crop growth.A. Causal factors:

1. Too little nitrogen or phosphorus.2. Timing of nutrient applications does not

coincide with plant nutrient need.B. Corrective actions (where applicable):

1. Increase application rate to supply nitro-gen and phosphorus.

2. Augment nitrogen and phosphorus ofwastewater with commercial fertilizerapplications.

3. Adjust application schedule to matchcrop need.

7. Irrigation pump has normal pressure, but aboveaverage flow rate.A. Causal factors:

1. Broken main, riser, or lateral.2. Leaking gasket.3. Sprinkler head or nozzle is missing.4. Too many distribution laterals are in ser-

vice at one time.B. Corrective actions (where applicable):

1. Locate and repair problems.2. Locate and replace defective gasket.3. Correct valving to adjust number of lat-

erals in service.8. Irrigation pump has above average pressure, but

below average flow.A. Causal factor:

1. Blockage in system.B. Corrective action:

1. Locate and correct blockage.9. Irrigation pump has below average pressure and

flow rate

A. Causal factors:1. Worn impeller.2. Partially clogged pump inlet screen.

B. Corrective actions (where applicable):1. Replace impeller.2. Clean screen.

10. Excessive erosion occurring.A. Causal factors:

1. Excessive application rates.2. Inadequate crop coverage.

B. Coverage actions (where applicable):1. Reduce application rate.

11. Odor complaints.A. Causal factors:

1. Wastes are turning septic during transportto treatment or irrigation site.

2. Storage reservoirs are septic.B. Corrective actions (where applicable):

1. Aerate or chemically treat wastes duringtransport.

2. Install cover over discharge point. Collectand treat gases before release.

3. Improve pretreatment.4. Aerate storage reservoirs.

12. Center pivot irrigation rigs stuck in mud.A. Causal factors:

1. Excessive application rates.2. Improper rig or tires.3. Poor drainage.

B. Corrective actions:1. Reduce application rate.2. Install tire with higher flotation capabili-

ties.13. Nitrate in groundwater near irrigation site is

increasing.A. Causal factors:

1. Nitrogen application rate does not bal-ance with crop need.

2. Applications are occurring during dor-mant periods.

3. Crop is not being properly harvested andremoved.

B. Corrective actions (where applicable):1. Change to crop with higher nitrogen

requirement.2. Adjust schedule to apply only during

active growth periods.3. Harvest and remove crop as required.

18.12.8 BIOLOGICAL NUTRIENT REMOVAL

Recent experience has shown that BNR systems are reli-able and effective in removing nitrogen and phosphorus.The process is based upon the principle that under specific



conditions, microorganisms will remove more phosphorusand nitrogen than is required for biological activity. Sev-eral patented processes are available for this purpose. Per-formance depends on the biological activity and the pro-cess employed.

18.13 SOLIDS (SLUDGE OR BIOSOLIDS) HANDLING

The wastewater treatment unit processes described to thispoint remove solids and BOD from the wastestream beforethe liquid effluent is discharged to its receiving waters.What remains to be disposed of is a mixture of solids andwastes, called process residuals; they are more commonlyreferred to as sludge or biosolids.

Note: Sludge is the commonly accepted name forwastewater solids. If wastewater sludge is usedfor beneficial reuse (e.g., as a soil amendmentor fertilizer), it is commonly called biosolids.

The most costly and complex aspect of wastewatertreatment can be the collection, processing, and disposalof sludge. This is the case because the quantity of sludgeproduced may be as high as 2% of the original volume ofwastewater, depending somewhat on the treatment processbeing used.

Because sludge can be as much as 97% water contentand the cost of disposal will be related to the volume ofsludge being processed, one of the primary purposes orgoals (along with stabilizing it so it is no longer objection-able or environmentally damaging) of sludge treatment isto separate as much of the water from the solids as pos-sible. Sludge treatment methods may be designed toaccomplish both of these purposes.

Sludge treatment methods are generally divided intothree major categories: thickening, stabilization, anddewatering. Many of these processes include complexsludge treatment methods (i.e., heat treatment, vacuumfiltration, incineration and others).

18.13.1 SLUDGE: BACKGROUND INFORMATION

When we speak of sludge or biosolids, we are speakingof the same substance or material; each is defined as thesuspended solids removed from wastewater during sedi-mentation and concentrated for further treatment anddisposal or reuse. The difference between the terms sludgeand biosolids is determined by the way they are managed.(Note: The task of disposing, treating or reusing waste-water solids is called sludge or biosolids management.)Sludge is typically seen as wastewater solids that are dis-posed. Biosolids is the same substance managed for reuse,commonly called beneficial reuse (e.g., for land applica-tion as a soil amendment, such as biosolids compost).

Note that even as wastewater treatment standards havebecome more stringent because of increasing environmen-tal regulations, the volume of wastewater sludge has alsoincreased.

Note also that before sludge can be disposed of orreused, it requires some form of treatment to reduce itsvolume, stabilize it, and inactivate pathogenic organisms.

Sludge forms initially as a 3 to 7% suspension ofsolids; with each person typically generating about 4 galof sludge per week, the total quantity generated each day,week, month, and year is significant. Because of the vol-ume and nature of the material, sludge management is amajor factor in the design and operation of all water pol-lution control plants.

Note: Wastewater solids account for more than halfof the total costs in a typical secondary treat-ment plant.

18.13.1.1 Sources of Sludge

Wastewater sludge is generated in primary, secondary, andchemical treatment processes. In primary treatment, thesolids that float or settle are removed. The floatable mate-rial makes up a portion of the solid waste known as scum.Scum is not normally considered sludge; however, it shouldbe disposed of in an environmentally sound way. The set-tleable material that collects on the bottom of the clarifieris known as primary sludge. Primary sludge can also bereferred to as raw sludge because it has not undergonedecomposition. Raw primary sludge from a typical domes-tic facility is quite objectionable and has a high percentageof water — two characteristics that make handling difficult.

Those solids not removed in the primary clarifier arecarried out of the primary unit. These solids are known ascolloidal suspended solids. The secondary treatment sys-tem (i.e., trickling filter, activated sludge, etc.) is designedto change those colloidal solids into settleable solids thatcan be removed. Once in the settleable form, these solidsare removed in the secondary clarifier. The sludge at thebottom of the secondary clarifier is called secondarysludge. Secondary sludges are light and fluffy and moredifficult to process than primary sludges. In short, second-ary sludges do not dewater well.

The addition of chemicals and various organic andinorganic substances prior to sedimentation and clarifica-tion may increase the solids capture and reduce the amountof solids lost in the effluent. This chemical addition resultsin the formation of heavier solids that trap the colloidalsolids or convert dissolved solids to settleable solids. Theresultant solids are known as chemical sludges. As chem-ical usage increases, so does the quantity of sludge thatmust be handled and disposed. Chemical sludges can bevery difficult to process; they do not dewater well andcontain lower percentages of solids.


610


18.13.1.2 Sludge Characteristics

The composition and characteristics of sewage sludge varywidely and can change considerably with time. Notwith-standing these facts, the basic components of wastewatersludge remain the same. The only variations occur inquantity of the various components as the type of sludgeand the process from which it originated changes.

The main component of all sludges is water. Prior totreatment, most sludges contain 95 to +99% water (seeTable 18.8). This high water content makes sludge han-dling and processing extremely costly in terms of bothmoney and time. Sludge handling may represent up to40% of the capital cost and 50% of the operation cost ofa treatment plant. As a result, the importance of optimumdesign for handling and disposal of sludge cannot be over-emphasized. The water content of the sludge is present ina number of different forms. Some forms can be removedby several sludge treatment processes, allowing the sameflexibility in choosing the optimum sludge treatment anddisposal method.

The various forms of water and their approximatepercentages for a typical activated sludge are shown inTable 18.9. The forms of water associated with sludgesare:

Free water water that is not attached to sludge solidsin any way. This can be removed by simplegravitational settling.

Floc water water that is trapped within the floc andtravels with them. Its removal is possible bymechanical dewatering.

Capillary water water that adheres to the individualparticles and can be squeezed out of shape andcompacted.

Particle water water that is chemically bound to theindividual particles and can’t be removed with-out inclination.

From a public health view, the second and probablymore important component of sludge is the solids matter.Representing from 1 to 8% of the total mixture, thesesolids are extremely unstable. Wastewater solids can beclassified into two categories based on their origin: organicand inorganic. Organic solids in wastewater, are materialsthat are or were at one time alive and that will burn orvolatilize at 550°C after 15 minutes in a muffle furnace.The percent of organic material within a sludge will deter-mine how unstable it is.

The inorganic material within a sludge will determinehow stable it is. The inorganic solids are those solids thatwere never alive and will not burn or volatilize at 550°Cafter 15 minutes in a muffle furnace. Inorganic solids aregenerally not subject to breakdown by biological actionand are considered stable. Certain inorganic solids, how-ever, can create problems when related to the environment(e.g., heavy metals such as copper, lead, zinc, mercury,and others). These can be extremely harmful if discharged.

Organic solids may be subject to biological decompo-sition in either an aerobic or anaerobic environment.Decomposition of organic matter (with its production ofobjectionable by-products) and the possibility of toxicorganic solids within the sludge compound the problemsof sludge disposal.

Note: Before moving on to a discussion of the funda-mentals of sludge treatment methods, it isimportant to begin by covering sludge pumpingcalculations. It is important to point out that itis difficult (if not impossible) to treat the sludgeunless it is pumped to the specific sludge treat-ment process.

TABLE 18.8Typical Water Content of Sludges

Water Treatment Process% Moisture of

Sludge GeneratedWater/lb

Sludge Solids

Primary Sedimentation 95 19Trickling Filter Humus (low rate) 93 13.3 Humus (high rate) 97 32.3Activated sludge 99 99

Source: U.S. Environmental Protection Agency, Operational Manual:Sludge Handling and Conditioning, EPA-430/9–78–002, 1978.

TABLE 18.9Distribution of Water in an Activated Sludge

Water Type % Volume

Free Water 75Floc Water 20Capillary Water 2Particle Water 2.5Solids 0.5Total 100

Source: U.S. Environmental Pro-tection Agency, OperationalManual: Sludge Handling andConditioning, EPA-430/9–78–002, 1978.



18.13.1.3 Sludge Pumping Calculations

While on shift, wastewater operators are often called uponto make various process control calculations. An importantcalculation involves sludge pumping. The sludge pumpingcalculations the operator may be required to make duringplant operations (and should be known for licensure exam-inations) are covered in this section.

18.13.1.3.1 Estimating Daily Sludge Production

The calculation for estimation of the required sludge-pumping rate provides a method to establish an initialpumping rate or to evaluate the adequacy of the currentwithdrawal rate:

EXAMPLE 18.59

Problem:

The sludge withdrawn from the primary settling tank con-tains 1.4% of solids. The unit influent contains 285 mg/LTSS and the effluent contains 140 mg/L TSS. If the influ-ent flow rate is 5.55 MGD, what is the estimated sludgewithdrawal rate in gallons per minute (assuming the pumpoperates continuously)?

Solution:

The following chart is used for Examples 18.60 to18.65.

18.13.1.3.2 Sludge Pumping Time

The sludge pumping time is the total time the pump oper-ates during a 24-h period in minutes:

(18.56)

EXAMPLE 18.60

Problem:

What is the pump operating time?

Solution:

18.13.1.3.3 Sludge Pumped in Gallons per day

Use Equation 18.57 to calculate the amount of sludgepumped in gallons per day:

EXAMPLE 18.61

Problem:

What is the amount of sludge pumped in gallons per day?

Solution:

18.13.1.3.4 Sludge Pumped in Pounds per Day

Use Equation 18.58 to calculate the amount of sludgepumped in pounds per day:

EXAMPLE 18.62

Problem:

What is the amount of sludge pumped in pounds per day?

Solution:

18.13.1.3.5 Solids Pumped in Pounds per Day

Use Equation 18.59 to calculate the amount of sludgepumped in pounds per day:

Operating time 15 min/cFrequency 24 c/dPump rate 120 gal/minSolids 3.7%Volatile matter 66%

Est. Pump Rate

Influent TSS Conc. Effluent TSS Conc. Flow

% Solids in Sludge min d

=

- ¥ ¥¥ ¥

( )

gpm

lb gal

.

.

8 34

8 34 1440

(18.55)

Sludge Rate, gpm min d

gpm

=- ¥ ¥

¥ ¥

=

( )285 140 5 55 8 34

0 014 8 34 1440

40

mg L mg L

lb gal

. .

. .

Pump Operating Time

Time c min Frequency c d

=

( ) ¥ ( )

Pump Operating Time min h c d

min d

= ¥

=

15 24

360

Sludge Pumped gal d Operating Time min d

Pump Rate gal min

( ) = ( ) ¥

( ) (18.57)

Sludge Pumped gal d min d gal min

gal d

( ) = ¥

=

360 120

43 200,

Sludge Pumped lb d Sludge Pumped gal d

lb gal

( ) = ( ) ¥

.8 34 (18.58)

Sludge Pumped lb d gal d lb gal

lb d

( ) = ¥

=

43 200 8 34

360 300

, .

,



EXAMPLE 18.63

Problem:

What is the amount of solid pumped in pounds per day?

Solution:

18.13.1.3.6 Volatile Matter Pumped in Pounds per Day

Use Equation 18.60 to calculate the amount of volatilematter pumped in pounds per day:

(18.60)

EXAMPLE 18.64

Problem:

What is the amount of volatile matter pumped in poundsper day?

Solution:

If you wish to calculate the pounds of solids or thepounds of volatile solids removed per day, the individualequations demonstrated above can be combined into asingle calculation:

EXAMPLE 18.65

Problem:

Use Equation 18.61 to calculate the amount of (1) solidsand (2) volatile matter removed in pounds per day.

Solution:

1. Amount of solids removed in pounds per day:

2. Amount of volatile solids in pounds per day:

18.13.1.3.7 Sludge Production in Pounds per Million Gallons

A common method of expressing sludge production is inpounds of sludge per million gallons of wastewatertreated:

(18.62)

EXAMPLE 18.66

Problem:

Records show that the plant has produced 85,000 gal ofsludge during the past 30 d. The average daily flow forthis period was 1.2 MGD. What was the plant’s sludgeproduction in pounds per million gallons?

Solution:

18.13.1.3.8 Sludge Production in Wet Tons per Year

Sludge production can also be expressed in terms of theamount of sludge (water and solids) produced per year.This is normally expressed in wet tons per year:

Solids Pumped lb d Sludge Pumped lb d

Solids

( ) = ( ) ¥

% (18.59)

Solids Pumped lb d lb d

lb d

( ) = ¥

=

360 300 0 0370

13 331

, .

,

VM lb d Solids Pumped lb d( ) = ( ) ¥ %VM

VM lb d lb d

lb d

( ) = ¥

=

13 331 0 66

8 798

, .

.

Solids lb d Pump Time min c Frequency c d

Rate gal min lb gal

Solids VM lb d

time min c Frequency c d

Rate gal min

%Solids %VM

( ) ( ) ( )

( )

( )

( ) ( )

( )

= ¥ ¥

¥ ¥

= ¥ ¥

¥ ¥

¥

.

%

.

8 34

8 34

(18.61)

Solids lb d min c c d gal min

lb d

( ) = ¥ ¥ ¥

¥

=

15 24 120

8 34 0 0370

13 331

. .

,

VM lb d min c c d gal min

lb d

( ) = ¥ ¥ ¥

¥ ¥

=

15 24 120

8 34 0 0370 0 66

8798

. . .

ST

Tludge lb MG

otal Sludge Production lb

otal Wastewater Flow MG( ) ( )

( )=

SMGD d

ludge lb MG85,000 gal 8.34 lb gal

lb MG

( ) =¥

¥

=

1 2 30

19 692

.

,



EXAMPLE 18.67

Problem:

The plant is currently producing sludge at the rate of16,500 lb/MG. The current average daily wastewater flowrate is 1.5 MGD. What will be the total amount of sludgeproduced per year in wet tons per year?

Solution:

18.13.1.4 Sludge Treatment: An Overview

The release of wastewater solids without proper treatmentcould result in severe damage to the environment. We musthave a system to treat the volume of material removed assludge throughout the system. Release without treatmentwould defeat the purpose of environmental protection. Adesign engineer can choose from many processes whendeveloping sludge treatment systems. No matter what thesystem or combination of systems chosen, the ultimatepurpose will be the same: the conversion of wastewater

sludges into a form that can be handled economically anddisposed of without damaging the environment or creatingnuisance conditions. Leaving either condition unmet willrequire further treatment. The degree of treatment willgenerally depend on the proposed method of disposal.

Sludge treatment processes can be classified into anumber of major categories. In this handbook, we discussthe processes shown in Figure 18.13: thickening, digestion(or stabilization), de-watering, incineration, and landapplication. Each of these categories has then been furthersubdivided according to the specific processes that areused to accomplish sludge treatment.

As mentioned, the importance of adequate, efficientsludge treatment cannot be overlooked when designingwastewater treatment facilities. The inadequacies of a sludgetreatment system can severely affect a plant’s overall perfor-mance capabilities. The inability to remove and process sol-ids as fast as they accumulate in the process can lead to thedischarge of large quantities of solids to receiving waters.

Even with proper design and capabilities in place, nosystem can be effective unless it is properly operated. Properoperation requires proper operator performance. Properoperator performance begins and ends with proper training.

18.13.2 SLUDGE THICKENING

The solids content of primary, activated, trickling-filter, oreven mixed sludge (i.e., primary plus activated sludge)varies considerably, depending on the characteristics ofthe sludge. Note that the sludge removal and pumpingfacilities and the method of operation also affect the solids

Sludge wet tons year

Sludge Production lb MG Average Daily Flow MGD 365 d year

000 lb ton

( )

( ) ( )

=

¥ ¥

2

(18.63)

Sludge wet tons year

16,500 lb MG 1 MGD 365 d year

000 lb ton

wet tons year

( )

=¥ ¥

=

.5

2

4517

FIGURE 18.13 Major solids handling processes. (From Spellman, F.R., Spellman’s Standard Handbook for Wastewater Operators,Vol. 1, Technomic Publ., Lancaster, PA, 1999.)

Solidshandling

Digestion

Land application

Thickening

Dewatering

Incineration



content. Sludge thickening (or concentration) is a unitprocess used to increase the solids content of the sludgeby removing a portion of the liquid fraction. By increasingthe solids content, more economical treatment of thesludge can be effected. Sludge thickening processesinclude:

1. Gravity thickeners2. Flotation thickeners3. Solids concentrators

18.13.2.1 Gravity Thickening

Gravity thickening is most effective on primary sludge. Inoperation, solids are withdrawn from primary treatment(and sometimes secondary treatment) and pumped to thethickener. The solids buildup in the thickener forms asolids blanket on the bottom. The weight of the blanketcompresses the solids on the bottom and “squeezes” thewater out. By adjusting the blanket thickness, the percentof solids in the underflow (solids withdrawn from thebottom of the thickener) can be increased or decreased.The supernatant (clear water) that rises to the surface isreturned to the wastewater flow for treatment.

Daily operations of the thickening process includepumping, observation, sampling and testing, process con-trol calculations, maintenance and housekeeping.

Note: The equipment employed in thickening dependson the specific thickening processes used.

Equipment used for gravity thickening consists of athickening tank, which is similar in design to the settlingtank used in primary treatment. Generally the tank is circularand provides equipment for continuous solids collection.The collector mechanism uses heavier construction thana settling tank’s because the solids being moved are moreconcentrated. The gravity thickener pumping facilities(i.e., pump and flow measurement) are used for with-drawal of thickened solids.

Performance of gravity thickeners (i.e., the solids con-centrations achieved) typically results in producing 8 to10% solids from primary underflow, 2 to 4% solids fromwaste activated sludge, 7 to 9% solids from trickling filterresiduals, and 4 to 9% from combined primary and sec-ondary residuals.

The performance of gravity thickening processesdepends on various factors, including:

1. Type of sludge2. Condition of influent sludge3. Temperature4. Blanket depth5. Solids loading6. Hydraulic loading

7. Solids retention time8. HDT

18.13.2.2 Flotation Thickening

Flotation thickening is used most efficiently for wastesludges from suspended-growth biological treatment pro-cess, such as the activated sludge process. In operation,recycled water from the flotation thickener is aeratedunder pressure. During this time the water absorbs moreair than it would under normal pressure. The recycled flowtogether with chemical additives (if used) is mixed withthe flow. When the mixture enters the flotation thickener,the excess air is released in the form of fine bubbles. Thesebubbles become attached to the solids and lift them towardthe surface. The accumulation of solids on the surface iscalled the float cake. As more solids are added to thebottom of the float cake, it becomes thicker and waterdrains from the upper levels of the cake. The solids arethen moved up an inclined plane by a scraper and dis-charged. The supernatant leaves the tank below the surfaceof the float solids and is recycled or returned to the wast-estream for treatment. Flotation thickener performance istypically 3 to 5% solids for WAS with polymer additionand 2 to 4% solids without polymer addition.

The flotation thickening process requires pressurizedair, a vessel for mixing the air with all or part of the processresidual flow, a tank for the flotation process to occur,solids collector mechanisms to remove the float cake (sol-ids) from the top of the tank, and accumulated heavy solidsfrom the bottom of the tank. Since the process normallyrequires chemicals be added to improve separation, chemicalmixing equipment, storage tanks, and metering equipmentto dispense the chemicals at the desired dose are required.

The performance of dissolved air-thickening processdepends on various factors that include:

1. Bubble size2. Solids loading3. Sludge characteristics4. Chemical selection5. Chemical dose

18.13.2.3 Solids Concentrators

Solids concentrators (belt thickeners) usually consist of amixing tank, chemical storage and metering equipment,and a moving porous belt. In operation, the process resid-ual flow is chemically treated and then spread evenly overthe surface of the moving porous belt. As the flow iscarried down the belt (similar to a conveyor belt) the solidsare mechanically turned or agitated and water drainsthrough the belt. This process is primarily used in facilitieswhere space is limited.




As with other unit treatment processes, proper operationof sludge thickeners depends on operator observation. Theoperator must make routine adjustment of sludge additionand withdrawal rates to achieve desired blanket thickness.Sampling and analysis of influent sludge, supernatant, andthickened sludge are also required. If possible, sludgeaddition and withdrawal should be continuous to achieveoptimum performance. Mechanical maintenance is alsorequired.

Expected performance ranges for gravity and dis-solved air flotation thickeners are listed below:

1. Primary sludge — 8 to 19% solids2. WAS — 2 to 4% solids3. Trickling filter sludge — 7 to 9% solids4. Combined sludges — 4 to 9% solids

Typical operational problems with sludge thickenersinclude odors, rising sludge, thickened sludge belowdesired solids concentration, a dissolved air concentrationthat is too low, an effluent flow containing excessive solids,and torque alarm conditions.

Gravity Thickener

1. Odors and rising sludge.A. Causal factors:

1. Sludge withdrawal rate is too low.2. Overflow rate is too low.3. Septicity in the thickener.

B. Corrective actions (where applicable):1. Increase sludge withdrawal rate.2. Increase influent flow rate.3. Add chlorine, permanganate, or peroxide

to influent.2. Thickened sludge is below desired solids con-

centration.A. Causal factors:

1. Overflow rate is too high.2. Sludge withdrawal rate is too high.3. Short-circuiting.

B. Corrective actions (where applicable):1. Decrease influent sludge flow rate.2. Decrease pump rate for sludge withdrawal.3. Identify cause and correct.

3. Torque alarm is activated.A. Causal factors:

1. Heavy sludge accumulation.2. Collector mechanism is jammed.

B. Corrective actions (where applicable):1. Agitate sludge blanket to decrease density.2. Increase sludge withdrawal rate.3. Attempt to locate and remove obstacle.4. Dewater tank and remove obstacle.

Dissolved Air Flotation Thickener

1. Float solids concentration is too low.A. Causal factors:

1. Skimmer speed is too high.2. Unit is overloaded.3. Insufficient polymer dose.4. Excessive air-to-solids ratio.5. Low dissolved air levels.

B. Corrective actions (where applicable):1. Adjust skimmer speed to permit concen-

tration to occur.2. Stop sludge flow through unit or purge with

recycles flow.3. Determine proper chemical dose and

adjust.4. Reduce airflow to pressurization tank.5. Identify malfunction and correct.

2. Dissolved air concentration is too low.A. Causal factor:

1. Mechanical malfunction.B. Corrective action:

1. Identify cause and correct.3. Effluent (subnatant) flow contains excessive

solids.A. Causal factors:

1. Unit is overloaded.2. Chemical dose is too low.3. Skimmer is not operating.4. Low air-to-solids ratio.5. Solids buildup in thickener.

B. Corrective actions (where applicable):1. Turn off sludge flow.2. Purge unit with recycle.3. Determine proper chemical dose and blow.4. Turn skimmer on.5. Adjust skimmer speed.6. Increase airflow to pressurization system.7. Remove sludge from tank.

18.13.2.3.2 Process Calculations (Gravity and Dissolved Air Flotation)

Sludge thickening calculations are based on the conceptthat the solids in the primary or secondary sludge are equalto the solids in the thickened sludge. Assuming a negligi-ble amount of solids are lost in the thickener overflow, thesolids are the same. Note that the water is removed tothicken the sludge and results in higher percent solids.

18.13.2.3.2.1 Estimating Daily Sludge Production

Equation 18.76 provides a method to establish an initialpumping rate or to evaluate the adequacy of the currentpump rate:


616


(18.64)

EXAMPLE 18.68

Problem:

The sludge withdrawn from the primary settling tank con-tains 1.5% of solids. The unit influent contains 280 mg/LTSS, and the effluent contains 141 mg/L. If the influentflow rate is 5.55 MGD, what is the estimated sludgewithdrawal rate in gallons per minute (assuming the pumpoperates continuously)?

Solution:

18.13.2.3.2.2 Surface Loading Rate

Surface loading rate (surface settling rate) is hydraulicloading — the amount of sludge applied per square footof gravity thickener:

(18.65)

EXAMPLE 18.69

Problem:

The 70-ft diameter gravity thickener receives 32,000 gal/dof sludge. What is the surface loading in gallons persquare foot per day?

Solution:

18.13.2.3.2.3 Solids Loading Rate

The solids loading rate is the pounds of solids per daybeing applied to 1 ft2 of tank surface area. The calculationuses the surface area of the bottom of the tank. It assumesthe floor of the tank is flat and has the same dimensionsas the surface.

EXAMPLE 18.70

Problem:

The thickener influent contains 1.6% of solids. The influ-

ent flow rate is 39,000 gal/d. The thickener is 50 ft in

diameter and 10 ft deep. What is the solid loading in

pounds per day?

Solution:

18.13.2.3.2.4 Concentration Factor

The concentration factor (CF) represents the increase inconcentration resulting from the thickener:

(18.67)

EXAMPLE 18.71

Problem:

The influent sludge contains 3.5% solids. The thickened

sludge solids concentration is 7.7%. What is the concen-

tration factor?

Solution:

18.13.2.3.2.5 Air-to-Solids Ratio

The air-to-solids ratio is the ratio between the pounds ofsolids entering the thickener and the pounds of air beingapplied:

Surf. Loading, gal d ft

Sludge Applied to the Thickener, gpdThickener Area, ft

2

2

=

Surface Loading

gpd ft2

=¥ ¥

=

32 000

0 785 70 70

8 32

,

.

.

gpd

ft ft


Sludge Applied to the Thickener gal d

Thickener Area ft

2

2

( ) =

( )( )


32,000 gal d

0.785 70 ft 70 ft

gal d ft

2

2

( )

=¥ ¥

= 8 32.

Solids Loading Rate lb d ft

%Sludge Solids Sludge Flow gal d 8.34 lb gal

Thickener Area ft

2

2

( ) =

¥ ( ) ¥( )

(18.66)

Solids Loading Rate

0.016 39,000 gal d 8.34 lb gal

0.785 50 ft 50 ft

lb d ft2

lb d ft2

2 7

( )

=¥ ¥

¥ ¥

= .

CFThickened Sludge Concentration %

Influent Sludge Concentration %= ( )

( )

CF 7.7%

.5% = =

32 2.

Air:Solids Ratio

Air Flow ft 0 lb ft

Sludge Flow gal min %Solids 8.34 lb gal

3 3

=

( ) ¥ ( )( ) ¥ ¥

min .075

(18.68)



EXAMPLE 18.72

Problem:

The sludge pumped to the thickener is 0.85% solids. Theairflow is 13 ft3/min. What is the air-to-solids ratio if thecurrent sludge flow rate entering the unit is 50 gal/min?

Solution:

18.13.2.3.2.6 Recycle Flow in PercentThe amount of recycle flow expressed as a percent:

(18.69)

EXAMPLE 18.73

Problem:

The sludge flow to the thickener is 80 gal/min. The recycleflow rate is 140 gal/min. What is the recycle flow?

Solution:

18.13.3 SLUDGE STABILIZATION

The purpose of sludge stabilization is to reduce volume,stabilize the organic matter, and eliminate pathogenicorganisms to permit reuse or disposal. The equipmentrequired for stabilization depends on the specific processused. Sludge stabilization processes include:

1. Aerobic digestion2. Anaerobic digestion3. Composting4. Lime stabilization5. Wet air oxidation (heat treatment)6. Chemical oxidation (chlorine oxidation)7. Incineration

18.13.3.1 Aerobic Digestion

Equipment used for aerobic digestion consists of an aer-ation tank (digester) which is similar in design to theaeration tank used for the activated sludge process. Either

diffused or mechanical aeration equipment is necessary tomaintain the aerobic conditions in the tank. Solids andsupernatant removal equipment is also required.

In operation, process residuals (sludge) are added tothe digester and aerated to maintain a DO concentrationof 1.0 mg/L. Aeration also ensures that the tank contentsare well mixed. Generally, aeration continues for approx-imately 20 d retention time. Aeration is periodicallystopped and the solids are allowed to settle. Sludge andthe clear liquid supernatant are withdrawn as needed toprovide more room in the digester. When no additionalvolume is available, mixing is stopped for 12 to 24 hbefore solids are withdrawn for disposal. Process controltesting should include alkalinity, pH, percent solids, per-cent volatile solids for influent sludge, supernatant,digested sludge, and digester contents.

Normal operating levels for an aerobic digester arelisted in Table 18.10.

A typical operational problem associated with an aer-obic digester is pH control. For example, when pH drops,this may indicate normal biological activity or low influentalkalinity. This problem is corrected by adding alkalinity(lime, bicarbonate, etc.).

18.13.3.1.1 Process Control Calculations: Aerobic Digester

Wastewater operators (who operate aerobic digesters) arerequired to make certain process control calculations.Moreover, licensing examinations typically include aerobicdigester problems for determining volatile solids loading,digestion time, digester efficiency, and pH adjustment.These process control calculations are explained in thefollowing sections:

18.13.3.1.1.1 Volatile Solids Loading

Volatile solids loading for the aerobic digester is expressedin pounds of volatile solids entering the digester per dayper cubic foot of digester capacity:

Air:Solids Ratio

13 ft 0 lb ft

50 gal min 0 8.34 lb gal

3 3

=¥

¥ ¥

=

min .

.

.

075

0085

0 28

Recycle Flow %

Recycle Flow Rate gal min 100

Sludge Flow gal min

( ) =

( ) ¥( )

Recycle Flow %140 gal min 100

80 gal min175%( ) =

¥=

TABLE 18.10Aerobic Digester Normal Operating Levels

Parameter Normal Levels

Detention time (d) 10–20Volatile solids loading 0.1–0.3lb/ft3/dDO (mg/L) 1.0pH 5.9–7.7Volatile Solids 40–50%Reduction

Source: Spellman, F.R., Spellman’s Stan-dard Handbook for Wastewater Operators,Vol. 1, Technomic Publ., Lancaster, PA, 1999.



(18.70)

EXAMPLE 18.74

Problem:

The aerobic digester is 25 ft in diameter and has an oper-

ating depth of 24 ft. The sludge added to the digester daily

contains 1350 lb of volatile solids. What is the volatile

solids loading in pounds per day per cubic foot?

Solution:

18.13.3.1.1.2 Digestion Time

Digestion time is the theoretical time the sludge remainsin the aerobic digester:

(18.71)

EXAMPLE 18.75

Problem:

Digester volume is 240,000 gal. Sludge is being added to

the digester at the rate of 13,500 gal/d. What is the diges-

tion time in days?

Solution:

18.13.3.1.1.3 Digester Efficiency

To determine digester efficiency or the percentage ofreduction, a two-step procedure is required. The percentvolatile matter reduction must first be calculated and thenthe percent moisture reduction:

1. Percent volatile matter reduction —Because ofthe changes occurring during sludge digestion,

the calculation used to determine percent vola-tile matter reduction is more complicated:

EXAMPLE 18.76

Problem:


Data:

Raw sludge volatile matter = 71%

Digested sludge volatile matter = 53%

2. Moisture reduction — Use Equation 18.73 tocalculate percent moisture reduction:

EXAMPLE 18.77

Problem:

Using the digester data provided below, determine the%moisture reduction for the digester (Note: %Moisture =100% – Percent Solids):

Solution:

Volatile Solids Loading lb d ft

Volatile Solids Added lb d

Digester Volume ft

3

3

( ) =

( )( )

Volatile Solids Loading lb d ft

1350 lb d

ft 24 ft

lb d ft

3

3

( )

=¥ ¥ ¥

=

0 785 25 25

0 11

.

.

ft

Digestion Time dDigester Volume gal

Sludge Added gal d( ) = ( )

( )

Digestion Time d240,000 gal

3,500 gal d d( ) ( )= =

117 8.

Raw Sludge %Solids 6%%Moisture 94% (100% – 6%)

Digested Sludge %Solids 15%%Moisture 85% (100% – 15%)

% Matter Reduction

% Volatile Matterin % Volatile Matterout

% Vol. Matter % Vol. Matter % Vol. Matterin in out

=

- ¥

- ¥

( )( )[ ]

100

(18.72)

%VM Reduction.53

.71 .53

or 54%

=- ¥

- ¥

=

[ ]( )[ ]

0 71 0 100

0 71 0 0

53 9

.

.

. %

%Moisture Reduction

%Moisture %Moisture

M M Min out

in in out

=

-[ ] ¥

- ¥( )[ ] % % %

100

oisture oisture oisture

(18.73)

%Moisture Reduction.85

.94 .85

4%

=- ¥

- ¥

=

[ ]( )[ ]

0 94 0 100

0 94 0 0

6

.

.



18.13.3.1.1.4 pH AdjustmentOccasionally, the pH of the aerobic digester will fall belowthe levels required for good biological activity. When thisoccurs, the operator must perform a laboratory test todetermine the amount of alkalinity required to raise thepH to the desired level. The results of the lab test mustthen be converted to the actual quantity of chemical (usu-ally lime) required by the digester:

EXAMPLE 18.78

Problem:

The lab reports that it took 225 mg of lime to increasepH of a 1-L sample of the aerobic digester contents topH 7.2. The digester volume is 240,000 gal. How manypounds of lime will be required to increase the digesterpH to 7.2?

Solution:

18.13.3.2 Anaerobic Digestion

Anaerobic digestion is the traditional method of sludgestabilization. It involves using bacteria that thrive in the

absence of oxygen and is slower than aerobic digestion.The advantage of anaerobic digestion is that only a smallpercentage of the wastes are converted into new bacterialcells. Most of the organics are converted into carbon diox-ide and methane gas.

Note: In an anaerobic digester, the entrance of airshould be prevented because of the potential forair mixed with the gas produced in the digesterthat could create an explosive mixture.

Equipment used in anaerobic digestion includes asealed digestion tank with either a fixed or a floating cover(see Figure 18.14), heating and mixing equipment, gasstorage tanks, solids and supernatant withdrawal equip-ment, and safety equipment (e.g., vacuum relief, pressurerelief, flame traps, explosion proof electrical equipment).

In operation, process residual (thickened or unthick-ened sludge) is pumped into the sealed digester. Theorganic matter digests anaerobically by a two-stage pro-cess. Sugars, starches, and carbohydrates are converted tovolatile acids, carbon dioxide, and hydrogen sulfide. Thevolatile acids are then converted to methane gas. Thisoperation can occur in a single tank (single stage) or intwo tanks (two stages). In a single-stage system, superna-tant and digested solids must be removed whenever flowis added. In a two-stage operation, solids and liquids fromthe first stage flow into the second stage each time freshsolids are added. Supernatant is withdrawn from the sec-ond stage to provide additional treatment space. Solids areperiodically withdrawn for dewatering or disposal. Themethane gas produced in the process may be used formany plant activities.

Note: The primary purpose of a secondary digester isto allow for solids separation.

Chem. Required, lbsChemical Used in Lab Test, mg

Sample Volume, Liters

Dig. Vol, MG

=

¥ ¥ .8 34 lb gal(18.74)

Chemical Required

mg 240,000 gal 3.785 L gal

L 454 g 1000 mg g

lb

=¥ ¥

¥ ¥

=

225

1

450

FIGURE 18.14 Floating cover anaerobic digester. (From Spellman, F.R., Spellman’s Standard Handbook for Wastewater Operators,Vol. 1, Technomic Publ., Lancaster, PA, 1999.)

Gas dome

Stabilizedthickened biosolids

Digestingbiosolids

Supernatant

Scum layer

Floatingcover

Gas

Biosolids inlet

Recirc to heater

Digested biosolids

Supernatant



Various performance factors affect the operation ofthe anaerobic digester. For example, percent volatile mat-ter in raw sludge, digester temperature, mixing, volatileacids-alkalinity ratio, feed rate, percent solids in rawsludge and pH are all important operational parametersthat the operator must monitor.

Along with being able to recognize normal and abnor-mal anaerobic digester performance parameters, waste-water operators must also know and understand normaloperating procedures. Normal operating proceduresinclude sludge additions, supernatant withdrawal, sludgewithdrawal, pH control, temperature control, mixing, andsafety requirements. Important performance parametersare listed in Table 18.11.

18.13.3.2.1 Sludge Additions

Sludge must be pumped (in small amounts) several timeseach day to achieve the desired organic loading and opti-mum performance.

Note: Keep in mind that in fixed cover operationsadditions must be balanced by withdrawals. Ifnot, structural damage occurs.

18.13.3.2.2 Supernatant Withdrawal

Supernatant withdrawal must be controlled for maximumsludge retention time. When sampling, sample all drawoffpoints and select level with the best quality.

18.13.3.2.3 Sludge Withdrawal

Digested sludge is withdrawn only when necessary.Always leave at least 25% seed.

18.13.3.2.4 pH Control

pH should be adjusted to maintain 6.8 to 7.2 pH by adjust-ing feed rate, sludge withdrawal, or alkalinity additions.

Note: The buffer capacity of an anaerobic digester isindicated by the volatile acid/alkalinity relation-ship. Decreases in alkalinity cause a correspond-ing increase in ratio.

18.13.3.2.5 Temperature ControlIf the digester is heated, the temperature must be con-trolled to a normal temperature range of 90 to 95∞F. Neveradjust the temperature by more than 1∞F per day.

18.13.3.2.6 MixingIf the digester is equipped with mixers, mixing should beaccomplished to ensure organisms are exposed to foodmaterials.

18.13.3.2.7 SafetyAnaerobic digesters are inherently dangerous; several cat-astrophic failures have been recorded. To prevent suchfailures, safety equipment such as pressure relief and vac-uum relief valves, flame traps, condensate traps, and gascollection safety devices are installed. It is important thatthese critical safety devices be checked and maintainedfor proper operation.

Note: Because of the inherent danger involved withworking inside anaerobic digesters, they areautomatically classified as permit-required con-fined spaces. All operations involving internalentry must be made in accordance with OSHA’sconfined space entry standard.

18.13.3.2.8 Process Control Monitoring, Testing, and Troubleshooting

During operation, anaerobic digesters must be monitoredand tested to ensure proper operation. Testing should beaccomplished to determine supernatant pH, volatile acids,alkalinity, BOD or COD, total solids and temperature.Sludge (in and out) should be routinely tested for percentsolids and percent volatile matter. Normal operatingparameters are listed in Table 18.12.

18.13.3.2.9 Anaerobic Digester: TroubleshootingAs with all other unit processes, the wastewater operatoris expected to recognize problematic symptoms withanaerobic digesters and effect the appropriate correctiveactions. Symptoms, causes, and corrective actions are dis-cussed below.

1. Symptom 1: Digester gas production is reduced,pH drops below 6.8, and volatile acids-alkalin-ity ratio increases.A. Causes:

1. Digester souring.2. Organic overloading.3. Inadequate mixing.4. Low alkalinity.5. Hydraulic overloading.6. Toxicity.7. Loss of digestion capacity.

B. Corrective actions:1. Add alkalinity (digested sludge, lime, etc.).2. Improve temperature control.

TABLE 18.11Anaerobic Digester — Sludge Parameters

Raw Sludge Solids Impact

<4% Solids Loss of alkalinity; decreased SRT; increased heating requirements; decreased volatile acids-alkalinity ratio

4–8% Solids Normal operation>8% Solids Poor mixing; organic overloading; decreased

volatile acids-alkalinity ratio

Source: Spellman, F.R., Spellman’s Standard Handbook for WastewaterOperators, Vol. 1, Technomic Publ., Lancaster, PA, 1999.



3. Improve mixing.4. Eliminate toxicity.5. Clean digester.

2. Symptom 2: Gray foam oozing from digester.A. Cause:

1. Rapid gasification.2. Foam producing organisms present.3. Foam producing chemical present.

B. Corrective actions:1. Reduce mixing.2. Reduce feed rate.3. Mix slowly by hand.4. Clean all contaminated equipment.

18.13.3.2.10 Anaerobic Digester: Process Control Calculations

Process control calculations involved with anaerobicdigester operation include determining the required seedvolume, volatile acid-alkalinity ratio, SRT, estimated gasproduction, volatile matter reduction, and percent mois-ture reduction in digester sludge. Examples on how tomake these calculations are provided in the followingsections.

18.13.3.2.10.1 Required Seed Volume in Gallons

Use Equation 18.75 to calculate the require seed volumein gallons:

(18.75)

EXAMPLE 18.79

Problem:

The new digester requires a 25% seed to achieve normaloperation within the allotted time. If the digester volumeis 266,000 gal, how many gallons of seed material willbe required?

Solution:

18.13.3.2.10.2 Volatile Acids-Alkalinity RatioThe volatile acids-alkalinity ratio can be used to controloperation of an anaerobic digester:

(18.76)

EXAMPLE 18.80

Problem:

The digester contains 240 mg/L volatile acids and 1860mg/L alkalinity. What is the volatile acids-alkalinity ratio?

Increases in the ratio normally indicate a potentialchange in the operation condition of the digester as shownin Table 18.13.

18.13.3.2.10.3 Sludge Retention TimeSRT is the length of time the sludge remains in thedigester:

(18.77)

TABLE 18.12 Anaerobic Digester: Normal Operating Ranges

Parameter Normal Range

Sludge retention timeHeated 30–60 dUnheated 180+ d

Volatile solids loading 0.04–0.1 lb VM/day/ft3

Operating temperatureHeated 90–95∞FUnheated Varies with season

MixingHeated (primary) YesUnheated (secondary) No%Methane in gas 60– 72%%Carbon dioxide in gas 28–40%pH 6.8–7.2Volatile acids-alkalinity ratio £0.1Volatile solids reduction 40–60%Moisture reduction 40–60%

Source: Spellman, F.R., Spellman’s Standard Handbook for Waste-water Operators, Vol. 1, Technomic Publ., Lancaster, PA, 1999.

Seed Volume gal Digester Volume %Seed( ) = ¥

TABLE 18.13

Operating Condition Volatile Acids-Alkalinity Ratio

Optimum £0.1Acceptable range 0.1–0.3%Carbon dioxide in gas increases ≥0.5pH decreases ≥0.8

Source: Spellman, F.R., Spellman’s Standard Handbook for WastewaterOperators, Vol. 1, Technomic Publ., Lancaster, PA, 1999.

Seed Volume gal

gal

( ) = ¥

=

266 000 0 25

66 500

, .

,

Acids:Alkalinity Ratio

Volatile Acids ConcentrationAlkalinity Concentration

=

Acids Alkalinity Ratio: . 240 mg L

860 mg L= =

10 13

SRT d

Digester Volume gal

Sludge Volume Added per Day gal d

( ) =

( )( )



EXAMPLE 18.81

Problem:

Sludge is added to a 525,000-gal digester at the rate of12,250 gal/d. What is the sludge retention time?

Solution:

18.13.3.2.10.4 Estimated Gas Production

The rate of gas production is normally expressed as thevolume of gas (ft3) produced per pound of volatile matterdestroyed. The total cubic feet of gas a digester will pro-duce per day can be calculated by:

(18.78)

EXAMPLE 18.82

Problem:

The digester receives 11,450 lb of volatile matter per day.The volatile matter reduction achieved by the digester is52%. The rate of gas production is 11.2 ft3 of gas perpound of volatile matter destroyed. What is the estimatedgas production per day?

Solution:

18.13.3.2.10.5 Percent Volatile Matter Reduction

Because of the changes occurring during sludge digestion,the calculation used to determine percent volatile matterreduction is more complicated:

EXAMPLE 18.83

Problem:


Data:

Raw sludge volatile matter = 74%Digested sludge volatile matter = 55%

18.13.3.2.10.6 Percent Moisture Reduction in Digested Sludge

Use Equation 18.92 to calculate the percent moisturereduction in digested sludge:

EXAMPLE 18.84

Problem:

Using the digester data provided below, determine thepercent moisture reduction and percent volatile matterreduction for the digester (Note: %Moisture = 100% –Percent Solids):

Solution:

Raw sludge %solids = 6%Digested sludge %solids = 14%

18.13.3.3 Other Sludge Stabilization Processes

Along the aerobic and anaerobic digestion, other sludgestabilization processes include composting, lime stabili-zation, wet air oxidation, and chemical (chlorine) oxida-tion. These other stabilization processes are brieflydescribed in this section.

18.13.3.3.1 Composting

The purpose of composting sludge is to stabilize theorganic matter, reduce volume, and eliminate pathogenicorganisms. In a composting operation, dewatered solidsare usually mixed with a bulking agent (i.e., hardwoodchips) and stored until biological stabilization occurs. Thecomposting mixture is ventilated during storage to providesufficient oxygen for oxidation and to prevent odors. Afterthe solids are stabilized, they are separated from the bulk-ing agent. The composted solids are then stored for curingand applied to farmlands or other beneficial uses.Expected performance of the composting operation for

SRT d525,000 gal

2,250 gal d d( ) = =

142 9.

Gas Production ft VM

%VM Reduction Production Rate ft

3in

3

d lb d

lb

( ) = ( ) ¥

¥ ( )

Gas Production ft lb d

ft

ft3

3

3

d

lb

d

( ) = ¥ ¥

=

11 450 0 52

11 2

66 685

, .

.

,

% Reduction

% Volatile Matter Volatile Matter

% Volatile Matter Volatile Matter % Volatile Matterin out

in in out

=

- ¥

- ¥

( )( )[ ]

%

%

100

(18.79)

%VM Reduction.55

.74 .55%=

- ¥- ¥

=[ ]( )[ ]

0 74 0 100

0 74 0 057

.

.

%Moisture Reduction

Moisture Moisture

M M Min out

in in out

=

-[ ] ¥

- ¥( )[ ] % %

% % %

100

oisture oisture oisture

(18.80)

%Moisture Reduction.86

.94 .86%=

- ¥- ¥

=[ ]( )[ ]

0 94 0 100

0 94 0 061

.

.



both percent volatile matter reduction and percent mois-ture reduction ranges from 40 to 60%+.

18.13.3.3.2 Lime StabilizationIn lime stabilization, process residuals are mixed with limeto achieve a pH of 12. This pH is maintained for at least2 h. The treated solids can then be dewatered for disposalor directly land applied.

18.13.3.3.3 Thermal TreatmentThermal treatment (or wet air oxidation) subjects sludgeto high temperature and pressure in a closed reactor vessel.The high temperature and pressure rupture the cell wallsof any microorganisms present in the solids and causeschemical oxidation of the organic matter. This processsubstantially improves dewatering and reduces the volumeof material for disposal. It also produces a very highstrength waste, which must be returned to the wastewatertreatment system for further treatment.

18.13.3.3.4 Chlorine OxidationChlorine oxidation also occurs in a closed vessel. In thisprocess, chlorine (100 to 1000 mg/L) is mixed with arecycled solids flow. The recycled flow and process residualflow are mixed in the reactor. The solids and water areseparated after leaving the reactor vessel. The water isreturned to the wastewater treatment system and the treatedsolids are dewatered for disposal. The main advantage ofchlorine oxidation is that it can be operated intermittently.The main disadvantage is production of extremely low pHand high chlorine content in the supernatant.

18.13.3.3.5 Stabilization: Operation and Performance

Depending on the stabilization process employed, theoperational components vary. In general, operationsinclude pumping, observations, sampling and testing, pro-cess control calculations, maintenance, and housekeeping.Performance of the stabilization process will also varywith the type of process used. Stabilization processes cangenerally produce 40 to 60% reduction of both volatilematter (organic content) and moisture.

18.13.4 SLUDGE DEWATERING

Digested sludge removed from the digester is still mostlyliquid. Sludge dewatering is used to reduce volume byremoving the water to permit easy handling and econom-ical reuse or disposal. Dewatering processes include sanddrying beds, vacuum filters, centrifuges, filter presses (beltand plate), and incineration.

18.13.4.1 Sand Drying Beds

Drying beds have been used successfully for years todewater sludge. Composed of a sand bed (consisting of agravel base, underdrains, and 8 to 12 in. of filter grade

sand), drying beds include an inlet pipe, splash pad con-tainment walls, and a system to return filtrate (water) fortreatment. In some cases, the sand beds are covered toprovide drying solids protection from the elements.

In operation, solids are pumped to the sand bed andallowed to dry by first draining off excess water throughthe sand and then by evaporation. This is the simplest andcheapest method for dewatering sludge. No special train-ing or expertise is required. There is a downside; dryingbeds require a great deal of manpower to clean beds, theycan create odor and insect problems, and they can causesludge buildup during inclement weather.

18.13.4.1.1 Performance FactorsIn sludge drying beds, various factors affect the length oftime required to achieve the desired solids concentrations.The major factors and their impact on drying bed perfor-mance include the following:

1. Climate — Drying beds in cold or moist cli-mates will require significantly longer dryingtime to achieve an adequate level of percentsolids concentrations in the dewatered sludge.

2. Depth of applied sludge — The depth of thesludge drawn onto the bed has a major impacton the required drying time. Deeper sludge lay-ers require longer drying times. Under idealconditions, a well-digested sludge drawn to adepth of approximately 8 in. will requireapproximately 3 weeks to reach the desired 40 to60% solids.

3. Type of sludge applied — The quality and sol-ids concentration of the drying media will affectthe time requirements.

4. Bed cover — Covered-drying beds preventrewetting of the sludge during storm events. Inmost cases, this reduces the average drying timerequired to reach the desired solids levels.


Although drying beds involve two natural processes —drainage and evaporation — that normally work wellenough on their own, a certain amount of preparation andoperator attention is still required to maintain optimumdrying performance. For example, in the preparation stage,all debris is removed from the raked and leveled mediasurface. Then, all openings to the bed are sealed.

After the bed is properly prepared, the sludge linesare opened, and sludge is allowed to flow slowly onto themedia. The bed is filled to desired operating level (8 to12 in.). The sludge line is closed and flushed, and the beddrain is opened. Water begins to drain. The sludge remainson the media until the desired percent solids (40 to 60%) isachieved. Later, the sludge is removed. In most operations,



manual removal is required to prevent damage to theunderdrain system. The sludge is disposed of in an approvedlandfill or by land application as a soil conditioner.

18.13.4.1.3 Operational ProblemsIn the operation of a sludge drying bed, the operatorobserves the operations and looks for various indicatorsof operational problems and makes process adjustmentsas required:

1. Sludge takes a long time to dewater.A. Causal factors:

1. Applied sludge is too deep.2. Sludge was applied to a dirty bed.3. The drain system is plugged or broken.4. Insufficient design capacity.5. Inclement weather or poor drying condi-

tions.B. Corrective actions (where applicable):

1. Allow bed to dry to minimum acceptable% solids and remove.

2. Use described procedure below to deter-mine appropriate sludge depth.a. Clean bed and apply smaller depth of

sludge (i.e., 6 to 8 in.).b. Measure the decrease in depth (draw-

down) at the end of 3 days of drying.c. Use a sludge depth equal to twice the

3-d drawdown depth for future appli-cations.

3. After sludge has dried, remove sludgeand 0.5 to 1.0 in. of sand. Add clean sand.

4. Allow sludge to dry to minimum allow-able percent solids and remove.

5. Use external water source (with back flowprevention) to slowly flush underdrains.

6. Repair or replace underdrains as required.7. Prevent damage to underdrains by drain-

ing during freezing weather.8. Use polymer to increase bed perfor-

mance.9. Cover or enclose the beds.

2. Influent sludge is very thin.A. Causal factor:

1. Coning is occurring in the digester.B. Corrective action:

1. Reduce rate of sludge withdrawal.3. Sludge feed lines plug frequently.

A. Causal factor:1. Solids or grit is accumulating in the line.

B. Corrective actions:1. Open lines fully at the start of each with-

drawal cycle.2. Flush lines at the end of each withdrawal

cycle.

4. Flies breeding in the drying sludge.A. Causal factors:

1. Inadequately digested sludge.2. Natural insect reproduction.

B. Corrective actions:1. Break sludge crust and apply a larvicide

(borax).2. Use insecticide (if approved) to remove

adult insects.3. Remove sludge as soon as possible.

5. Objectionable odors are present when sludge isapplied to bed.A. Causal factor:

1. Raw or partially digested sludge is beingapplied to the bed.

B. Corrective actions:1. Add lime to the sludge to control odors

and potential insect and rodent problems.2. Remove the sludge as quickly as possible.3. Identify and correct the digester problem.

18.13.4.2 Rotary Vacuum Filtration

Rotary vacuum filters have also been used for many yearsto dewater sludge. The vacuum filter includes filter media(belt, cloth or metal coils), media support (drum), vacuumsystem, chemical feed equipment, and conveyor belts totransport the dewatered solids.

In operation, chemically treated solids are pumped toa vat or tank in which a rotating drum is submerged. Asthe drum rotates, a vacuum is applied to the drum. Solidscollect on the media and are held there by the vacuum asthe drum rotates out of the tank. The vacuum removesadditional water from the captured solids. When solidsreach the discharge zone, the vacuum is released and thedewatered solids are discharged onto a conveyor belt fordisposal. The media are then washed prior to returning tothe start of the cycle.

18.13.4.2.1 Types of Rotary Vacuum FiltersThe three principal types of rotary vacuum filters arerotary drum, coil, and belt.

The rotary drum filter consists of a cylindrical drumrotating partially submerged in a vat or pan of conditionedsludge. The drum is divided length-wise into a number ofsections that are connected through internal piping to portsin the valve body (plant) at the hub. This plate rotates incontact with a fixed valve plate with similar parts that areconnected to a vacuum supply, a compressed air supply,and an atmosphere vent. As the drum rotates, each sectionis connected to the appropriate service.

The coil type vacuum filter uses two layers of stainlesssteel coils arranged in corduroy fashion around the drum.After a dewatering cycle, the two layers of springs leavethe drum bed and are separated from each other so that



the cake is lifted off the lower layer and is dischargedfrom the upper layer. The coils are then washed and reap-plied to the drum. The coil filter is used successfully forall types of sludges; sludges that have extremely fine par-ticles or are resistant to flocculation de-water poorly withthis system.

The media on a belt filter leave the drum surface atthe end of the drying zone and passes over a small diam-eter discharge roll to aid cake discharge. Washing of themedia occurs next. Then the media are returned to thedrum and to the vat for another cycle. This type of filternormally has a small-diameter curved bar between thepoint where the belt leaves the drum and the dischargeroll. This bar primarily aids in maintaining belt dimen-sional stability.

18.13.4.2.1.1 Filter MediaDrum and belt vacuum filters use natural or synthetic fibermaterials. On the drum filter, the cloth is stretched andsecured to the surface of the drum. In the belt filter, thecloth is stretched over the drum and through the pulleysystem. The installation of a blanket requires several days.The cloth will (with proper care) last several hundred toseveral thousand hours. The life of the blanket dependson the cloth selected, the conditioning chemical, backwashfrequency, and cleaning (i.e., acid bath) frequency.

18.13.4.2.1.2 Filter DrumThe filter drum is a maze of pipe work running from ametal screen and wooden skeleton and connecting to arotating valve port at each end of the drum. The drum isequipped with a variable speed drive to turn the drum from1/8 to 1 r/min. Normally, solids pickup is indirectly relatedto the drum speed. The drum is partially submerged in avat containing the conditioned sludge. Normally, submer-gence is limited to 1/5 or less of filter surface at a time.

18.13.4.2.1.3 Chemical ConditioningSludges that are dewatered using vacuum filtration arenormally chemically conditioned just prior to filtration.Sludge conditioning increases the percentage of solidscaptured by the filter and improves the de-watering char-acteristics of the sludge. Conditional sludge must be fil-tered as quickly as possible after chemical addition toobtain these desirable results.


In operation, the rotating drum picks up chemically treatedsludge. A vacuum is applied to the inside of the drum todraw the sludge onto the outside of the drum cover. Thisporous outside cover or filter medium allows the filtrateor liquid to pass through into the drum and the filter cake(dewatered sludge) to stay on the medium. In the cakerelease or discharge mode, slight air pressure is appliedto the drum interior. Dewatered solids are lifted from themedium and scraped off by a scraper blade. Solids drop

onto a conveyor for transport for further treatment or dis-posal. The filtrate water is returned to the plant for treatment.

While in operation, the operator observes drum speed,sludge pickup, filter cake thickness and appearance, chem-ical feed rates, sludge depth in vat, and overall equipmentoperation. Sampling and testing are routinely performedon influent sludge solids concentration, filtrate BOD andsolids, and sludge cake solids concentration. We cover theindicators and observations of vacuum filter operationalproblems and causal factors, along with recommendedcorrective actions in the following list:

1. High solids in filtrate.A. Causal factors:

1. Improper coagulant dosage.2. Filter media binding.

B. Corrective actions (where applicable):1. Adjust coagulant dosage.2. Recalibrate coagulant feeder.3. Clean synthetic cloth with steam and

detergent.4. Clean steel coil with acid bath.5. Clean cloth with water or replace cloth.

2. Thin filter cake and poor dewatering.A. Causal factors:

1. Filter media binding.2. Improper chemical dosage.3. Inadequate vacuum.4. Drum speed is too high.5. Drum submergence is too low.

B. Corrective actions (where applicable):1. Clean synthetic cloth with steam and

detergent.2. Clean steel cloth with acid bath.3. Clean cloth with water or replace cloth.4. Adjust coagulant dosage.5. Recalibrate coagulant feeder.6. Repair vacuum system.7. Reduce drum speed.8. Increase drum submergence.

3. Vacuum pump stops.A. Causal factors:

1. Power to drive motor is off.2. Lack of seal water.3. Broken drive belt.

B. Corrective action (where applicable):1. Reset heater, breaker, etc. and restart.2. Starts seal water flow.3. Replace drive belt.

4. Drum stops rotating.A. Causal factor:

1. Power to drive motor is off.B. Corrective action:

1. Reset heater, breaker, etc. and restart5. Receiver vibrating.



A. Causal factors:1. Filtrate pump is clogged.2. Loose bolts and gasket around inspection

plate.3. Worn ball check valve in filtrate pump.4. Air leaks in suction line.5. Dirty drum face.6. Seal strips are missing.

B. Corrective actions (where applicable):1. Clear pump.2. Tighten bolts and gasket.3. Replace ball check.4. Seal leaks.5. Clean face with pressure hose.6. Replace missing seal strips.

6. High vat level.A. Causal factors:

1. Improper chemical conditioning.2. Feed Rate is too high.3. Drum speed is too slow.4. Filtrate pump is off or clogged.5. Drain line is plugged.6. Vacuum pump has stopped.7. Seal strips are missing.

B. Corrective actions:1. Change coagulant dosage.2. Reduce feed rate.3. Increase drum speed.4. Turn on or clean pump.5. Clean drain line.6. Replace seal strips.

7. Low vat level.A. Causal factors:

1. Feed rate is too low.2. Vat drain valve is open.

B. Corrective actions (where applicable):1. Increase feed rate.2. Close vat drain valve.

8. Vacuum pump is drawing high amperage.A. Causal factors:

1. Filtrate pump is clogged.2. Improper chemical conditioning.3. High vat level.4. Cooling water flow to vacuum pump is

too high.B. Corrective actions (where applicable):

1. Clear pump clog.2. Adjust coagulant dosage.3. Decrease cooling water flow rate.

9. Scale buildup on vacuum pump seals.A. Causal factor:

1. Hard, unstable water.B. Corrective action:

1. Add sequestering agent.

18.13.4.2.3 Process Control CalculationsProbably the most frequent calculation vacuum filter oper-ators have to make is for determining filter yield. Example18.85 illustrates how this calculation is made.

18.13.4.2.3.1 Filter Yield: Vacuum Filter

EXAMPLE 18.85

Problem:

Thickened thermally conditioned sludge is pumped to avacuum filter at a rate of 50 gal/min. The vacuum area ofthe filter is 12 ft wide with a drum diameter of 9.8 ft. Ifthe sludge concentration is 12%, what is the filter yieldin pounds per hour per square foot? Assume the sludgeweighs 8.34 lb/gal.

Solution:

Calculate the filter surface area:

Calculate the pounds of solids per hour:

Divide the two results:

18.13.4.3 Pressure Filtration

Pressure filtration differs from vacuum filtration in thatthe liquid is forced through the filter media by a positivepressure instead of a vacuum. Several types of presses areavailable, but the most commonly used types are plate andframe presses and belt presses.

Filter presses include the belt or plate and frame types.The belt filter includes two or more porous belts, rollers,and related handling systems for chemical makeup andfeed. It also includes supernatant and solids collection andtransport (see Figure 18.15).

The plate and frame filter consists of a support frame,filter plates covered with porous material, hydraulic ormechanical mechanism for pressing plates together, andrelated handling systems for chemical makeup and feed.It also includes supernatant and solids collection andtransport.

Area of a cylinder side Diameter Length

= 3.14 9.8 ft 12 ft

= 369.3 ft 2

= ¥ ¥

¥ ¥

3 14.

50 60 8 34 0 12

3002 4

gal min min h lb gal

lb h

¥ ¥ ¥ =. .

.

3002.4 lb h

369.3 ft lb h ft2

28 13= .



In the plate and frame filter, solids are pumped (sand-wiched) between plates. Pressure (200 to 250 psi) isapplied to the plates and water is squeezed from the solids.At the end of the cycle, the pressure is released and as theplates separate the solids drop out onto a conveyor beltfor transport to storage or disposal.

Performance factors for plate and frame pressesinclude feed sludge characteristics, type and amount ofchemical conditioning, operating pressures, and the typeand amount of precoat.

In operation, the belt filter uses a coagulant (polymer)mixed with the influent solids. The chemically treated solidsare discharged between two moving belts. First water drainsfrom the solids by gravity. The two belts then move betweena series of rollers, and pressure squeezes additional waterout of the solids. The solids are then discharged onto aconveyor belt for transport to storage or disposal.

Performance factors for the belt press include sludgefeed rate, belt speed, belt tension, belt permeability, chem-ical dosage, and chemical selection.

Filter presses have lower operation and maintenancecosts than vacuum filters or centrifuges. They typicallyproduce a good quality cake and can be batch operated.The downside is that construction and installation costsare high. Moreover, chemical addition is required and thepresses must be operated by skilled personnel.


Most plate and filter press operations are partially or fullyautomated. Operation consists of observation, mainte-nance, and sampling and testing.

Operation of belt filter presses consists of preparationof conditioning chemicals, chemical feed rate adjust-ments, sludge feed rate adjustments, belt alignment, beltspeed and belt tension adjustments, sampling and testing,and maintenance.

We include common operational problems, causal fac-tors, and recommended corrective actions for the platepress and belt filter press in the following list.

Plate Press

1. Plates fail to seal.A. Causal factors:

1. Poor alignment.2. Inadequate shimming.

B. Corrective actions (where applicable):1. Realign parts.2. Adjust shimming of stay bosses.

2. Cake discharge is difficult.A. Causal factors:

1. Inadequate precoat.2. Improper conditioning.

B. Corrective actions (where applicable):1. Increase precoat and feed at 25 to 40 psig.2. Change conditioner type or dosage (use

filter leaf test to determine).3. Filter cycle times are excessive.

A. Causal factors:1. Improper conditioning.2. Feed solids are low.

B. Corrective Actions (where applicable):1. Change chemical dosage.2. Improve thickening operation.

4. Filter cake sticks to conveyors.A. Causal factor:

1. Improper conditioning chemical or dosage.B. Corrective action:

1. Increase inorganic conditioner dose.5. Precoat pressures gradually increase.

A. Causal factors:1. Improper sludge conditioning.2. Improper precoat feed.

FIGURE 18.15 Belt filter press. (From Spellman, F.R., Spellman’s Standard Handbook for Wastewater Operators, Vol. 1, TechnomicPubl., Lancaster, PA, 1999.)

Sludge

Polymer

Water

Return fortreatment

Disposal



3. Filter media plugged.4. Calcium buildup in media.

B. Corrective actions (where applicable):1. Change chemical dosage2. Decrease feed for a few cycles and

optimize.3. Wash filter media.4. Wash media with inhibited hydrochloric

acid.6. Frequent media binding.

A. Causal factors:1. Inadequate precoat.2. Initial feed rate too high (no precoat).

B. Corrective actions (where applicable):1. Increase precoat.2. Reduce feed rate or develop initial cake

slowly.7. Excessive moisture in cake.

A. Causal factors:1. Improper conditioning.2. Filter cycle too short.

B. Corrective actions (where applicable):1. Change chemical dosage.2. Lengthen filter cycle.

8. Sludge blowing out of press.A. Causal factor:

1. Obstruction between plates.B. Corrective Action:

1. Shut down feed pump, hit press closuredrive, restart feed pump, and clean aftercycle.

9. Plate Press: Leaks around Lower Faces of PlatesA. Causal factor:

1. Wet cake soiling media on lower faces.B. Corrective actions (where applicable):

1. Change chemical dosage.2. Lengthen filter cycle.

Belt Press

1. Filter cake discharge is difficult.A. Causal factors:

1. Wrong conditioning chemical selected.2. Improper chemical dosage.3. Changing sludge characteristics.4. Wrong application point.

B. Corrective actions (where applicable):1. Change conditioning chemical.2. Adjust chemical dosage.3. Change chemical or sludge4. Adjust application point.

2. Sludge leaking from belt edges.

A. Causal factors:1. Excessive belt tension.2. Belt speed too low.3. Excessive sludge feed rate.

B. Corrective actions (where applicable):1. Reduce belt tension.2. Increase belt speed.3. Reduce sludge feed rate.

3. Excessive moisture in filter cake.A. Causal factors:

1. Improper belt speed or drainage time.2. Wrong conditioning chemical.3. Improper chemical dosage.4. Inadequate belt washing.5. Wrong belt weave or material.

B. Corrective actions (where applicable):1. Adjust belt speed.2. Change conditioning chemical.3. Adjust chemical dosage.4. Clear spray nozzles or adjust sprays.5. Replace belt.

4. Excessive belt wear along edges.A. Causal factors:

1. Roller misalignment.2. Improper belt tension.3. Tension or alignment in control system.

B. Corrective actions (where applicable):1. Correct roller alignment.2. Correct tension.3. Repair tracking and alignment system

controls.5. Belt shifts or seizes.

A. Causal factors:1. Uneven sludge distribution.2. Inadequate or uneven belt washing.

B. Corrective actions:1. Adjust feed for uniform sludge distribu-

tion.2. Clean and adjust belt-washing sprays.

18.13.4.3.2 Process Control Calculations: Filter Presses

As part of the operating routine for filter presses, operatorsare called upon to make certain process control calcula-tions. The process control calculation most commonly usedin operating the belt filter press determines the hydraulicloading rate on the unit. The most commonly used processcontrol calculation used in operation of plate and filterpresses determines the pounds of solids pressed per hour.Both of these calculations are demonstrated below.



18.13.4.3.2.1 Hydraulic Loading Rate: Belt Filter Press

EXAMPLE 18.86

Problem:

A belt filter press receives a daily sludge flow of 0.30 gal.If the belt is 60 in. wide, what is the hydraulic loadingrate on the unit in gallons per minute for each foot of beltwidth?

Solution:

18.13.4.3.2.2 Pounds of Solids Pressed per Hour: Plate and Frame Press

EXAMPLE 18.87

Problem:

A plate and frame filter press can process 850 gal of sludgeduring its 120-min operating cycle. If the sludge concen-tration is 3.7%, and if the plate surface area is 140 ft2,how many pounds of solids are pressed per hour for eachsquare foot of plate surface area?

Solution:

18.13.4.4 Centrifugation

Centrifuges of various types have been used in dewateringoperations for at lease 30 years and appear to be gainingin popularity. Depending on the type of centrifuge usedand the centrifuge pumping equipment for solids feed andcentrate removal, chemical makeup and feed equipmentand support systems for removal of dewatered solids arerequired.


Generally, the centrifuge spins at a very high speed whenoperating. The centrifugal force it creates throws the solidsout of the water. Chemically conditioned solids arepumped into the centrifuge. The spinning action throwsthe solids to the outer wall of the centrifuge. The centrate(water) flows inside the unit to a discharge point. Thesolids held against the outer wall are scraped to a dischargepoint by an internal scroll moving slightly faster or slowerthan the centrifuge speed of rotation.

In the operation of the continuous feed, solids bowl,conveyor type centrifuge (this is the most common typecurrently used), and other commonly used centrifuges,solid and liquid separation occurs as a result of rotatingthe liquid at high speeds to cause separation by gravity.

In the solid bowl type, the solid bowl has a rotatingunit with a bowl and a conveyor (see Figure 18.16). Theunit has a conical section at one end that acts as a drainagedevice. The conveyor screw pushes the sludge solids tooutlet ports and the cake to a discharge hopper. The sludgeslurry enters the rotating bowl through a feed pipe leadinginto the hollow shaft of the rotating screw conveyor. Thesludge is distributed through ports into a pool inside therotating bowl. As the liquid sludge flows through the hol-low shaft toward the overflow device, the fine solids settleto the wall of the rotating bowl. The screw conveyorpushes the solids to the conical section where the solidsare forced out of the water and the water drains back inthe pool.

The expected percent solids for centrifuge dewateredsludges is in the range of 10 to 15%. The expected

0.30 MGD 1,000,000 gal MG

min d gal min

in.1 ft

12 in. ft

208.3 gal

5 ft gal min ft

¥=

¥ =

=

1440208 3

60 5

41 7

.

.

850 262 3

262 3

120131 2

0 94

gal 0.037 8.34 lb gal b

lb

min

60 min

1 h lb h

131.2 lb h

140 ft lb h ft

2

2

¥ ¥ =

¥ =

=

.

..

.

l

FIGURE 18.16 Centrifuge. (From Spellman, F.R., Spellman’s Standard Handbook for Wastewater Operators, Vol. 1, TechnomicPubl., Lancaster, PA, 1999.)

Centratedischarge

DamSolids

discharge

Screwconveyor

Rotating bowl

Inlet



performance is depended on the type of sludge beingdewatered, as shown in Table 18.14.

Centrifuge operation is dependent upon various per-formance factors:

1. Bowl design (length-diameter ratio and flowpattern)

2. Bowl speed3. Pool volume4. Conveyor design5. Relative conveyor speed6. Type and condition of sludge7. Type and amount of chemical conditioning8. Operating pool depth9. Relative conveyor speed (if adjustable)

Centrifuge operators often find that the operation ofcentrifuges can be simple, clean, and efficient. In mostcases, chemical conditioning is required to achieve optimumconcentrations. Operators soon discover that centrifugesare noisemakers; units run at very high speed and producehigh-level noise, which can cause loss of hearing withprolonged exposure. When working in an area where acentrifuge is in operation, special care must be taken toprovide hearing protection.

Actual operation of a centrifugation unit requires theoperator to perform the following tasks:

1. Control and adjust chemical feed rates.2. Observe unit operation and performance.3. Control and monitor centrate returned to treat-

ment system.4. Perform required maintenance as outlined in the

manufacturer’s technical manual.

The centrifuge operator must be trained to observeand recognize (as with other unit processes) operationalproblems that may occur with centrifuge operation. Wecover several typical indicators and observations of cen-trifuge problems, along with causal factors and suggested

corrective actions (troubleshooting procedures) in the fol-lowing sections.

1. Poor centrate clarity.A. Causal factors:

1. Feed rate is too high.2. Wrong plate dam position.3. Worn conveyor flights.4. Speed is too high.5. High feed sludge solids concentration.6. Improper chemical conditioning.

B. Corrective actions (where applicable):1. Adjust sludge feed rate.2. Increase pool depth.3. Repair or replace conveyor.4. Change pulley setting to obtain lower

speed.5. Dilute sludge feed.6. Adjust chemical dosage.

2. Solids cake is not dry enough.A. Causal factors:

1. Feed rate is too high.2. Wrong plate dam position.3. Speed is too low.4. Excessive chemical conditioning.5. Influent is too warm.

B. Corrective actions (if applicable):1. Reduce sludge feed rate.2. Decrease pool depth to increase dryness.3. Change pulley setting to obtain higher

speed.4. Adjust chemical dosage.5. Reduce influent temperature.

3. Torque control keeps tripping.A. Causal factors:

1. Feed rate is too high.2. Feed solids concentration is too high.3. Foreign material (i.e., tramp iron) in

machine.4. Gear unit is misaligned.5. Gear unit has mechanical problem.

B. Corrective actions (where applicable):1. Reduce flows.2. Dilute flows.3. Remove conveyor or clear foreign mate-

rials.4. Correct gear unit alignment.5. Repair gear unit.

4. Excess vibration.A. Causal factors:

1. Improper lubrication.2. Improper adjustment of vibration isolators.3. Discharge funnels are contacting centri-

fuge.

TABLE 18.14Expected Percent Solids for Centrifuge Dewatered Sludges

Type of Sludge %Solids

Raw sludge 25–35%Anaerobic digestion 15–30%Activated sludge 8–10%Heat treated 30–50%

Source: Spellman, F.R., Spellman’s Stan-dard Handbook for Wastewater Operators,Vol. 1, Technomic Publ., Lancaster, PA, 1999.



4. Portion of conveyor flights may beplugged (causing an imbalance).

5. Gear box improperly aligned.6. Pillow box bearings are damaged.7. Bowl is out of balance.8. Parts are not tightly assembled.9. Uneven wear on conveyor.

B. Corrective actions (where applicable):1. Lubricate according to manufacturer’s

instructions.2. Adjust isolators.3. Reposition slip joints at funnels.4. Flush centrifuge.5. Align gearbox.6. Replace bearings.7. Return rotating parts to factory for rebal-

ancing8. Tighten parts.9. Resurface and rebalance.

5. Sudden increase in power consumption.A. Causal factors:

1. Contact between bowl exterior and accu-mulated solids in case.

2. Effluent pipe is plugged.B. Corrective actions (where applicable):

1. Apply hard surfacing to areas with wear.2. Clear solids discharge.

6. Gradual increase in power consumption.A. Causal factor:

1. Conveyor blade wear.B. Corrective action:

1. Replace blades.7. Spasmodic surging of solids discharge.

A. Causal factors:1. Pool depth too low.2. Conveyor helix is rough.3. Feed pipe too near drainage deck.4. Excessive vibration.

B. Corrective Actions (where applicable):1. Increase pool depth.2. Refinish conveyor blade area.3. Move feed pipe to effluent end.

8. Centrifuge shuts down or will not start.A. Causal factors:

1. Blown fuses.2. Overload relay is tripped.3. Motor overheated or thermal protectors

are tripped.4. Torque control is tripped.5. Vibration switch is tripped.

B. Corrective actions (where applicable):1. Replace fuses.2. Flush centrifuge and reset relay.3. Flush centrifuge and reset thermal protec-

tors.

18.13.4.5 Sludge Incineration

Not surprisingly, incinerators produce the maximum sol-ids and moisture reductions. The equipment requireddepend on whether the unit is a multiple hearth or fluid-bed incinerator. Generally, the system will require a sourceof heat to reach ignition temperature, solids feed systemand ash handling equipment. It is important to note thatthe system must also include all required equipment (e.g.,scrubbers) to achieve compliance with air pollution con-trol requirements.

In operation, solids are pumped to the incinerator. Thesolids are dried and ignited (burned). As they burn theorganic matter is converted to carbon dioxide and watervapor and the inorganic matter is left behind as ash orfixed solids. The ash is then collected for reuse of disposal.

18.13.4.5.1 Process DescriptionThe incineration process first dries then burns the sludge.The process involves the following steps:

1. The temperature of the sludge feed is raised to212°F.

2. Water evaporates from the sludge.3. The temperature of the water vapor and air mix-

ture increases.4. The temperature of the dried sludge volatile

solids raises to the ignition point.

Note: Incineration will achieve maximum reductionsif sufficient fuel, air, time, temperature, and tur-bulence are provided.

18.13.4.5.2 Incineration Processes

18.13.4.5.2.1 Multiple Hearth FurnaceThe multiple hearth furnace consists of a circular steel shellsurrounding a number of hearths. Scrappers (rabble arms)are connected to a central rotating shaft. Units range from4.5 to 21.5 ft in diameter and have from 4 to 11 hearths.

In operation, dewatered sludge solids are placed onthe outer edge of the top hearth. The rotating rabble armsmove them slowly to the center of the hearth. At the centerof the hearth, the solids fall through ports to the secondlevel. The process is repeated in the opposite direction.Hot gases generated by burning on lower hearths dry thesolids. The dry solids pass to the lower hearths. The hightemperature on the lower hearths ignites the solids. Burn-ing continues to completion. Ash materials discharge tolower cooling hearths where they are discharged for dis-posal. Air flowing inside center column and rabble armscontinuously cools internal equipment.

18.13.4.5.2.2 Fluidized Bed FurnaceThe fluidized bed incinerator consists of a vertical circularsteel shell (reactor) with a grid to support a sand bed andan air system to provide warm air to the bottom of the



sand bed. The evaporation and incineration process takesplace within the super-heated sand bed layer.

In operation, air is pumped to the bottom of the unit.The airflow expands (fluidizes) the sand bed inside. Thefluidized bed is heated to its operating temperature(1200 to 1500°F). Auxiliary fuel is added when needed tomaintain operating temperature. The sludge solids areinjected into the heated sand bed. Moisture immediatelyevaporates. Organic matter ignites and reduces to ash.Residues are ground to fine ash by the sand movement.Fine ash particles flow up and out of the unit with exhaustgases. Ash particles are removed using common air pol-lution control processes. Oxygen analyzers in the exhaustgas stack control the airflow rate.

Note: Because these systems retain a high amount ofheat in the sand, the system can be operated aslittle as four hours per day with little or noreheating.


The operator of an incinerator monitors various perfor-mance factors to ensure optimal operation. These perfor-mance factors include feed sludge volatile content, feedsludge moisture content, operating temperature, sludgefeed rate, fuel feed rate, and air feed rate.

Note: To ensure that the volatile material is ignited, thesludge must be heated between 1400 and 1700°F.

In ensuring operating parameters are in the correctrange, the operator monitors and adjusts sludge feed rate,airflow, and auxiliary fuel feed rate.

All maintenance conducted on an incinerator shouldbe in accordance with manufacturer’s recommendations.

18.13.4.5.3.1 Operational ProblemsThe operator of a multiple hearth or fluidized bed incin-erator must be able to recognize operational problemsusing various indicators and observations. We discussthese indicators and observations, causal factors, and rec-ommended corrective actions in the following list:

Multiple Hearths

1. Incinerator temperature is too high.A. Causal factors:

1. Excessive fuel feed rate.2. Greasy solids.3. Thermocouple has burned out.

B. Corrective actions (where applicable):1. Decrease fuel feed rate.2. Reduce sludge feed rate.3. Increase air feed rate.4. Replace thermocouple.

2. Furnace temperature is too low.A. Causal factors:

1. Moisture content of the sludge hasincreased.

2. Fuel system malfunction.3. Excessive air feed rate.4. Flame is out.

B. Corrective actions (where applicable):1. Increase fuel feed rate until dewatering

operation improves.2. Establish proper fuel feed rate.3. Decrease air feed rate.4. Increase sludge feed rate.5. Relight furnace.

3. Oxygen content of stack gas is too high.A. Causal factors:

1. Sludge feed rate is too low.2. Sludge feed system blockage.3. Air feed rate is too high.

B. Corrective actions (where applicable):1. Increase sludge feed rate.2. Clear any feed system blockages.3. Decrease air feed rate.

4. Oxygen content of stack gas is too low.A. Causal factors:

1. Volatile or grease content of the sludge hasincreased.

2. Air feed rate is too low.B. Corrective actions (where applicable):

1. Increase air feed rate.2. Decrease sludge feed rate.3. Increase air feed rate.

5. Furnace refractories have deteriorated.A. Causal factor:

1. Rapid start-up or shutdown of furnace.B. Corrective actions:

1. Repair furnace refractories.2. Follow specified start-up or shutdown

procedures.6. Unusually high cooling effect.

A. Causal factor:1. Air leak.

B. Corrective action:1. Locate and repair leak.

7. Short hearth life.A. Causal factor:

1. Uneven firing.B. Corrective action:

1. Fire hearths equally on both sides.8. Center shaft shear pin failure.

A. Causal factors:1. Rabble arm is dragging on hearth.2. Debris is caught under the arm.

B. Corrective actions (where applicable):



1. Adjust rabble arm to eliminate rubbing.2. Remove debris.

9. Scrubber temperature is too high.A. Causal factor:

1. Low water flow to scrubber.B. Corrective action:

1. Adjust water flow to proper level.10. Stack gas temperatures are too low.

A. Causal factors:1. Inadequate fuel feed supply.2. Excessive sludge feed rate.

B. Corrective actions (where applicable):1. Increase fuel feed rate.2. Decrease sludge feed rate.

11. Stack gas temperatures are too high.A. Causal factors:

1. Sludge has higher volatile content (heatvalue).

2. Excessive fuel feed rate.B. Corrective actions (where applicable):

1. Increase air feed rate.2. Decrease sludge feed rate.3. Decrease fuel feed rate.

12. Furnace burners are slagging up.A. Causal factor:

1. Burner design.B. Corrective action:

1. Replace burners with newer designs thatreduce slagging.

13. Rabble arms are dropping.A. Causal factors:

1. Excessive hearth temperatures.2. Loss of cooling air.

B. Corrective actions (where applicable):1. Maintain temperatures within proper

range.2. Discontinue injection of scum into the

hearth.3. Repair cooling air system immediately.

14. Excessive air pollutants are in stack gas.A. Causal factors:

1. Incomplete combustion (insufficient air).2. Air pollution control malfunction.

B. Corrective actions (where applicable):1. Raise air-fuel ratio.2. Repair or replace broken equipment.

15. Flashing or explosions.A. Causal factor:

1. Scum or grease additions.B. Corrective action:

1. Remove scum or grease before incinera-tion.

Fluidized Beds

1. Bed temperature is falling.A. Causal factors:

1. Inadequate fuel supply.2. Excessive sludge feed rate.3. Excessive sludge moisture levels.4. Excessive air flow.

B. Corrective actions (where applicable):1. Increase fuel supply.2. Repair fuel system malfunction.3. Decrease sludge feed rate.4. Correct sludge de-watering process prob-

lem.5. Decrease airflow rate.

2. Low (<3%) oxygen in exhaust gas.A. Causal Factors:

1. Low air flow rate.2. Fuel feed rate is too high.

B. Corrective actions (where applicable):1. Increase blower air feed rate.2. Reduce fuel feed rate.

3. Excessive (>6%) oxygen in exhaust gas.A. Causal factor:

1. Sludge feed rate is too low.B. Corrective actions (where applicable):

1. Increase sludge feed rate.2. Adjust fuel feed rate to maintain steady

bed temperature.4. Erratic bed depth on control panel.

A. Causal factor:1. Bed pressure taps are plugged with solids.

B. Corrective actions (where applicable):1. Tap a metal rod into pressure tap pipe

when the unit is not in operation.2. Apply compressed air to pressure tap

while the unit is in operation (followmanufacturer’s safety guidelines).

5. Preheat burner fails and alarm sounds.A. Causal factors:

1. Pilot flame is not receiving fuel.2. Pilot flame is not receiving spark.3. Defective pressure regulators.4. Pilot flame ignites, but flame scanner

malfunctions.B. Corrective actions (where applicable):

1. Correct fuel system problem.2. Replace defective part.3. Replace defective regulators.4. Clear scanner sight glass.5. Replace defective scanner.

6. Bed temperature is too high.A. Causal factors:



1. Bed gun fuel feed rate is too high.2. Grease or high organic content in sludge

(high heat value).B. Corrective Actions (where applicable):

1. Reduce bed gun fuel feed rate.2. Increase airflow rate.3. Decrease sludge fuel rate.

7. Bed temperature reads off scale.A. Causal factor:

1. Thermocouple has burned out.B. Corrective action:

1. Replace thermocouple.8. Scrubber inlet shows high temperature.

A. Causal factors:1. No water flowing in scrubber.2. Spray nozzles are plugged.3. Ash water not recirculating.

B. Corrective actions (where applicable):1. Open valves to provide water.2. Correct system malfunction to provide

required pressure.3. Clear nozzles and strainers.4. Repair or replace recirculation pump.5. Unclog scrubber discharge line.

9. Poor bed fluidization.A. Causal factor:

1. Sand leakage through support plate dur-ing shutdown.

B. Corrective actions (where applicable):1. Clear wind box.2. Clean wind box at least once per month.

18.13.4.6 Land Application of Biosolids

The purpose of land application of biosolids is to disposeof the treated biosolids in an environmentally sound man-ner by recycling nutrients and soil conditioners. In orderto be land applied, wastewater biosolids must comply withstate and federal biosolids management and disposal reg-ulations. Biosolids must not contain materials that aredangerous to human health (i.e., toxicity, pathogenicorganisms, etc.) or dangerous to the environment (i.e.,toxicity, pesticides, heavy metals, etc.).

Treated biosolids are land applied by either directinjection or application and plowing in (incorporation).

18.13.4.6.1 Process Control: Sampling and Testing

Land application of biosolids requires precise control toavoid problems. The quantity and the quality of biosolidsapplied must be accurately determined. For this reason,the operator’s process control activities include biosolidssampling and testing functions.

Biosolids sampling and testing includes determinationof percent solids, heavy metals, organic pesticides and

herbicide, alkalinity, total organic carbon, organic nitro-gen, and ammonia nitrogen.

18.13.4.6.2 Process Control CalculationsProcess control calculations include determining disposalcost, plant available nitrogen (PAN), application rate (drytons and wet tons per acre), metals loading rates, maxi-mum allowable applications based upon metals loading,and site life based on metals loading.

18.13.4.6.2.1 Disposal CostThe cost of disposal of biosolids can be determined bythe following equation:

(18.81)

EXAMPLE 18.88

Problem:

The treatment system produces 1925 wet tons of biosolidsfor disposal each year. The biosolids are 18% solids. Acontractor disposes of the biosolids for $28.00 per dryton. What is the annual cost for sludge disposal?

Solution:

18.13.4.6.2.2 Plant Available NitrogenOne factor considered when land applying biosolids is theamount of nitrogen in the biosolids available to the plantsgrown on the site. This includes ammonia nitrogen andorganic nitrogen. The organic nitrogen must be mineral-ized for plant consumption. Only a portion of the organicnitrogen is mineralized per year. The mineralization factor(f1) is assumed to be 0.20. The amount of ammonia nitrogenavailable is directly related to the time elapsed betweenapplying the biosolids and incorporating (plowing) thesludge into the soil. We provide volatilization rates basedupon this example below:

wheref1 = Mineral rate for organic nitrogen (assume

0.20)V1 = Volatilization rate ammonia nitrogenV1 = 1.00 if biosolids are injected

Cost Wet Tons Biosolids Produced Year

% Solids Cost Dry Ton

= ¥

¥

Cost = ¥ ¥

=

1925 0 18 28 00

9702

wet tons year dry ton. $ .

$

PAN lb dry ton Organic Nitrogen mg kg

Ammonia Nitrogen mg kg

lb dry ton

( ) ( )( )[( )( )]

= ¥ +

¥ ¥

f

V

1

1

0 002

. (18.82)



V1 = 0.85 if biosolids are plowed in within 24 hV1 = 0.70 if biosolids are plowed in within 7 d

EXAMPLE 18.89

Problem:

The biosolids contain 21,000 mg/kg of organic nitrogenand 10,500 mg/kg of ammonia nitrogen. The biosolids areincorporated into the soil within 24 h after application.What is the PAN per dry ton of solids?

Solution:

18.13.4.6.2.3 Application Rate Based on Crop Nitrogen Requirement

In most cases, the application rate of domestic biosolids tocrop lands will be controlled by the amount of nitrogenthe crop requires. The biosolids application rate based uponthe nitrogen requirement is determined by the following:

1. Using an agriculture handbook to determine thenitrogen requirement of the crop to be grown

2. Determining the amount of sludge in dry tonsrequired to provide this much nitrogen

(18.83)

EXAMPLE 18.90

Problem:

The crop to be planted on the land application site requires150 lb of nitrogen per acre. What is the required biosolidsapplication rate if the PAN of the biosolids is 30 lb/dryton?

Solution:

(18.84)

18.13.4.6.2.4 Metals LoadingWhen biosolids are land applied, metals concentrationsare closely monitored and their loading on land applica-tion sites are calculated:

EXAMPLE 18.91

Problem:

The biosolids contain 14 mg/kg of lead. Biosolids arecurrently being applied to the site at a rate of 11 drytons/acre. What is the metals loading rate for lead inpounds per acre?

Solution:

18.13.4.6.2.5 Maximum Allowable Applications Based upon Metals Loading

If metals are present, they may limit the total number ofapplications a site can receive. Metals loading are nor-mally expressed in terms of the maximum total amountof metal that can be applied to a site during its use:

EXAMPLE 18.92

Problem:

The maximum allowable cumulative lead loading is48.0 lb/acre. Based upon the current loading of0.35 lb/acre, how many applications of biosolids can bemade to this site?

Solution:

18.13.4.6.2.6 Site Life Based on Metals Loading

The maximum number of applications based upon metalsloading and the number of applications per year can beused to determine the maximum site life:

PAN lb dry ton

mg kg

lb PAN dry ton

( )

( ) ( )[ ]= ¥ + ¥ ¥

=

21 000 0 20 10 500 0 85 0 002

26 3

, . , . .

.

Dry tons acre

Plant Nitrogen Requirement lb acre

PAN lb dry ton

=

( )( )

Dry tons acre150 lb nitrogen acre

3 lb dry ton

dry tons acre

=

=

0

5

Loading Rate lb acre

Metal Concentration mg kg lb dry ton

Applied Rate dry tons acre

( ) =

( ) ¥ ¥

( )

.

0 002

(18.85)

Loading Rate lb acre

mg kg lb dry ton dry tons

lb acre

( )= ¥ ¥

=

14 0 002 11

0 31

.

.

Applications

Maximum Allowable Cumulative Load for the Metal lb acre

Metal Loading lb acre application

=

( )( )

(18.86)

A

applications

pplications 48 lb acre

.35 lb acre application

=

=

0

137



(18.87)

EXAMPLE 18.93

Problem:

Biosolids is currently applied to a site twice annually.Based upon the lead content of the biosolids, the maxi-mum number of applications is determined to be135 applications. Based upon the lead loading and theapplication rate, how many years can this site be used?

Solution:

Note: When more than one metal is present, the cal-culations must be performed for each metal.The site life would then be the lowest valuegenerated by these calculations.

18.14 PERMITS, RECORDS, AND REPORTS

Permits, records, and reports play a significant role inwastewater treatment operations. In fact, in regards to thepermit, one of the first things any new operator quicklylearns is the importance of “making permit” each month.In this section, we briefly cover National Pollutant Dis-charge Elimination System (NPDES) permits and otherpertinent records and reports the wastewater operator mustbe familiar with.

Note: The discussion that follows is general in nature;it does not necessarily apply to any state inparticular, but instead is an overview of permits,records, and reports that are an important partof wastewater treatment plant operations. Forspecific guidance on requirements for yourlocality, refer to your state water control boardor other authorized state agency for informa-tion. In this handbook, the term board signifiesthe state-reporting agency.

18.14.1 DEFINITIONS

There are several definitions that should be discussed priorto discussing the permit requirements for records andreporting:

Average daily limitation the highest allowable aver-age over a 24-h period, calculated by adding all

of the values measured during the period anddividing the sum by the number of values deter-mined during the period.

Average hourly limitation the highest allowableaverage for a 60-min period, calculated by add-ing all of the values measured during the periodand dividing the sum by the number of valuesdetermined during the period.

Average monthly limitation the highest allowableaverage over a calendar month, calculated byadding all of the daily values measured duringthe month and dividing the sum by number ofdaily values measured during the month.

Average weekly limitation the highest allowableaverage over a calendar week, calculated byadding all of the daily values measured duringthe calendar week and dividing the sum by thenumber of daily values determined during theweek.

Daily discharge the discharge of a pollutant measuredduring a calendar day or any 24-h period thatreasonably represents the calendar for the pur-pose of sampling. For pollutants with limitationsexpressed in units of weight, the daily dischargeis calculated as the total mass of the pollutantdischarged over the day. For pollutants withlimitations expressed in other units, the dailydischarge is calculated as the average measure-ment of the pollutant over the day.

Discharge monitoring report forms used in the report-ing of self-monitoring results of the permittee.

Discharge permit State Pollutant Discharge Elimina-tion System (state-PDES) permit that specifiesthe terms and conditions under which a pointsource discharge to state waters is permitted.

Effluent limitation any restriction by the state boardon quantities, discharge rates, or concentrationsof pollutants that are discharged from pointsources into state waters.

Maximum daily discharge the highest allowable valuefor a daily discharge.

Maximum discharge the highest allowable value forany single measurement.

Minimum discharge the lowest allowable value forany single measurement.

Point source any discernible, defined, and discreteconveyance, including but not limited to anypipe, ditch, channel, tunnel, conduit, well, dis-crete fissure, container, rolling stock, vessel, orother floating craft, from which pollutants areor may be discharged. This definition does notinclude return flows from irrigated agriculturalland.

Site Life years

Maximum Allowable Applications Number of Applications Planned Year

( ) =

Site Life years135 applications

2 applications year68 years( ) = =



18.14.2 NPDES PERMITS

In the U.S., all treatment facilities that discharge to statewaters must have a discharge permit issued by the statewater control board or other appropriate state agency. Thispermit is known on the national level as the NationalNPDES permit and on the state level as the state-PDESpermit. The permit states the specific conditions that mustbe met to legally discharge treated wastewater to statewaters. The permit contains general requirements (applyingto every discharger) and specific requirements (applyingonly to the point source specified in the permit).

A general permit is a discharge permit that covers aspecified class of dischargers. It is developed to allowdischargers with the specified category to discharge underspecified conditions.

All discharge permits contain general conditions.These conditions are standard for all dischargers and covera broad series of requirements. Read the general condi-tions of the treatment facility’s permit carefully.

Permittees must retain certain records. These recordsinclude:

Monitoring:

1. Date, time, and exact place of sampling ormeasurements

2. Names of the individuals performing samplingor measurement

3. Dates and times analyses were performed4. Names of the individuals who performed the

analyses5. Analytical techniques or methods used6. Observations, readings, calculations, bench

data, and results7. Instrument calibration and maintenance8. Original strip chart recordings for continuous

monitoring9. Information used to develop reports required by

the permit10. Data used to complete the permit application

Note: All records must be kept at least 3 years (longerat the request of the state board).

18.14.2.1 Reporting

In general, reporting must be made under the followingconditions and situations (requirements may vary depend-ing upon state regulatory body with reporting authority):

1. Unusual or extraordinary discharge reports —The board must be notified by telephone within

24 h of occurrence and submit written reportwithin 5 d. The report must include:A. Description of the non-compliance and its

cause.B. Noncompliance dates, times, and duration.C. Steps planned or taken to reduce or elimi-

nate occurrence.D. Steps planned or taken to prevent reoccur-

rence.2. Anticipated noncompliance — The board must

be notified at least 10 d in advance of anychanges to the facility or activity that may resultin noncompliance.

3. Compliance schedules — Compliance or non-compliance with any requirements contained incompliance schedules must be reported no laterthan 14 d following scheduled date for comple-tion of the requirement.

4. 24-h Reporting — Any noncompliance thatmay adversely affect state waters or may endan-ger public health must be reported orally with24 h of the time the permittee becomes awareof the condition. A written report must be sub-mitted within 5 d.

5. Discharge monitoring reports (DMRs) — Thesereports consist of self-monitoring data generatedduring a specified period (normally 1 month).When completing the DMR, remember:A. More frequent monitoring must be reported.B. All results must be used to complete

reported values.C. Pollutants monitored by an approved

method, but not required by the permit mustbe reported.

D. No empty blocks on the form should be leftblank.

E. Averages are arithmetic unless noted other-wise.

F. Appropriate significant figures should beused.

G. All bypasses and overflows must be reported.H. The licensed operator must sign the report.I. Responsible official must sign the report.J. Department must receive by the 10th of the

following month.

18.14.2.2 Sampling and Testing

The general requirements of the permit specify minimumsampling and testing that must be performed on the plantdischarge. The permit will also specify the frequency ofsampling, sample type, and length of time for compositesamples.

Unless a specific method is required by the permit, allsample preservation and analysis must be in compliance



with the requirements set forth in the Code of FederalRegulations, Guidelines Establishing Test Procedures forthe Analysis of Pollutants Under the Clean Water Act (40CFR 136).

Note: All samples and measurements must be repre-sentative of the nature and quantity of the dis-charge.

18.14.2.3 Effluent Limitations

The permit sets numerical limitations on specific param-eters contained in the plant discharge. Limits may beexpressed as:

1. Average monthly quantity (kg/d)2. Average monthly concentration (mg/L)3. Average weekly quantity (kg/d)4. Average weekly concentration (mg/L)5. Daily quantity (kg/d)6. Daily concentration (mg/L)7. Hourly average concentration (mg/L)8. Instantaneous minimum concentration (mg/L)9. Instantaneous maximum concentration (mg/L)

18.14.2.4 Compliance Schedules

The facility may require additional construction or othermodifications to fully comply with the final effluent limi-tations. If this is the case, the permit will contain a scheduleof events to be completed to achieve full compliance.

18.14.2.5 Special Conditions

Any special requirements or conditions set for approvalof the discharge will be contained in this section. Specialconditions may include:

1. Monitoring required to determine effluent tox-icity

2. Pretreatment program requirements

18.14.2.6 Licensed Operator Requirements

The permit will specify, based on the treatment systemcomplexity and the volume of flow treated, the minimumlicense classification required to be the designated respon-sible charge operator.

18.14.2.7 Chlorination or Dechlorination Reporting

Several reporting systems apply to chlorination or chlori-nation followed by dechlorination. It is best to reviewthis section of the specific permit for guidance. Contactthe appropriate state regulatory agency for any neededclarification.

18.14.2.8 Reporting Calculations

Failure to accurately calculate report data will result inviolations of the permit. The basic calculations associatedwith completing the DMR are covered below.

18.14.2.8.1 Average Monthly Concentration

The average monthly concentration (AMC) is the averageof the results of all tests performed during the month:

(18.88)

where N = tests during a month.

18.14.2.8.2 Average Weekly Concentration (AWC)

The average weekly concentration (AWC) is the resultsof all the tests performed during a calendar week. A cal-endar week must start on Sunday and end on Saturdayand be completely within the reporting month. A weeklyaverage is not computed for any week that does not meetthese criteria:

(18.89)

where N = tests during a calendar week.

18.14.2.8.3 Average Hourly Concentration

The average hourly concentration (AHC) is the averageof all test results collected during a 60-min period:

(18.90)

where N = tests during a 60-min period.

18.14.2.8.4 Daily Quantity

Daily quantity (DQ) is the quantity of a pollutant in kilo-grams per day discharged during a 24-h period:

A

N

MC mg L

Test Test Test Test1 2 3 n

( ) =

+ + + +Â

K

A

N

WC mg L

Test Test Test Test

1 2 3 n

( ) =

+ + + +Â

K

A

N

HC mg L

Test Test Test Test

1 2 3 n

( ) =

+ + + +Â

K

DQ kg d Concentration mg L MGD

kg MG mg L

( ) = ( ) ¥ ( ) ¥Q

.3 785 (18.91)



18.14.2.8.5 Average Monthly QuantityAverage monthly quantity (AMQ) is the average of all theindividual daily quantities determined during the month:

(18.92)

where N = tests during a month.

18.14.2.8.6 Average Weekly QuantityThe average weekly quantity (AWQ) is the average of allthe daily quantities determined during a calendar week. Acalendar week must start on Sunday and end on Saturdayand be completely within the reporting month. A weeklyaverage is not computed for any week that does not meetthese criteria:

(18.93)

where N = tests during a calendar week.

18.14.2.8.7 Minimum ConcentrationThe minimum concentration is the lowest instantaneousvalue recorded during the reporting period.

18.14.2.8.8 Maximum ConcentrationMaximum concentration is the highest instantaneous valuerecorded during the reporting period.

18.14.2.8.9 Bacteriological ReportingBacteriological reporting is used for reporting fecalcoliform test results. To make this calculation the geomet-ric mean calculation is used and all monthly geometricmeans are computed using all the test values. Note thatweekly geometric means are computed using the sameselection criteria discussed for average weekly concentra-tion and quantity calculations. The easiest method used inmaking this calculation requires a calculator, which canperform logarithmic (log) or Nth root functions:

where N = number of tests.or

18.15 CHAPTER REVIEW QUESTIONS AND PROBLEMS

18.1. Who must sign the DMR?18.2. What does the COD test measure?18.3. Give three reasons for treating wastewater.18.4. Name two types of solids based on physical

characteristics.18.5. Define organic and inorganic.18.6. Name four types of microorganisms that may

be present in wastewater.18.7. When organic matter is decomposed aerobi-

cally, what materials are produced?18.8. Name three materials or pollutants, which

are not removed by the natural purificationprocess.

18.9. What are the used water and solids from acommunity that flow to a treatment plantcalled?

18.10. Where do disease-causing bacteria in waste-water originate?

18.11. What does the term pathogenic mean?18.12. What is wastewater called that comes from

the household?18.13. What is wastewater called that comes from

industrial complexes?18.14. The lab test indicates that a 500-g sample of

sludge contains 22 g of solids. What are thepercent solids in the sludge sample?

18.15. The depth of water in the grit channel is28 in. What is the depth in feet?

18.16. The operator withdraws 5250 gal of solidsfrom the digester. How many pounds of sol-ids have been removed?

18.17. Sludge added to the digester causes a1920–ft3 change in the volume of sludge inthe digester. How many gallons of sludgehave been added?

18.18. The plant effluent contains 30 mg/L solids.The effluent flow rate is 3.4 MGD. How manypounds per day of solids are discharged?

18.19. The plant effluent contains 25 mg/L BOD.The effluent flowrate is 7.25 MGD. How many kilograms per day of BOD are beingdischarged?

18.20. The operator wishes to remove 3280 lb/d ofsolids from the activated sludge process. Thewaste activated sludge concentration is3250 mg/L. What is the required flow rate inmillion gallons per day?

18.21. The plant influent includes an industrial flowthat contains 240 mg/L BOD. The industrialflow is 0.72 MGD. What is the populationequivalent for the industrial contribution inpeople per day?

AMQ

DQ DQ DQ DQ

N

n

kg d( ) =

+ + + º +Â

1 2 3

AWQ

DQ DQ DQ DQ

N

n

kg d( ) =

+ + + º +Â

1 2 3

Geometric Mean =

+ + + º +

log log log logX X X X

Nn1 2 3

(18.94)

Geometric Mean = + + º +X X Xnn

1 2



18.22. The label of hypochlorite solution states thatthe specific gravity of the solution is 1.1288.What is the weight of 1 gal of the hypochlo-rite solution?

18.23. What must be done to the cutters in a com-minutor to ensure proper operation?

18.24. What is grit? Give three examples of materialwhich is considered to be grit.

18.25. The plant has three channels in service. Eachchannel is 2 ft wide and has a water depthof 3 ft. What is the velocity in the channelwhen the flow rate is 8 MGD?

18.26. The grit from the aerated grit channel has astrong hydrogen sulfide odor upon standingin a storage container. What does this indicateand what action should be taken to correctthe problem?

18.27. What is the purpose of primary treatment?18.28. What is the purpose of the settling tank in the

secondary or biological treatment process?18.29. The circular settling tank is 90 ft in diameter

and has a depth of 12 ft. The effluent weirextends around the circumference of thetank. The flow rate 2.25 MGD. What is thedetention time in hours, surface loading ratein gallons per day per square foot and weiroverflow rate in gallons per day per foot?

18.30. Give three classifications of ponds basedupon their location in the treatment system.

18.31. Describe the processes occurring in a rawsewage stabilization pond (facultative).

18.32. How do changes in the season affect the qual-ity of the discharge from a stabilization pond?

18.33. What is the advantage of using mechanicalor diffused aeration equipment to provideoxygen?

18.34. Name three classifications of trickling filters.Identify the classification that produces thehighest quality effluent.

18.35. Microscopic examination reveals a predom-inance of rotifers. What process adjustmentdoes this indicate is required?

18.36. Increasing the wasting rate will __________the MLSS, ______________ the return con-centration, ______________ the MCRT,__________ the F:M ratio, and __________the SVI.

18.37. The plant currently uses 45.8 lb of chlorineper day. Assuming the chlorine usage willincrease by 10% during the next year, howmany 2000-lb cylinders of chlorine will beneeded for the year (365 days)?

18.38. The plant has 6 2000-lb cylinders on hand.The current dose of chlorine being used todisinfect the effluent is 6.2 mg/L. The aver-

age effluent flow rate is 2.25 MGD. Allowing15 days for ordering and shipment, whenshould the next order for chlorine be made?

18.39. The plant feeds 38 lb of chlorine per day anduses 150-lb cylinders. Chlorine use isexpected to increase by 11% next year. Thechlorine supplier has stated that the currentprice of chlorine ($0.170/lb) will increase by7.5% next year. How much money shouldthe town budget for chlorine purchases forthe next year (365 days)?

18.40. The sludge pump operates 30 min every 3 h.The pump delivers 70 gal/min. If the sludge is5.1% solids and has a volatile matter contentof 66%, how many pounds of volatile solidsare removed from the settling tank each day?

18.41. The aerobic digester has a volume of 63,000gal. The laboratory test indicates that 41 mgof lime were required to increase the pH ofa 1-L sample of digesting sludge from 6 tothe desired 7.1. How many pounds of limemust be added to the digester to increase thepH of the unit to 7.4?

18.42. The digester has a volume of 73,500 gal.Sludge is added to the digester at the rate of2750 gal/d. What is the SRT in days?

18.43. The raw sludge pumped to the digester con-tains 72% volatile matter. The digestedsludge removed from the digester contains48% volatile matter. What is the percent vol-atile matter reduction?

18.44. The acronym NPDES stands for______________________________.

18.45. How can primary sludge be freshened goinginto a gravity thickener?

18.46. A neutral solution has a pH value of _______.18.47. Why is the seeded BOD test required for

some samples?18.48. What is the foremost advantage of the COD

over the BOD?18.49. High mixed liquor concentration is indicated

by a ____________________ aeration tankfoam.

18.50. What typically happens to the activity level ofbacteria when the temperature is increased?

18.51. List three factors other than food that affectsthe growth characteristics of activatedsludge.

18.52. What are the characteristics of facultativeorganisms?

18.53. BOD measures the amount of ___________material in wastewater.

18.54. The activated sludge process requires______________________ in the aerationtank to be successful.



18.55. The activated sludge process can not be suc-cessfully operated with a ________ clarifier.

18.56. The activated biosolids process can success-fully remove ___________ BOD.

18.57. Successful operation of a complete mix reac-tor in the endogenous growth phase is______________.

18.58. The bacteria in the activated biosolids pro-cess are either __________ or ___________.

18.59. Step feed activated biosolids processes have___________ mixed liquor concentrations indifferent parts of the tank.

18.60. An advantage of contact stabilization com-pared to complete mix is ___________ aer-ation tank volume.

18.61. Increasing the _____________ of waste-water increases the BOD in the activated bio-solids process.

18.62. Bacteria need phosphorus to successfullyremove _________ in the activated biosolidsprocess.

18.63. The growth rate of microorganisms is con-trolled by the _________ ratio.

18.64. Adding chlorine just before the __________can control alga growth.

18.65. What is the purpose of the secondary clarifierin an activated biosolids process?

18.66. The ____________ growth phase shouldoccur in a complete mix activated biosolidsprocess.

18.67. The typical DO value for activated biosolidsplants is between ______ and ______ mg/L.

18.68. In the activated biosolids process, whatchange would an operator normally expectto make when the temperature decreasesfrom 25°C to15°C?

18.69. In the activated biosolids process, whatchange must be made to increase the MLVSS?

18.70. In the activated biosolids process, whatchange must be made to increase the F:M?

18.71. What does the Gould sludge age assume to bethe source of the MLVSS in the aeration tank?

18.72. What is one advantage of complete mix overplug flow?

18.73. The grit in the primary sludge is causing exces-sive wear on primary treatment sludge pumps.The plant uses an aerated grit channel. Whataction should be taken to correct this problem?

18.74. When the MCRT increases, the MLSS con-centration in the aeration tank ___________.

18.75. Exhaust air from a chlorine room should betaken from where?

18.76. If chlorine costs $0.21/lb, what is the dailycost to chlorinate a 5-MGD flow rate at chlo-rine feed rate of 2.6 mg/L?

18.77. What is the term that describes a normallyaerobic system from which the oxygen hastemporarily been depleted?

18.78. The ratio that describes the minimumamount of nutrients theoretically required foran activated sludge system is 100:5:1. Whatare the elements that fit this ratio?

18.79. A flotation thickener is best used for whattype of sludge?

18.80. True of false: Drying beds are an example ofa sludge stabilization process.

18.81. The minimum flow velocity in collectionsystems should be ___________________.

18.82. What effect will the addition of chlorine,acid, alum, carbon dioxide, or sulfuric acidhave on the pH of wastewater?

18.83. An amperometric titrater is used to measure___________________.

18.84. The normal design detention time for pri-mary clarifier is _________________.

18.85. The volatile acids-alkalinity ratio in ananaerobic digester should be approximately________.

18.86. The surface loading rate in a final clarifiershould be approximately _____________.

18.87. In a conventional effluent chlorination sys-tem the chlorine residual measured is mostlyin the form of ___________.

18.88. For a conventional activated biosolids pro-cess, the Food:Microorganism (F:M) ratioshould be in the range of ___ to ____.

18.89. Denitrification in a final clarifier can causeclumps of sludge to rise to the surface. Thesludge flocs attach to small sticky bubbles of_________ gas.

18.90. An anaerobic digester is covered and keptunder positive pressure to do what?

18.91. During the summer months, the major sourceof oxygen added to a stabilization pond is___________.

18.92. Which solids cannot be removed by vacuumfiltration?

18.93. The odor recognition threshold for H2S isreported to be as low as:

REFERENCES

1. Metcalf & Eddy, Inc. Wastewater Engineering: Treat-ment, Disposal, Reuse, 3rd ed., McGraw-Hill, NewYork, 1991, pp. 29–31.

2. Metcalf & Eddy, Inc., Wastewater Engineering: Treat-ment, Disposal, Reuse, 3rd ed., McGraw-Hill, NewYork, 1991.


Documents

Process of wastewater treat. / Proces preciscavanja otpadnih voda