ENVIRONMENTAL MICROBIAL

BIOTECHNOLOGY

MICR 307

DOMESTIC WASTES AND WASTE TREATMENT

PROF A.O. OLANIRAN F3 03-028

Collection systems for human wastes date back to Roman times but simply discharged into the nearest body of water, i.e. lake, stream, or ocean.

Sewage treatment is a relatively modern practice,

Fish and other wildlife were affected by the depletion of oxygen and the threat of waterborne disease increased as the size of human population grew.

Outbreak of cholera by mid 19th century brought about concern about the control of domestic waste.

Need for proper collection and disposal of wastes.

DOMESTIC WASTEWATER

Modern sewage treatment practices began at the turn of the 20th century

Involves treatment of organic matter in domestic wastes before disposal in water.

Because of increasing demands on limited water supplies and the need to reuse domestic wastes, treatment now include:

reduction of pathogenic mos and

the removal of toxic substances.

Domestic wastewater is primarily a combination of human feces, urine, and “greywater”.

Also, water from various industries and businesses.

Major constituents of untreated domestic sewage are shown in Table 1 (page 14).

The amount of organic matter in domestic wastes determines the degree of biological treatment required.

The amount of organic matter in wastewater

is assessed by three tests:

Total organic carbon (TOC)

Biochemical oxygen demand (BOD)

Chemical oxygen demand (COD)

BOD

BOD is the amount of dissolved oxygen consumed

by microorganisms during the biochemical

oxidation of organic and inorganic matter.

The major objective of domestic waste treatment is to

reduce the BOD.

The 5-day BOD test (BOD5) is a measure of the amount

of oxygen consumed by a mixed population of

heterotrophic bacteria in the dark at 20oC over a period

of 5 days.

Place aliquots of wastewater in BOD bottle

Dilute in phosphate buffer (pH 7.2) containing other

inorganic elements (N, Ca, Mg, Fe) and saturated with

oxygen

Add a nitrification inhibitor (sometimes) to the sample

to determine only carbonaceous BOD

Determine DO concentration at time 0 and after 5-day

incubation using an oxygen electrode (chemical

process) or a nanometric BOD apparatus

The BOD test is carried out on a series of dilutions of

the sample, depending on the sample source.

BOD Measurement

BOD value is expressed in mg/L according to the following equation (APHA, 1995).

BOD (mg/L) = D1 – D5 / p

D1 = initial DO, D5 = DO at day 5, and p = decimal volumetric fraction of wastewater utilized.

See pg 14 for example.

To determine the amount of oxygen that will be required for biological treatment of the organic matter present in a wastewater.

To determine the size of waste treatment facility needed.

To assess the efficiency of treatment processes.

To determine compliance with wastewater discharge permits.

The typical BOD5 of raw sewage ranges from 110 to 440 mg/L and conventional sewage treatment will reduce this by 95%.

Applications of the BOD5 test

The amount of oxygen necessary to oxidize all of the organic carbon completely to CO2 and H2O.

Measured by oxidation with potassium dichromate (K2Cr2O7) in the presence of sulfuric acid and silver.

Expressed in mg/L.

The ratio BOD/COD is approximately 0.5, normally.

When the ratio falls below 0.3, it means that the sample contains large amounts of organic compounds that are not easily biodegradable.

COD

Determined by;

oxidation of the organic matter with heat and

oxygen

measurement of the CO2 liberated with an

infrared analyzer.

Both TOC and COD represent concentration

of both biodegradable and non-biodegradable

organics in water.

TOC

PATHOGENIC MICROORGANISMS

IN DOMESTIC WASTE WATER

Almost always present in domestic wastewater (Table 2) because of excretions from infected individuals (Symptomatic & Asymptomatic).

The concentration of enteric pathogens in raw wastewater varies depending on;

the incidence of the infection in the community.

the socioeconomic status of the population.

the time of year.

the per-capita water consumption.

The peak incidence of many enteric infections is seasonal in temperate climates. For example;

Enterovirus – late summer & early fall, rotavirus – early winter; Cryptosporidium – early spring & fall

This may be due to:

survival of different agents in the environment during the different seasons.

excretion differences among animal reservoirs.

greater exposure to contaminated water, as in swimming, for increased incidence in summer.

Certain populations are more susceptible to infection.

Enteric infection is common in;

children because of usual lack of previous protective immunity

Lower socioeconomic groups, particularly where lower standards of sanitary conditions prevail

Concentration of enteric pathogens are much greater in sewage in the developing world than the industrialized world.

E.g. 103 per liter enteric viruses estimated in the US compare to 105 per liter observed in Africa and Asia

MODERN WASTEWATER

TREATMENT

The primary goal is the removal and degradation of organic matter under controlled conditions

Complete sewage treatment comprises three major steps;

Primary Treatment – Physical Process

Secondary Treatment – Biological Process

Tertiary Treatment – Chemical Process

Source: blog.pennlive.com/.../2008/07/Waste70908.jpg

Primary treatment

The first step in municipal sewage treatment.

A physical process that involves the separation of large debris, by passing the raw sewage through a metal grate.

The waste stream is then pumped into the primary settling tank (sedimentation tank or clarifier) where about half the suspended organic solids settle to the bottom as sludge or biosolids.

The resulting sludge is referred to as primary sludge.

Microbial pathogens are not effectively removed.

Secondary treatment

Consists of biological degradation, in which the

remaining suspended solids are decomposed and

the number of pathogens is reduced.

The effluent from primary treatment may be pumped

into a trickling filter bed (an aeration tank), or a

sewage lagoon.

A disinfection step is generally included at the end

of the treatment.

Trickling Filters

The TFB is a bed of stones or corrugated plastic sheets through which water drips.

Microorganisms reside on the bed, intercept the organic material and decompose it aerobically.

As the organic matter passes through the trickling filter, it is converted to microbial biomass which forms a biofilm on the filter medium surfaces.

The biofilm that forms on the surface of the filter medium is called the zooleal film.

It is composed of bacteria, fungi, algae, and protozoa.

Example of a Trickling Filter

Increase in biofilm thickness, over time, leads to limited oxygen diffusion to the deeper layers of the biofilm

This create an anaerobic environment near the filter medium surface.

The organisms eventually slough from the surface and a new biofilm is formed.

BOD removal by TF is approximately 85%.

Enteric pathogens’ removal by TF is low and erratic.

Removal of enteric viruses and probably other pathogenic MOS is affected by filtration rates.

Example of a Trickling Filter

Trickling Filter

Conventional Activated Sludge

Activated sludge process is also known as Aeration-tank digestion.

Effluent from 1o treatment is pumped into a tank and mixed with a bacteria-rich slurry, activated sludge

Air or pure oxygen is pumped through the mixture to promote bacterial growth and organic matter decomposition.

It then goes to 2o settling tank, where water is siphoned off the top of the tank and sludge is removed from the bottom.

Conventional Activated Sludge

Some of the sludge is used as an inoculum for the

incoming 1o effluent, and the remainder known as the

2o sludge is removed.

There is reduction in the concentration of

pathogens.

The detention time for sewage in the aeration basin

varies from 4 to 8 hours.

The content of the aeration tank is referred to as

mixed-liquor suspended solids (MLSS).

The organic part of the MLSS is called the mixed-

liquor volatile suspended solids (MLVSS).

This includes non-microbial organic matter as well as dead and living microorganisms and debris.

Proper ratio of food-to-microorganisms (F/M) must be maintained in the activated sludge process.

This is expressed as BOD per kg per day as follows:

F/M = Q x BOD

MLSS x V

Q = flow rate of sewage in million gallons per day (MGD).

BOD = 5-day biochemical oxygen demand (mg/L).

MLSS = mixed-liquor suspended solids (mg/L).

V = Volume of aeration tank (gallons).

F/M is controlled by the rate of activated sludge wasting.

Wastage or 2o sludge is sludge that is not returned as activated sludge.

The higher the wasting rate, the higher the F/M ratio.

For conventional aeration tanks, F/M ratio is 0.2-0.5 Ib BOD5/day/Ib MLSS

It can be higher (up to 1.5) for activated sludge when high-purity oxygen is used.

A low F/M ratio means that the MOS in the aeration

tank are starved, leading to more efficiency.

The operation of an activated sludge process is

controlled by the following;

organic loading rates

oxygen supply

control and operation of the final settling tank which

has the function of clarification and thickening

For routine operation, sludge settleability is

determined by use of the sludge volume index

(SVI).

SVI is determined by measuring the sludge volume

after it has settled for 30 minutes as follows:

SVI = V x 1000

MLSS

V = volume of settled sludge after 30 minutes (ml/L).

Poor settling may be caused by sudden changes in

temperature, pH, absence of nutrients, and

presence of toxic metals and organics.

A common problem is filamentous bulking, which

causes of slow settling and poor compaction of solids

in the clarifier. This is caused by excessive growth of

filamentous MOS.

A high SVI (> 150 ml/g) indicate bulking conditions.

Filamentous bacteria can be controlled by treating the

return sludge with chlorine or H2O2 which kill

filamentous MOS selectively.

TERTIARY TREATMENT

Involves a series of additional steps after secondary treatment to further reduce organics, turbidity, nitrogen, phosphorus, metals, and pathogens.

Normally practiced for additional protection of wildlife after discharge into rivers or lakes or when the wastewater is to be used for:

irrigation (e.g. food crops, golf courses)

recreational purposes (e.g. lakes, estuaries).

For drinking water.

Involve some type of physicochemical

treatment such as;

coagulation

filtration

activated carbon adsorption of organics

additional disinfection

Schematic representation of wastewater treatment process

Figure 1: Schematic representation of the treatment processes typical of modern

wastewater treatment

Nitrogen removal by the activated

sludge process (ASP)

ASP can be modified for nitrogen removal to encourage

nitrification followed by denitrification.

Establishment of nitrifying population in the activated sludge depends on the wasting rate of the sludge: BOD load, MLSS, and retention time.

The growth rate of nitrifying bacteria must be higher than the growth of heterotrophs in the system.

This is achieved by maintaining a long sludge retention time (above 4 days) for the conversion of ammonia to nitrate (nitrification), followed by denitrification.

Processes involved in the modification of ASP to encourage denitrification include:

Single sludge system (Fig. 2):

comprises a series of aerobic and anaerobic tanks in lieu of a single aeration tank.

Methanol or settled sewage serves as the source of carbon for denitrifiers.

Figure 2: Denitrification system: single sludge system

Multisludge system (Fig. 3):

Carbonaceous oxidation, nitrification and denitrification are carried out in three separate systems.

Methanol or settled sewage serves as the source of carbon for denitrifiers.

Figure 3: Denitrification system: multisludge system

Bardenpho process (Fig. 4):

Consists of two aerobic and two anoxic tanks

followed by a sludge settling tank.

Tank 1 – Anoxic: For denitrification, with wastewater

used as a carbon source.

Tank 2 – Aerobic: For both carbonaceous oxidation and nitrification. The mixed liquor from this tank, which contains NO3 is returned to tank 1

Tank 3 – Anoxic: Removes the nitrate remaining in the effluent by denitrification.

Tank 4 – Aerobic: Used to strip the nitrogen gas that results from denitrification, thus improving mixed liquor settling

Figure 4: Denitrification system: Bardenpho system

Phosphorus removal by the

activated sludge process (ASP)

Depends on the uptake of phosphorus by the

microbes during the aerobic stage and subsequent release during the anaerobic stage.

Examples of such systems are:

A/O (aerobic/oxid) process (Fig. 5):

includes an anaerobic zone (detention time 0.5-1 hour) upstream of the conventional aeration tank (detention time 1-3 hours). As in Fig. 6:

Clarifier

Return sludge

Figure 5: A/O process for phosphorous removal

Figure 6: Microbiology of the A/O process

Polyphosphate

ATP, ADP

Pi = Inorganic

phosphate

Under anaerobic conditions:

Stored phosphorus is released by MOS to

generate energy.

The energy liberated is used for the uptake of

BOD from wastewater.

Removal efficiency is high when the

BOD/phosphorus ratio exceeds 10.

When aerobic conditions are restored:

MOS exhibit phosphorus uptake levels above those normally required to support the cell maintenance, synthesis, and transport reactions required for BOD oxidation.

Excess phosphorus is stored as polyphosphates within the cell.

Bardenpho process: Removes nitrogen as well as phosphorus by a nitrification-denitrification process.

Figure 6: Microbiology of the A/O process

Polyphosphate

ATP, ADP

Pi = Inorganic

phosphate

Removal of pathogens by sewage

treatment processes

Significant removal of pathogens, especially enteric

bacteria can be achieved.

Disinfection and/or advanced tertiary treatment is

necessary for many reuse applications.

Recognition of the importance of water and food in

the transmission of new emerging enteric pathogens

have created a need for information on the ability of

treatment processes to remove those pathogens.

Compared with other biological treatment methods (e.g.

trickling filters), activated sludge is relatively efficient in

reducing the numbers of pathogens in raw wastewater.

Activated sludge typically removes 90% of the enteric

bacteria, 80 to 90-99% of the enteroviruses and

rotaviruses and 90% Giardia and Cryptosporidium.

Tertiary treatment can be effective in further reducing

the concentration of pathogens and enhancing the

effectiveness of disinfection processes by the removal

of soluble and particulate organic matter.

Filtration is probably the most common tertiary treatment process.

Mixed-media filtration is most effective in the reduction of protozoan parasites.

Coagulation, particularly with lime, can result in significant reduction of pathogens, especially enteric viruses due to the denaturation of the viral protein coat at high pH conditions (pH 11-12).

Reverse osmosis and ultrafiltration also result in significant reductions in enteric pathogens, e.g. removal of enteric viruses in excess of 99.9%.

During aeration, pathogens are removed by;

Antagonistic mos

Environmental factors e.g. Temp

Adsorption or entrapment of the organisms

within the biological flocs

SLUDGE PROCESSING

The sludge or biosolids resulting from the various

stages of sewage processing also require treatment to

stabilize their organic matter and reduce their water

content.

Primary sludge from the primary clarifier contains 3-8%

solids, and secondary sludge contains 0.5-2%.

Treating the organic matter prevents odor formation

and decreases the number of pathogens.

Reducing the water content reduces the weight

of the sludge, making it more economical to

transport it to its final disposal site.

Sludge treatment involves several steps.

Achieved by settling or centrifugation

- Anaerobic or aerobic process

-Microbial process generating high temp

-Anaerobic digestion is common (2-3 wks) & CH4 is also

produced. Affected by temp, retention time, WW

chemical composition, competition with SRB & presence

of toxic substances (e.g. heavy metals)

- Aerobic digestion (12-30 days) – low capital costs, easy

operation, & production of odorless, stabilized sludge.

H/v, greater amount of waste sludge is produced

Achieved by air drying in spreading

basins, centrifugation, or vacuum filtration

Alum; FeCl3

or lime

addition to

aggregate

suspended

particles –

Effective in

removing P,

suspended

matter &

viruses

Anaerobic Digestor

Sludges may be disposed off after digestion, but they

are usually treated further to reduce the volume of

water.

This is accomplished by “conditioning”.

Coagulation with alum, lime, or polyelectrolytes is

effective in removing suspended matter and

phosphorus from sludges. It is also effective in

removing viruses.

The PPT that forms is removed and the residual effluent

is usually passed through sand or mixed-media filters.

Pathogen Occurrence and Fate in

Biosolids

Significant numbers of the pathogens present in raw sewage often remain in sewage biosolids.

The concentration of pathogens in sewage biosolids can be fairly high because of settling and adsorption.

Most microbial species found in raw sewage are concentrated in sludge during 1o sedimentation.

The average densities of pathogenic and indicator organisms in primary sludge (biosolids) is shown in Table 4.

Different biosolids may contain significantly greater or

smaller numbers of any organism, depending on the

kind of sewage from which the biosolid was derived.

The quantities of pathogenic species also vary

depending on which kinds are present at the time.

Microbial populations in biosolids after the biological

treatment of wastewater depend on:

the initial concentrations in the Wastewater

die-off or growth during treatments

association of the organisms with the biosolids

The densities of pathogenic and indicator microbial

species in 2o sludge biosolids is shown below

Reduction of pathogens by anaerobic digestion is

both time and temperature dependent.

Thermophilic digestion (50-60 oC) and longer

detention times favor greater reduction of pathogens.

Aerobic digestion temperatures are usually

mesophilic (37oC) with a mean retention time of 10-20

days.

Conversion of organic matter into CO2 and H2O leads to decreased carbon sources for bacteria; hence reduction in number because of nutrient deprivation.

After digestion, sludges may be air-dried or treated with lime to reduce the concentration of pathogens.

Lime stabilization raises the pH of the liquid sludge to 12.0 for at least 2 hours.

NH4 ion is deprotonated, resulting in the production of ammonia (NH3) gas.

High pH and NH3 can reduce enteroviruses by 4

orders of magnitude and coliforms by 2 to 7 orders

of magnitude.

Sludge is also being treated using composting

technology.

Other nonconventional treatment or disinfection

processes such as heat drying, pasteurization, heat

treatment, and gamma irradiation also act to reduce

the numbers of pathogens before disposal.

LAND DISPOSAL OF BIOSOLIDS

Landfarming is used for disposing biosolids

produced by wastewater plants on agricultural land.

It may be added to the soil as either solids or liquids.

It adds nutrients and water to the soil.

Two categories – class A and B have been

established by U.S. Environmental Protection

Agency.

Sewage sludge applied to lawns and home gardens and is sold or given away in bags or other containers must meet the criteria for class A (Table 21.9).

All sewage sludge that is land applied must meet class B requirements (Table 21.10).

Class B sludges may be treated by “processes which significantly reduce pathogens” or PSRP (Table 21.11).

Sludges treated by PSRP may be land applied if certain restrictions are met with regard to:

Crop production: no food crops should be grown within 18 months after application.

Animal grazing: prevention of grazing for at least 1 month by animals that provide products consumed by humans.

Public access to the treated site: public access must be controlled for at least 12 months.

There are no restrictions if the sludge is treated by “processes to further reduce pathogens” or PFRP (Table 21.12).

PFRP are required if sludge is applied to edible crops.

OXIDATION PONDS (OP)

Sewage lagoons are often referred to as oxidation

or stabilization ponds.

The oldest of the wastewater treatment systems.

About a hectare in area and a few meters deep.

They are natural “stewpots” where the wastewater is

detained while organic matter is degraded.

It takes 1-4 weeks for complete degradation of the

organic matter. Light, heat, and settling of the solids

can also effectively reduce the number of pathogens

in WW.

There are four categories of oxidation ponds:

Aerobic ponds: (Fig. 9)

Naturally mixed and must be shallow

Depends on light penetration to stimulate

algal growth that promotes subsequent O2

generation

Detention time of WW is generally 3-5 days

Aerobic Waste Pond

Anaerobic ponds: (Fig. 10)

may be 1-10 m deep

require long detention time of 20-50 days

do not require expensive mechanical aeration

generate small amounts of sludge

often serve as a pretreatment step for high-BOD organic wastes rich in protein and fat with a heavy concentration of suspended solids

Anaerobic Waste Pond

Facultative ponds: (Fig. 11)

Most common for domestic waste treatment

Waste treatment provided by both aerobic and

anaerobic processes

Range in depth from 1 to 2.5 m and are subdivided

in three layers: an upper aerated zone, a middle

facultative zone, and a lower anaerobic zone

Detention time varies between 5 and 30 days

Oxidation Pond

Aerated ponds:

mechanically aerated

may be 1-2 m deep

detention time: less than 10 days

In general, treatment depends on the aeration time,

temperature and the type of wastewater

E.g. An aeration period of 5 days at 20oC result in

85% BOD removal

Sewage lagoons require minimum technology and

are relatively low in cost.

Most common in developing countries where land

is available at reasonable prices.

H/v, organic matter and turbidity are not as

effectively reduced as in activated sludge treatment.

Oxidation Pond (OP) cause significant reductions in

the concentration of enteric pathogens, given

sufficient retention times.

Inactivation and/or removal of pathogens in OP is

controlled by temperature, sunlight, pH,

bacteriophage, predation by other MOS, and

adsorption to or entrapment by settleable solids.

SEPTIC TANKS (ST)

Many rural families and residents of towns and small

cities depend on pit toilets or “outhouses” for waste

disposal.

The pit toilets often allowed untreated wastes to

seep into groundwater, allowing pathogens to

contaminate drinking water supplies.

Development of septic tanks and properly

constructed drain fields necessary owing to public

health risk.

ST serves as repositories where solids are

separated from incoming wastewater and biological

digestion of the waste organic matter can take place

under anaerobic conditions.

Typically, the wastewater and sewage enter a tank

made of concrete, metal, or fiber glass (Fig 21.17).

Grease and oils rise to the top as scum, and solids

settle to the bottom.

Anaerobic bacterial decomposition then takes place,

resulting in the production of sludge.

The WW usually remains in the septic tank for just

24-72 hours and then channeled out to a drainfield.

The drainfield or leachfield is composed of small

perforated pipes that are embedded in gravel

below the surface of the soil.

The residual septage in the septic tank is

periodically pumped out into a tank truck and taken to

a treatment plant for disposal.

The concentration of contaminants in septic tank

septage is typically much greater than that found in

domestic wastewater (Table 21.13).

ST can be an effective method of waste disposal

- where land is available and

- population densities are not too high.

They are widely used in rural and suburban areas.

STs are not appropriate for every area of a country.

STs do not work well;

in cold, rainy climates, where the drainfield may be

too wet for proper evaporation.

in areas where the water table is shallow.

High densities of STs can lead to nitrate

contamination of groundwater.

Most of the waterborne disease outbreaks associated

with groundwater in the US result from contamination

by septic tanks.

Septic Tank

LAND APPLICATION OF

WASTEWATER

Apart from discharging treated domestic wastewater

into bodies of water, it may also be disposed of via land application;

for crop irrigation and

as a means of additional treatment or disposal.

There are 3 basic methods involved.

The choice of method depends on the prevailing conditions at the site which include:

loading rates

Irrigation methods

Crops

Expected treatment

Low-rate irrigation (LRI):

Sewage effluents are applied by sprinkling or by surface application at a rate of 1.5 to 10 cm /per wk.

2/3 of the water is taken up by crops or lost by evaporation, and the remainder percolates through the soil matrix.

The system must be designed to maximize denitrification (to avoid nitrate contamination).

Phosphorus is immobilized within the soil matrix by fixation or precipitation.

LRI method is used primarily by small communities.

Requires large areas, generally 5-6 hec/1000 people.

Overland flow (OF) method:

Wastewater effluents are allowed to flow for a

distance of 50-100 m along a 2-8% vegetated slope and collected in a ditch.

The loading rate of WW ranges from 5 to 14 cm/wk.

About 10% of the water percolates through soil, while 60% runs off into the ditch.

The remainder is lost as evapotranspiration.

OF system requires clay soils with low permeability and infiltration.

High-rate infiltration:

Also referred to as soil aquifer treatment (SAT) or rapid infiltration extraction (RIX).

Treats WW at loading rates exceeding 50 cm/wk.

The treated water is used for groundwater recharge and may be recovered for irrigation.

The system requires less land than other methods.

Drying periods are often necessary to aerate the soil system and avoid problems with clogging.

The selection of a site for land application depends

on:

soil types

drainability and depth

distance to groundwater

groundwater movement

slope

underground formations

the degree of isolation of the site from the public

WETLANDS AND AQUACULTURE

SYSTEMS

Wetlands are areas that support aquatic vegetation

and foster the growth of emergent plants.

They also provide important wetland habitat for many animal species.

Wetlands are means of additional treatment for secondary effluents.

The vegetation provides surfaces for the attachment of bacteria and aids in the filtration and removal of wastewater contaminants.

Constructed or artificial wetlands have received greater attention than the natural wetlands because of regulatory requirements.

Two types of constructed wetlands in general use:

Free water surface systems (FWSS): similar to a natural marsh because the water surface is exposed to the atmosphere.

Subsurface flow systems (SFS): consists of channels or trenches with relatively impermeable bottoms filled with sand or rock media to support emergent vegetation.

Constructed Wetland Treatment System (FWSS)

Source: www.rothecologicaldesign.com/projects.php

Constructed Wetland Treatment System (SFS)

Source: www.1southtasmania.com/wetland.html

During wetland treatment, the wastewater is usable

e.g. to grow aquatic plants and/or to raise fish for

human consumption.

Constructed wetlands have a higher degree of

biological activity than most ecosystems

Results in transformation of pollutants into

harmless byproducts or essential nutrients for plant

growth.

Aquaculture system

Source: www.41south-aquaculture.com/wetland.html

Aquaculture system

Source: www.41south-aquaculture.com/wetland.html

SOLID WASTE

Two types of solid waste are generated and

must either be disposed off or treated and reused.

Municipal solid waste (MSW): originates in households as garbage, in commercial establishments, or at construction sites.

Sludge (biosolids): From municipal sewage treatment plants.

Municipal Solid Waste

More than 200 million metric tons of MSW is

produced yearly by the U.S. (or 2 kg of trash per

person per day).

About 17% is recycled, while 83% is mostly

disposed of in landfills or incinerated.

Most landfill materials are non-degradable and

remain 30-40 years later because of unfavourable

conditions for the biodegradation process.

E.g. low moisture, low oxygen concentration and high heterogeneity of materials.

Many landfills serve as “waste repositories”, releasing pollutants to the groundwater and the atmosphere.

MSW may contain a variety of enteric pathogens originating from pet feces, food waste, garden waste, and disposable diapers.

38%

7%8%7%

8%

18%

14%Paper

Food waste

Plastic

Glass

Metal

Yard waste

Others

Nature of household trash (MSW) in the US

(on a volume basis). Data from U.S.EPA, 1994

Municipal Sanitary Landfill

Modern Sanitary Landfills

Old landfills were usually located in old quarries, mines, natural depressions, or excavated holes in abandoned land.

Formation and movement of leachate, produced by the infiltration of water through the waste material results in water pollution.

Landfill leachate consists of water containing dissolved chemicals such as salts, heavy metals etc.

Anaerobic microbial processes in landfills also generate greenhouse gases, such as CH4, N2O & CO2.

Modern sanitary landfill is designed in order to meet exact standards with respect to containment of all materials, including leachates and gases.

It is designed based on the desire for minimal impact on the environment (short and long term), with particular emphasis on groundwater protection.

Landfill site selection is based on geology and soil type, as well as depth of the water table and use.

A new landfill is normally located in an excavated depression, and fresh garbage is covered daily with a layer of soil.

The bottom of the landfill is lined with a low-

permeability liner made out of high density plastic or

clay.

Provisions are also made to collect and analyze

leachate and gases emanating from the landfill.

High temperature generated inactivates most

enteric pathogens but abundance of nutrients

allows growth of indicator bacteria.

Composting of Biosolids and

Domestic Solid Waste

Involves biological decomposition of the organic

matter of solid waste under controlled aerobic conditions.

Heat produced during the decomposition destroys human pathogens, including many that survive other treatment methods.

Composting results in products that can be safely handled, easily stored, and readily applied to the land without adverse environmental effect.

Large debris and easily recycled materials such as metals (e.g. aluminum cans, batteries), plastics, etc must be removed first during composting process.

The remaining solids are then ground and mixed.

The composting mixture consists of raw sewage sludge mixed with wood chips as a bulking agent.

The mix is aerated and biological processes decompose the organic matter, thus generating temperatures high enough to destroy any pathogenic MOS.

There are three categories of composting system:

Windrow systems (Fig. 21.23)

The MSW mixtures are

composted in long rows

(called windrows) and

aerated by convective

air movement and

diffusion.

The mixtures are turned

periodically by mechanical

means to expose the

organic matter to ambient

oxygen.

Advantages

Rapid drying of the compost.

Easier separation of bulking agent from the compost during screening and relatively high rates of recovery for bulking materials.

High volume of material can be utilized.

Good product stabilization.

Relatively low capital investment.

Static pile (or forced-

aeration) system

Piles of MSW mixture are

aerated by using a forced-aeration system.

The aeration system is installed under the piles to maintain a minimum oxygen level throughout the compost mass.

Advantages

Low capital costs.

A high degree of pathogen destruction. The insulation over the pile and uniform aeration throughout the pile help maintain pile temperatures that destroy pathogens.

In-vessel (mechanical or enclosed

reactor) systems

Takes place in a partially or

completely enclosed

container in which

environmental conditions

can be controlled.

It may incorporate the

features of windrow and/or

static pile methods of

composting.

Advantages

Better odor control than

windrow composting

Good product

stabilization.

Space efficiency.

Better process control

than outdoor operations.

Protection from adverse

climatic conditions.

Potential heat recovery

depending on system

design.

In-Vessel Composting site

In-Vessel Composting

COMPOST

The end product of the composting process.

It is a stable, humus-like substance with valuable properties as a soil conditioner.

It contains several macro- and micronutrients favorable to plant growth but not enough nitrogen to be considered as fertilizer.

Although, usually pathogen free, it is not completely stabilized (i.e. the organic matter is not 100% degraded).

It is stabilized enough to reduce the potential for odor generation, thus allowing the product to be stored and marketed.

The oxygen required for biological processes can be supplied in two ways:

By mechanical turning of the mixture to periodically expose the compost to atmospheric oxygen as in windrow approach

By using a blower to force or draw air through the mix, as in the static pile approach

Composting usually takes 3-4 weeks, followed by curing of the samples for about 30 days.

This leads to further decomposition, stabilization, and degassing.

Additional drying stage varying from a few days to

several months is included in some systems.

During composting, fungi, especially the genus

Aspergillus may grow to large numbers.

A. fumigatus is commonly found in composting

vegetation, wood chip piles, MSW compost, refuse

sludge compost, and moldy hay.

It grows over a range of 20 to about 50oC.

Spores of the fungus can cause bronchopulmonary hypersensitivity, marked by asthmatic spasm, fever, and malaise.

A. fumigatus constitutes 75% of the total viable microflora of air at the composting site.

Composting is a thermophilic process generating temperature (55 to 70oC) adequate to kill enteric pathogens.

Viruses could survive 25 days of composting if the composting mass did not achieve adequately high temperatures.

High concentration of fecal coliforms can also be present if maximum composting temperature is less than 50oC.

Temperature maintained above 53oC for 3 days are sufficient to eliminate enteric pathogens.