Activated Sludge Processes CE - 370. Basic Process The basic AS process consists of The basic AS process consists of A reactor in which the microorganisms

Activated Sludge ProcessesActivated Sludge Processes

CE - 370CE - 370

Basic ProcessBasic Process

The basic AS process consists ofThe basic AS process consists ofA reactor in which the microorganisms responsible for A reactor in which the microorganisms responsible for

treatment are kept in suspension and aeratedtreatment are kept in suspension and aeratedLiquid-solids separation, usually sedimentation tankLiquid-solids separation, usually sedimentation tankA recycle system for returning solids removed from the A recycle system for returning solids removed from the

liquid-solids separation unit back to the reactor liquid-solids separation unit back to the reactor

Important feature of the AS process is:Important feature of the AS process is:Formation of flocculent settleable solids that can be Formation of flocculent settleable solids that can be

removed by gravity settlingremoved by gravity settling

Activated Sludge process utilizes:Activated Sludge process utilizes: Fluidized microorganismsFluidized microorganisms Mixed growth microorganismsMixed growth microorganisms Aerobic conditionsAerobic conditions

MicroorganismsMicroorganisms Use organic materials in wastewater as substratesUse organic materials in wastewater as substrates Thus, they remove organic materials by microbial respiration and Thus, they remove organic materials by microbial respiration and

synthesissynthesis

MLSSMLSS Ranges between 2000 and 4000 mg/lRanges between 2000 and 4000 mg/l

FlowsFlows Feed wastewater (Q)Feed wastewater (Q) Waste activated sludge (QWaste activated sludge (Qww))

Recycled activated sludge (R)Recycled activated sludge (R) Prior to entering aeration tankPrior to entering aeration tank OR immediately after enteringOR immediately after entering

Oxygen SupplyOxygen SupplyDiffused compressed airDiffused compressed airMechanical surface aerationMechanical surface aerationPure oxygenPure oxygen

Purposes of aerationPurposes of aerationProvides oxygen required for aerobic bio-oxidationProvides oxygen required for aerobic bio-oxidationProvides sufficient mixing for adequate contact Provides sufficient mixing for adequate contact

between activated sludge and organic substancesbetween activated sludge and organic substances In order to maintain the desired MLSS in the In order to maintain the desired MLSS in the

aeration tank, R/Q ratio must be calculatedaeration tank, R/Q ratio must be calculated

Calculate (R / Q) RatioCalculate (R / Q) Ratio

Calculate the Sludge Density Index (SDI)Calculate the Sludge Density Index (SDI) Sample MLSS from downstream of aeration tankSample MLSS from downstream of aeration tank Determine SS in MLSSDetermine SS in MLSS Place 1 liter of the MLSS in 1-liter graduate cylinderPlace 1 liter of the MLSS in 1-liter graduate cylinder Settle the sludge for 30 minutesSettle the sludge for 30 minutes Measure volume occupied by settled sludgeMeasure volume occupied by settled sludge Compute SS in settled sludge in mg/lCompute SS in settled sludge in mg/l SS represents SDISS represents SDI The test approximates the settling that occurs in final clarifierThe test approximates the settling that occurs in final clarifier

If SDI = 10,000 mg/l and MLSS must be 2,500 mg/lIf SDI = 10,000 mg/l and MLSS must be 2,500 mg/l Then, Q(0) + R(10,000) = (Q+R)(2500)Then, Q(0) + R(10,000) = (Q+R)(2500) R/Q = (2500)/(7500) = (1/3) = 33.3 %R/Q = (2500)/(7500) = (1/3) = 33.3 % So, R is 33.3% of feed wastewater (Q)So, R is 33.3% of feed wastewater (Q)

Sludge Volume Index (SVI) = 1/ SDISludge Volume Index (SVI) = 1/ SDIIs the volume in ml occupied by 1 gram of settled Is the volume in ml occupied by 1 gram of settled

activated sludgeactivated sludgeIt is a measure of settling characteristics of sludgeIt is a measure of settling characteristics of sludgeIs between 50 and 150 ml/gm, if process is operated Is between 50 and 150 ml/gm, if process is operated

properly properly

Why QWhy Qww??Microbes utilize organic substances for respiration and Microbes utilize organic substances for respiration and

synthesis of new cellssynthesis of new cellsThe net cell production (QThe net cell production (Qww) must be removed from the ) must be removed from the

system to maintain constant MLSSsystem to maintain constant MLSSQw is usually 1 to 6 % of feed wastewater flowrate (Q)Qw is usually 1 to 6 % of feed wastewater flowrate (Q)

Common organic materials in municipal wastewater Common organic materials in municipal wastewater are:are:

Carbohydrates (C, H, O0Carbohydrates (C, H, O0 Fats (C, H, O)Fats (C, H, O) Proteins (C, H, O, N, S, P)Proteins (C, H, O, N, S, P) Urea (C, H, O, N)Urea (C, H, O, N) Soaps (C, H, O)Soaps (C, H, O) Detergents (C, H, O, P)Detergents (C, H, O, P) Traces of Traces of

PesticidesPesticides HerbicidesHerbicides Other agricultural chemicalsOther agricultural chemicals

Activated sludge can be represented by:Activated sludge can be represented by: CC55HH77OO22NN

Has a molecular weight of 113Has a molecular weight of 113

DesignDesign To design of AS, the following must be determined:To design of AS, the following must be determined:

Volume of reactorVolume of reactor Number of basinsNumber of basins Dimensions of each basinDimensions of each basin

Volume of reactor is determined from:Volume of reactor is determined from: Kinetic relationshipsKinetic relationships Space loading relationshipsSpace loading relationships Empirical relationshipsEmpirical relationships

Sludge production per day (XSludge production per day (Xww), kg/day), kg/day

Oxygen required per day (OOxygen required per day (Orr), kg/day), kg/day

Final clarifierFinal clarifier Number of basinsNumber of basins

Biological KineticsBiological Kinetics

1. Michaelis – Menten Concept1. Michaelis – Menten Concept

(1/X)(ds/dt) = specific rate of substrate utilization(1/X)(ds/dt) = specific rate of substrate utilization (ds/dt) = rate of substrate utilization(ds/dt) = rate of substrate utilization kkss = maximum rate of substrate utilization = maximum rate of substrate utilization

KKmm = substrate concentration when the rate of utilization is half = substrate concentration when the rate of utilization is half

maximum ratemaximum rate S = substrate concentrationS = substrate concentration

SK

Sk

dt

dS

X ms

1

If S is very large, Km can be If S is very large, Km can be neglected, therefore S neglected, therefore S cancels out and the reaction cancels out and the reaction is zero order in substrate. K is zero order in substrate. K is the rate constant for zero-is the rate constant for zero-order reaction.order reaction.

If S is relatively small, it If S is relatively small, it can be neglected in the can be neglected in the denominator and the denominator and the reaction is first-order in reaction is first-order in substrate. K is the rate substrate. K is the rate constant for the first-order constant for the first-order reactionreaction

)2......(1

Kkdt

dS

X s

)3......()(1

KSSK

k

dt

dS

X m

s

)1.......(1

SK

Sk

dt

dS

X ms

Rearrange and integrate Equation (2)Rearrange and integrate Equation (2)

X = average cell mass concentration during the biochemical X = average cell mass concentration during the biochemical reaction, that is reaction, that is X = (XX = (X00 + X + Xtt)/2)/2

SStt = substrate concentration at time t = substrate concentration at time t

SS00 = substrate concentration at time t = 0 = substrate concentration at time t = 0

)4......(0

0

00

tXKSS

or

tXKSS

yields

dtXKdS

t

t

tS

S

t

Rearrange and integrate Equation (3)Rearrange and integrate Equation (3)

X = average cell mass concentration during the biochemical X = average cell mass concentration during the biochemical reaction, that is reaction, that is X = (XX = (X00 + X + Xtt)/2)/2

SStt = substrate concentration at time t = substrate concentration at time t

SS00 = substrate concentration at time t = 0 = substrate concentration at time t = 0

)5......(lnln

lnln

0

0

00

tXKSS

or

tXKSS

yields

dtXKS

dS

t

t

tS

S

t

Equations (4) and (5) are in the form ofEquations (4) and (5) are in the form ofy = mx + by = mx + bPlotting SPlotting Stt on y-axis versus on y-axis versus Xt on the x-axis on Xt on the x-axis on

arithmetical paper produce a straight line with a arithmetical paper produce a straight line with a slope of –Kslope of –K

Plotting SPlotting Stt on y-axis versus on y-axis versus Xt on the x-axis on Xt on the x-axis on

semilog paper produce a straight line with a slope semilog paper produce a straight line with a slope of -Kof -K

The substrate could beThe substrate could beThe BODThe BOD55

Biodegradable part of CODBiodegradable part of CODBiodegradable fraction of TOCBiodegradable fraction of TOCBiodegradable of any other organic matterBiodegradable of any other organic matter

Rate Constant, KRate Constant, KDepends on the specific wastewaterDepends on the specific wastewaterFor domestic wastewater, it ranges between 0.1 to 1.25 For domestic wastewater, it ranges between 0.1 to 1.25

liter/(gram MLSS)(hr) using BODliter/(gram MLSS)(hr) using BOD55

Should be determined using lab-scale or pilot-scale Should be determined using lab-scale or pilot-scale studiesstudies

In the absence of studies, K between 0.1 and 0.4 In the absence of studies, K between 0.1 and 0.4 liter/(gram MLSS)(hr) is recommendedliter/(gram MLSS)(hr) is recommended

Example on Biochemical KineticsExample on Biochemical Kinetics

Food to Microorganism Ratio (F/M)Food to Microorganism Ratio (F/M)

F/M ratio is equal to the specific rate of substrate F/M ratio is equal to the specific rate of substrate utilization (1/X)(dS/dt)utilization (1/X)(dS/dt)

The units of F/M ratio are (mass substrate) / (mass The units of F/M ratio are (mass substrate) / (mass microbes) microbes) (time) (time)

(kg BOD(kg BOD55/kg MLVSS-day)/kg MLVSS-day)

tX

S

M

F

Mean Cell Residence Time (Mean Cell Residence Time (cc))

It is defined as:It is defined as:

X = active biological solids in the reactorX = active biological solids in the reactor X = active biological solids in the waste activated sludge flowX = active biological solids in the waste activated sludge flow

Units of Units of cc is days is days Mean cell residence time is sometimes referred to as Mean cell residence time is sometimes referred to as

sludge agesludge age

wc X

X

F/M Ratio and F/M Ratio and cc

Both parameters are used characterize the Both parameters are used characterize the performance of the activated sludge processperformance of the activated sludge process

A high F/M ratio and a low A high F/M ratio and a low cc produce filamentous produce filamentous

growth that have poor settling characteristicsgrowth that have poor settling characteristicsA low F/M ratio and a high A low F/M ratio and a high cc can cause the biological can cause the biological

solids to undergo excessive endogenous degradation and solids to undergo excessive endogenous degradation and cell dispersioncell dispersion

For municipal wastewaterFor municipal wastewatercc should be at least 3 to 4 days should be at least 3 to 4 days

If nitrification is required, If nitrification is required, cc should be at least 10 days should be at least 10 days


Relationship between Relationship between cc and F/M ratio can be derived and F/M ratio can be derived by starting with the equation of cell production, as by starting with the equation of cell production, as follows:follows:

((X/X/t) = rate of cell production, mass/timet) = rate of cell production, mass/time Y = cell yield coefficient, mass cell created/mass substrate removedY = cell yield coefficient, mass cell created/mass substrate removed kkee = endogenous decay, mass cells/(total mass cells) = endogenous decay, mass cells/(total mass cells) (time) (time) X = average cell concentration, massX = average cell concentration, mass

Xkt

SY

t

Xe


Divide by Divide by XX

c is the average time a cell remains in the system, c is the average time a cell remains in the system, thusthus

ekX

tSY

X

tX

//

tX

Xc

/


The F/M ratio is the rate of substrate removal per unit The F/M ratio is the rate of substrate removal per unit weight of the cells, thusweight of the cells, thus

ThusThus

X

tS

M

F

/

ec

kM

FY

1


Since F/M was also expressed as:Since F/M was also expressed as:

Then, Then,

tX

S

M

F

ec

ktX

SY

1

Types of ReactorsTypes of Reactors

Plug-flow reactorsPlug-flow reactors Dispersed plug-flow reactorsDispersed plug-flow reactors Completely-mixed reactorsCompletely-mixed reactors

Plug-flow and Dispersed-flow Plug-flow and Dispersed-flow ReactorsReactors

In plug-flow reactors, there is negligible In plug-flow reactors, there is negligible diffusion along the flow path through the diffusion along the flow path through the reactorreactor

In dispersed-flow reactors, there is significant In dispersed-flow reactors, there is significant diffusion along the flow path through the diffusion along the flow path through the reactorreactor

Both types of reactors are used in conventional Both types of reactors are used in conventional and tapered aeration activated sludgeand tapered aeration activated sludge

Conventional Activated SludgeConventional Activated Sludge

Rectangular aeration tankRectangular aeration tank F/M = 0.2 to 0.4 (kg BODF/M = 0.2 to 0.4 (kg BOD55/kg MLSS-day)/kg MLSS-day)

Space loading = 0.3 to 0.6 (kg BODSpace loading = 0.3 to 0.6 (kg BOD55/day-m/day-m33) )

cc = 5 to 15 (days) = 5 to 15 (days)

Retention time (aeration tank) = 4 to 8 (hours)Retention time (aeration tank) = 4 to 8 (hours) MLSS = 1500 to 3000 (mg/l)MLSS = 1500 to 3000 (mg/l) Recycle ratio (R/Q) = 0.25 to 1.0Recycle ratio (R/Q) = 0.25 to 1.0 Plug-flow and Dispersed-flowPlug-flow and Dispersed-flow BOD removal = 85 to 95 (%)BOD removal = 85 to 95 (%)

Tapered AerationTapered Aeration

It is a modification of the conventional processIt is a modification of the conventional process F/M = 0.2 to 0.4 (kg BODF/M = 0.2 to 0.4 (kg BOD55/kg MLSS-day)/kg MLSS-day)


cc = 5 to 15 (days) = 5 to 15 (days)

Retention time (aeration tank) = 4 to 8 (hours)Retention time (aeration tank) = 4 to 8 (hours) MLSS = 1500 to 3000 (mg/l)MLSS = 1500 to 3000 (mg/l) Recycle ratio (R/Q) = 0.25 to 1.0Recycle ratio (R/Q) = 0.25 to 1.0 Plug-flow and Dispersed-flowPlug-flow and Dispersed-flow BOD removal = 85 to 95 (%)BOD removal = 85 to 95 (%)

Oxygen Demand versus Reactor Oxygen Demand versus Reactor Length for Municipal WastewaterLength for Municipal Wastewater

Quarter of ReactorQuarter of ReactorOO22 Demand/total O Demand/total O22 for for

entire reactor (%)entire reactor (%)

11stst3535

22ndnd2626

33rdrd2020

4th4th1919

PerformancePerformance

is the detention time for the plug-flow reactoris the detention time for the plug-flow reactor

The volume of the plug-flow or dispersed-flow The volume of the plug-flow or dispersed-flow reactor is given by:reactor is given by:

XKt eS

S 0

)( RQV

Completely Mixed ReactorsCompletely Mixed Reactors

Usually circular and square aeration tanksUsually circular and square aeration tanks F/M = 0.1 to 0.6 (kg BODF/M = 0.1 to 0.6 (kg BOD55/kg MLSS-day)/kg MLSS-day)


cc = 5 to 30 (days) = 5 to 30 (days)

Retention time (aeration tank) = 3 to 6 (hours)Retention time (aeration tank) = 3 to 6 (hours) MLSS = 2500 to 4000 (mg/l)MLSS = 2500 to 4000 (mg/l) Recycle ratio (R/Q) = 0.25 to 1.5Recycle ratio (R/Q) = 0.25 to 1.5 Completely mixedCompletely mixed BOD removal = 85 to 95 (%)BOD removal = 85 to 95 (%)

Design ParametersDesign Parameters

The retention time and reactor volume for completely The retention time and reactor volume for completely mixed reactors can be determined by:mixed reactors can be determined by:

QV

SXK

SS

t

ti

Process ModificationsProcess Modifications

Objective of modificationsObjective of modifications ModificationsModifications

Step aerationStep aerationModified aerationModified aerationContact stabilizationContact stabilizationHigh-rate aerationHigh-rate aerationExtended aerationExtended aeration

Objectives of ModificationObjectives of Modification

Several modifications of the activated sludge Several modifications of the activated sludge process were made to attain a particular or process were made to attain a particular or design objectivedesign objective

Step AerationStep Aeration

It was developed to even out the oxygen It was developed to even out the oxygen demand of the MLSS throughout the length of demand of the MLSS throughout the length of the reactorthe reactor

It uses plug-flow and dispersed plug-flow It uses plug-flow and dispersed plug-flow reactors with step inputs of the feed flow (Q) reactors with step inputs of the feed flow (Q)

Design ParametersDesign Parameters = 3-5 hrs; = 3-5 hrs; cc = 5-15 days; R/Q = 25-75%; MLSS = = 5-15 days; R/Q = 25-75%; MLSS =

2000-3500 mg/l; BOD2000-3500 mg/l; BOD55 and SS removal = 85-95%; F/M and SS removal = 85-95%; F/M = 0.2-0.4 kg/kg-day; space loading = 0.6-1.0 kg = 0.2-0.4 kg/kg-day; space loading = 0.6-1.0 kg BODBOD55/day-m/day-m33

Modified AerationModified Aeration

Designed to provide a lower degree of Designed to provide a lower degree of treatment than the other activated sludge treatment than the other activated sludge processesprocesses

It uses plug-flow and dispersed plug-flow It uses plug-flow and dispersed plug-flow reactorsreactors

Design ParametersDesign Parameters = 1.5-3 hrs; = 1.5-3 hrs; cc = 0.2-0.5 days; R/Q = 5-15%; MLSS = = 0.2-0.5 days; R/Q = 5-15%; MLSS =

200-500 mg/l; BOD200-500 mg/l; BOD55 and SS removal = 60-75%; F/M = and SS removal = 60-75%; F/M =

1.5-5.0 kg/kg-day; space loading = 1.2-2.4 kg 1.5-5.0 kg/kg-day; space loading = 1.2-2.4 kg BODBOD55/day-m/day-m33

Contact StabilizationContact Stabilization

Designed to provide two reactors, one for the Designed to provide two reactors, one for the sorption of organic matter and for the bio-sorption of organic matter and for the bio-oxidation of the sorbed materialsoxidation of the sorbed materials


Design ParametersDesign Parameters = 0.5-6 hrs; = 0.5-6 hrs; cc = 5-15 days; R/Q = 50-150%; MLSS = = 5-15 days; R/Q = 50-150%; MLSS =

1000-10000 mg/l; BOD1000-10000 mg/l; BOD55 and SS removal = 80-90%; and SS removal = 80-90%;

F/M = 0.2-0.6 kg/kg-day; space loading = 1.0-1.2 kg F/M = 0.2-0.6 kg/kg-day; space loading = 1.0-1.2 kg BODBOD55/day-m/day-m33

High-Rate AerationHigh-Rate Aeration

Designed to provide a lower degree of Designed to provide a lower degree of treatment than the other activated sludge treatment than the other activated sludge processesprocesses

It uses completely mixed reactorIt uses completely mixed reactor Design ParametersDesign Parameters

= 2-4 hrs; = 2-4 hrs; cc = 5-10 days; R/Q = 100-500%; MLSS = = 5-10 days; R/Q = 100-500%; MLSS =

4000-10000 mg/l; BOD4000-10000 mg/l; BOD55 and SS removal = 75-90%; and SS removal = 75-90%;

F/M = 0.4-1.5 kg/kg-day; space loading = 1.6-16 kg F/M = 0.4-1.5 kg/kg-day; space loading = 1.6-16 kg BODBOD55/day-m/day-m33

Extended AerationExtended Aeration

Designed to minimize waste activated sludge Designed to minimize waste activated sludge production by providing a large endogenous production by providing a large endogenous decay of the sludge massdecay of the sludge mass


Design ParametersDesign Parameters = 18-36 hrs; = 18-36 hrs; cc = 20-30 days; R/Q = 75-150%; MLSS = 20-30 days; R/Q = 75-150%; MLSS

= 3000-6000 mg/l; BOD= 3000-6000 mg/l; BOD55 and SS removal = 75-95%; and SS removal = 75-95%;

F/M = 0.05-0.15 kg/kg-day; space loading = 0.16-0.4 kg F/M = 0.05-0.15 kg/kg-day; space loading = 0.16-0.4 kg BODBOD55/day-m/day-m33

Pure Oxygen ProcessPure Oxygen Process

Designed to reduce retention time, decrease Designed to reduce retention time, decrease the amount of waste activated sludge, increase the amount of waste activated sludge, increase sludge settling characteristics and reduce land sludge settling characteristics and reduce land requirementrequirement

It uses completely mixed reactorsIt uses completely mixed reactors Design ParametersDesign Parameters

= 1-3 hrs; = 1-3 hrs; cc = 8-20 days; R/Q = 25-50%; MLSS = = 8-20 days; R/Q = 25-50%; MLSS =

3000-8000 mg/l; BOD3000-8000 mg/l; BOD55 and SS removal = 85-95%; F/M and SS removal = 85-95%; F/M

= 0.25-1.0 kg/kg-day; space loading = 1.6-3.2 kg = 0.25-1.0 kg/kg-day; space loading = 1.6-3.2 kg BODBOD55/day-m/day-m33

Effect of Temperature on Growth RateEffect of Temperature on Growth Rate

Arrhenius relationshipArrhenius relationship

KK11 = reaction rate constant at temperature T = reaction rate constant at temperature T11

KK22 = reaction rate constant at temperature T = reaction rate constant at temperature T22

= temperature correction coefficient= temperature correction coefficient TT11 = temperature of MLSS for K = temperature of MLSS for K11

TT22 = temperature of MLSS for K = temperature of MLSS for K22

12

1

2 TT

K

K

Effect of Temperature on Endogenous Effect of Temperature on Endogenous Degradation Rate Constant (kDegradation Rate Constant (kee))

The relationshipThe relationship

kke1e1 = endogenous degradation rate constant at temperature T = endogenous degradation rate constant at temperature T11

keke22 = endogenous degradation rate constant at temperature T = endogenous degradation rate constant at temperature T22

= temperature correction coefficient= temperature correction coefficient TT11 = temperature of MLSS for k = temperature of MLSS for ke1e1

TT22 = temperature of MLSS for k = temperature of MLSS for ke2e2

12

1

2 TT

e

e

k

k

Other Kinetic RelationshipsOther Kinetic Relationships

2. The Monod Equation2. The Monod Equation

= growth rate constant, time-1= growth rate constant, time-1 maxmax = maximum growth rate constant, time-1 = maximum growth rate constant, time-1 S = substrate concentration in solutionS = substrate concentration in solution KKss = substrate concentration when the growth rate constant = substrate concentration when the growth rate constant

is half the maximum rate constant.is half the maximum rate constant.

SK

S

smax

Monod observed that the microbial growth is Monod observed that the microbial growth is represented by:represented by:

dX/dt = rate of cell productiondX/dt = rate of cell production X = number or mass of microbes presentX = number or mass of microbes present = growth rate constant= growth rate constant

Xdt

dX

Generalized substrate consumption and biomass growth with time.

Documents

Activated Sludge Processes CE - 370. Basic Process The basic AS process consists of The basic AS process consists of A reactor in which the microorganisms