Lecture 7-Wastewater & Activated Sludge Treatment

7/25/2019 Lecture 7-Wastewater & Activated Sludge Treatment

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WASTEWATER TREATMENT WASTEWATER MICROBIOLOGY OVERVIEW

CLASIFICATION OF MICROORGANISMS:

By kingdoms: animals, plants, protista, fungi, bacteria.

By energy and carbon source:

Carbonis the basic building block for cell synthesisexamples

Heterotrophic organic material as carbon source.

AutotrophicCO2as carbon source

Energyfor cell synthesis

PhototrophsSolar energy

ChemotrophsOrganic or inorganic oxidation /reduction


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WASTE WATER MICROBIOLOGY

OVERVIEW(contd.) By their relationship to oxygen

Obligate aerobes: Must have oxygen as the terminal electron acceptor Aerobic system

Obligate anaerobes: Cannot survive in the presence of oxygen Anaerobic system

Facultative anaerobes Can grow in the absence and presence of oxygen

Anoxic Facultative anaerobes (denitrifiers) Utilizes nitrites (N02-) and nitrates (N03-) as terminal electron

acceptor By their preferred temperature regime :

Psychrophiles 25 to 40C Thermophiles > 60C


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BACTERIAL METABOLISM


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DECOMPOSITION OF WASTE

The type of electron acceptor available for

catabolism determines the type of decomposition. AEROBIC DECOMPOSITION: Applied to large quantities of dilute wastewater

(BOD5


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DECOMPOSITION OF WASTE (contd.)

ANOXIC DECOMPOSITION: Uses nitrogen e.g. N02- , N03- as terminal electron

acceptor in absence of molecular oxygen Used to reduce nitrogen content of water to control

water pollution

ANAEROBIC DECOMPOSITION : Molecular Oxygen and nitrate must be absent as

terminal electron acceptors Decomposition is a two stage process

complex organics are fermented to low molecular weight fatty acids (volatile acids)

Organic acids are converted to methane Process is reduction with electron acceptance e . g

CO2, SO4 Low energy is released Cell production is low Characterized by obnoxious odor.


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DECOMPOSITION OF WASTE

(contd.)


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BACTERIAL GROWTH REQUIREMENTS

Major requirements to be satisfied for bacterial growth : A terminal electron acceptor Macronutrients Carbon to build cells Nitrogen to build cells Phosphorus to ATP (energy carrier) and DNA Micronutrients Trace metals Vitamins required by some bacteria Appropriate Environment

Moisture Temperature pH


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BACTERIAL GROWTH IN MIXED

CULTURES

In wastewater treatment a mixture of species competeand survive within the limits set by the environmentgoverned by the principles of synergy and symbiosis.

Pure cultures of microorganisms do not exist Population dynamics is the description of the time

varying success of the various species in competitionwith each other.

The dynamics of microbial population is governed by;

Competition for food Predator prey relationship


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CULTURES (population dynamics)

Population dynamics in activated sludge process(closed system)


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CULTURES (population dynamics) contd.

Population dynamics in activated sludge process (opensystem)


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MUNICIPAL WASTEWATER

TREATMENT SYSTEMS


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TREATMENT UNITS Pre- treatment: Bar rack , Grit chamber , equalization basin

Purpose : To provide protection to the wastewater treatmentplant (WWTP) equipment that follow.

Primary treatment: Pre treatment plus primary setting. Main goal : To remove pollutants that will either settle or

floatable. To remove about 60% suspended solids and 35% BOD in raw

sewage. Secondary treatment:Primary treatment plus biological

treatment. Major goal : Remove the soluble BOD that escapes primary

treatment processes and to provide additional removal ofsuspended solids . These goals are achieved by usingbiological processes.

Tertiary treatment: Major goal : Removal of excess nitrogen , phosphorus , heavy

metals , pathogenic microorganisms.

Process may involve physical e.g. filtration , chemicaltreatment , biological land application etc.


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MUNICIPAL WASTEWATER TREATMENT

IN MALAYSIA


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TREATMENT UNITS(contd.)

UNIT PROCESSES OF SECONDARY TREATMENT:

The major purpose of secondary treatment is :

To remove the soluble BOD5 that escapesprimary treatment .

Further removal of suspended solids. Common approaches used are :

Trickling filters (TF)

Activated sludge (AS)

Oxidation ponds (Lagoons)

Rotating Biological Contactors (RBC) a hybridof TF + AS.


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TRICKLING FILTER

Description : Consist of a bed of coarse material e.g. stones , slats ,

plastic materials over which wastewater is applied

A microbial growth is established on the surface of the stoneor packing forming a fixed film.

The wastewater passes over the stationary fixed filmmicrobial population which utilizes the organic content forgrowth and energy.

Excess growth of microorganisms wash from the mediasurface and goes into the sedimentation basin known assecondary clarifier or final clarifier

Disinfection is carried out in some cases after which theeffluent is discharged into water bodies.

Sludge accumulated in the final clarifier is taken out forfurther treatment and disposal.


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TRICKLING FILTER (contd.)

Some portion of the effluent is returned into the trickling filterknown as Recirculation

The ratio of returned flow to incoming is called recirculationratio (Qr)

Some benefits of recirculation

To increase contact efficiency To dampen variation in plant loadings To increase the DO of the influent. To improve distribution over the surface , thus reducing

the tendency to clog the pipes as well as reduce flies. To prevent the biological slimes from drying out and

dying during low flows at night. Two stage filters may be used to improve the

performance of filters. The second stage acts as apolishing step for the effluent.


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Two Stage Trickling Filter

Provides a means of improving the performance offilters.

The second stage is a polishing step by providingadditional contact time between the waste and themicroorganisms

Both stages may have the same media or differentmedia.

Selection is based on desired treatment efficiencies and

an economic analysis of the alternatives.


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ACTIVATED SLUDGE Description : Principle :

A mixture of wastewater and biological sludge(microorganisms) is mixed and agitated and aerated.

Biological solids are separated from the treated wastewaterand aeration process.

Method : Air is injected to mix with the wastewater and activated

sludge to supply oxygen needed by the organisms to breakdown the organics.

The mixture of activated sludge and wastewater in theaeration tank is known ass MIXED LIQUOR

The mixed liquor flows into the secondary clarifier wherethe activated sludge settles out.

Most of the settled sludge is returned to the aeration tank.This helps in maintaining a high population of microbes toperform rapid breakdown of the organics.

The remaining activated sludge called waste activatedsludge (WAS) is sent for treatment and disposal.


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ACTIVATED SLUDGE

Conventional Activated Sludge Treatment System


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DESIGN OF ACTIVATED SLUDGE

TREATMENT SYSTEMS

Assumption made in the development of AS designequations :

The influent and effluent biomass concentrations arenegligible compared to that in the reactor.

The influent food (So) is immediately diluted to thereactor (homogeneously mixed)

All biochemical reactions occur in the CSTR.

Complete mixed activated sludge design principle(CSTR)

It involves the mass balance application of thekinetics equation of microbial growth


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TREATMENT SYSTEMS

Completely mixed Biological Reactor with Solids Recycle


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DESIGN OF ACTIVATED SLUDGE TREATMENT

SYSTEMS (contd.)

Mass balance for biomass Biomass accumulated= Influent Biomass-Biomass in effluent +

Biomass wasted

Q= wastewater flow rate into aeration tank , m3/d

Xo = microorganism concentration (volatile suspended solids , VSS) enteringthe aeration tank , mg/L V = Volume of aeration tank , m3

m= maximum growth rate constant , d-1

S= soluble BOD5in aeration tank and effluent , mg/L X = microorganism concentration (mixed liquor volatile suspended solids

,MLVSS) in aeration tank mg/L

ks= half velocity constant = soluble BOD5concentration at the maximumgrowth rate ,mg/L

kd= decay rate of microorganisms d-1

Qw= flow rate of wasted activated sludge , m3/d

Xe= microorganism concentration (VSS) in effluent , mg/L Xr= microorganism concentration (VSS) in wasted activated sludge , mg/L

rwewd

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TREATMENT SYSTEMS (contd.)

Assumption that the influent and effluent biomass concentrationsare negligible compared to that in the reactor eliminates QXoand(Q-Qw)Xe

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IMPORTANT NOTES : Once the mean cell residence Ocis selected the

concentration of soluble BOD5,S in the effluent is fixed :

The soluble BOD5, S , is only affected by the mean cell-residence time and the amount of BOD5 entering thereactor.

To achieve a desired effluent quality both soluble andinsoluble fractions of BOD5must be considered.

To use equation some estimate of the BOD5 of thesuspended solids must be made first . This value is thensubtracted from the total allowable BOD5to find theallowable S ,

S = Total BOD5allowed BOD5in suspended solids

1)(

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dmc

cds

k

kKS



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The concentration of microorganisms in the aeration tank is a functionof the mean cell-residence time, hydraulic detention time, and thedifference between the influent and effluent substrate concentrations :

SLUDGE RETURN FLOW RATE (Qr)

Q = wastewater flow rate m3/d Qr = return sludge flow rate m3/d

X=mixed liquor suspended solids (MLSS) gm/m

3

Xrr = maximum return sludge concentration g/m3

Qw= sludge wasting flow rate

)1(

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Sludge Volume Index relationship with sludge return

concentration X


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TREATMENT SYSTEMS (contd.)SVI & Qr


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DESIGN OF ACTIVATED SLUDGE TREATMENT

SYSTEMS (contd.) SVI & Temperature


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SLUDGE PRODUCTION

Px = net waste activated sludge produced each day in terms ofVSS , kg/d

Yobs= observed yield , kg MLVSS/kgBOD5removed

Note: VSS is equal to 60 80 % MLVSS

)/10)(( 3 gkgSSQYPoobsx

cd

ob s

k

YY

1


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OXYGEN REQUIREMENTS

Mass of Oxygen required assuming the cell biochemicaloxidation reaction,

C5H7NO2+ 5O25CO2+2H2O + NH3 + energy

f= conversion factor for converting BOD5to ultimate BODL

Note : Take account of the purity of Oxygen in air.

Take account of the Oxygen transfer efficiency(dissolution of Oxygen) into the wastewater.

x

o

Pf

SSQMO 42.110

)(3

2


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Process Design Considerations

The SRT (c) selected for the design is a function of the degree oftreatment required.

A high SRT( or older sludge age) results in higher quantity of solids beingcarried in the system and higher degree of treatment being achieved.

A long SRT also results in the production of less waste sludge. Municipal wastewater generally contain sufficient nitrogen and phosphorus

to support biological growth. Industrial wastes are generally deficient in nutrients and supplemental

nitrogen and phosphorus may be required. The ratio of nitrogen to BOD5should be 1:32 The ration of phosphorous to BOD5should be 1:150 Toxic metals and organics should be removed during the pretreatment

program to avoid precipitation into the waste sludges leading to hazardouswaste generation.

Volatile organics may be stripped from solution into the atmosphere

creating an air pollution problem. Surface skimming equipment should be installed in the secondary

treatment system to remove oil and grease. Grease balls on the surface of the aeration tank cannot be degraded by

the microorganisms because it is not in the water to enable physicalcontact to take place.


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Sludge problems

Bulking Sludge This is sludge with poor settling characteristics and poor

compactability

Types of bulking sludge:

Caused by filamentous organisms

Water trapped in the bacterial floc causing reduction indensity of the agglomerate and resulting in poor settling.

Long slow moving collection system transport

Low availability of ammonia due to high organic loading

Low pH which favours acid-favouring fungi which tend topredominate

Lack of micro-nutrients, which stimulates predomination ofthe filamentous actinomycetes over the normal floc-forming bacteria

Lack of nitrogen also favours slime-producing bacteriawhich have low specific gravity.


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Sludge problems (contd.)

Rising Sludge

Sludge which floats to the surface after an apparently goodsettling.

Results from denitrification i.e. reduction in nitrates andnitrites to nitrogen gas in the sludge blanket(layer).

The gas is trapped in the sludge blanket causing globs ofsludge to rise to the surface to the effluent leading to waterpollution

Solution of rising sludge problem

Increasing the rate of return of sludge flow(Qr)

Increasing the speed of the sludge collecting mechanism Decreasing the mean cell residence time

Decreasing the flow from the aeration tank to the clarifier/settling tank


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ADVANCED WASTEWATER TREATMENT

Used to remove pollutants which escape secondarytreatment e.g nitrogen compounds , phosphoruscompounds , soluble CBOD , heavy metals etc.

Carried out to achieve high quality effluent which canbe used as a source of raw water for treatment topotable levels (Class A water according to DOEclassification)


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(contd.)

TREATMENT PROCESSES Filtration

Removes residual suspended solids , unsettledmicroorganisms , residual BOD5.

Carbon Adsorption Removal of refractory organics by using activated

carbon

Chemical Precipitation

Removal of phosphorus by using ferric chloride, lime

, alum.

Conversion of NH3to NH4by pH increase and to N2 byair stripping


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(contd.)

TREATMENT PROCESSES (contd.) Biological process

Nitrification /denitrification Two stage process

Conversion of NH3, NH4 into N03by nitrifyingbacteria

Anoxic denitrification by bacteria by convertingN03to nitrogen gas N2

Land treatment Application of effluents from secondary treatment on land

using water application methods employed in irrigation e.gslow rate , overland flow , rapid infiltration

The wastewater and nutrients its contains is exploited as aresource.

Land treatment of effluent provide moisture and nutrientsnecessary for crop growth .This would be a valuableresource in arid and semi arid areas of the world.

L d t t t


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Land treatment


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SLUDGE TREATMENT Sources of sludge and characteristics

Grit : made up of sand , broken glass cloth etc collected in thegrit chamber.

These are non biodegradable so they are disposed off in asanitary landfill site.

Primary or raw sludge: Sludge at the bottom of primary clarifiers. Contains 3 to 3 % solids Contains about 70% organic. Rapidly turns anaerobic.

Secondary Sludge Contains microorganisms and inert materials from

clarifiers. Contains 0.5 to 2 % solids for WAS and 2 to 5% for TF.

Contains about 90% organic matter. Tertiary Sludges:

Depends on the nature of the process i.e physical,biological or chemical


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SLUDGE TREATMENT....

Thickening:Separating as much water as possible bygravity or flotation

Stabilization: Converting organics solids to more inert formfor easy handling or for use as soil conditioners i.e.

digestion( biochemical oxidation processes). Conditioning: Treating sludge with chemicals or heat so

that the water can be readily separated.

Dewatering: Separating water by subjecting the sludge tovacuum, pressure or drying.

Reduction: Converting the solids to a stable form by wetoxidation or incineration: Chemical oxidation processessludge volume decrease).


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SLUDGE TREATMENT...

UTILIZATION OF SLUDGE

Bioconversion into useful products e,g. composting ,activated carbon , bio-fuels e.g. methane, bio-hydrogen bio-ethanol,

Chemical recovery e.g. lime recovery

Controlled crops : Using irrigation techniques.

SLUDGE TREATMENT


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SLUDGE TREATMENT

& DISPOSALPROCESSES


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Assignment on Activated Sludge (AS) Systems.

The town of Gatesville has been directed to upgrade itsprimary WWTP to a secondary plant that can meet an effluentstandard of 30 mg/L BOD and 30 mg/L suspended solids (SS).They have selected a completely mixed activated sludgesystem. Assuming that the BOD of the estimated SS may beestimated equal to 63% of the SS concentration, estimate the

required volume of the aeration tank. The following data areavailable from the existing primary plant.Existing plant effluent characteristicsFlow = 0.15 m3/sBOD = 84 mg/LAssume the following values for the growth constants:

Ks= 100 mg/L BOD; m= 2.5 day-1; kd= 0.050 day-1; Y =0.50 mg VSS/ mg BOD removed; MLVSS = 2000 mg/L

Documents

Lecture 7-Wastewater & Activated Sludge Treatment