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Biological Treatment of Wastewater – Secondary
Treatment Process – Activated Sludge Process
Sudipta Sarkar
Pradeep Kumar
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Typical Process flow Diagram –
Different Treatment Blocks
Bar Screens Grit RemovalPrimary Clarifier
O2
Aeration
tank
Secondary
ClarifierNutrient
Removal
D
I
S
P
O
S
A
L
Dewatered
Sludge to
landfill
AnaerobicDigester
Gravity Sludgethickener
Filter Press
Screenings Grit
PRELIMINARY PRIMARY SECONDARY TERTIARY
Advanced
Treatments
SLUDGE PROCESSING
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BIOLOGICAL TREATMENT PROCESSES - OVERVIEW
Domestic sewage and some industrial or agricultural wastewater
contains high concentrations of biodegradable organic matter. The
organic material if discharged untreated, act as a food source formicroorganisms. If the discharge is large, problems occur leading
to large scale pollution.
The preliminary and primary treatment of wastewater together
remove almost 60 percent of solids loading and 40 percent of
BOD load that is influent to the wastewater treatment plant. The
solids removed mostly are inorganic in nature, as the specific
gravity and size of the commonly occurring inorganic solids are
higher than their organic counterparts.
The removal of the BOD, coagulation of non-settleable colloidal
solids, and the stabilization of organics are accomplished
biologically using a variety of microorganisms.
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Importance and Objectives of Biological Treatment
• Use organic matter as a food supply to support the growth of
biomass• Also use organic material to provide energy for growth
resulting in production of CO2 and other metabolic byproducts
thereby reducing total BOD
4
• Biological treatment is used to remove the most of the
contaminants remaining in regular sewage or industrial
wastewater that contains biodegradable materials. The
biodegradable part may be in either particulate (solid) or
dissolved form.
• Biological treatment is targeted to remove the contaminants
by: a) coagulation and sedimentation and b) stabilization oforganic matter so that organic content is reduced.
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Types of Microbial Communities
• Aerobic
–
utilize oxygen• Anaerobic
– grow in absence of oxygen
• Facultative
– can grow either with or without oxygen
– metabolism changes as environment changes from
aerobic to anaerobic
5
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Aerobic Organisms
• require oxygen to perform their metabolic activities
• Require high rates of oxygen supply for wastewater treatment
processes
6
Aerobic Processes
1. presence of oxygen 2. rapid conversion of BOD 3. release lots
of energy
InorganicEssential nutrients: N, S, P, K,
Mg, Ca, Fe, Na, Cl
Micro-nutrients: Zn, Mn,
Mo, Se, Co, Cu, Ni, V and W
Organic nutrients (growth factor)
Amino acids
Purines and pyrimidines
vitamins
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Microbial Growth
General Growth patterns in Pure Cultures:
7
Binary Fission Exponential Growth
Generation Time : 20min to less than a day
Condition: unlimited supply of food, unlimited supply of nutrientsand abundance of dissolved oxygen in water
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L o g V i a b
l e
C e l l C o u n t
Time
Lag
Phase
Exponential
Growth Phase
Stationary
Phase
Log Death
Phase
Microbial growth pattern in a batch reactor
8
Condition: Finite amount of food and nutrient supply
Bacteria acclimate to
the new environment
Excess food surrounding the bacteria;
rate of metabolism and growth is a
function of the ability of microorganismto process the substrate
Growth rate and
death rate of
bacteria are thesame as the food
becomes limited
Food is limited; bacteria
metabolize own protoplasm,
death rate far exceeds the
production of new cells
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• Cells have abundant food and grow without limit during thisphase
– X is cell concentration (mass dry wt/vol)
– X 0 is cell concentration at start of exponential phase
– μ is the specific growth rate (time
-1
) – t is time
Exponential Growth Phase
t e X X 0
9
dt dX
In other words, in both batch and continuous culture system,the rate of the growth of bacteria can be given by,
g r X
Is it a constant?
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Substrate (Food) Limited Growth
• Specific growth rate is a function of environmental conditionsfor the organism, including substrate (food) concentration
• there is a maximum rate at which organisms can grow evenwith plenty of nutrients available ( μmax)
• as substrate becomes limited, growth slows down
• a simple equation describing this behavior is called the Monod
model
10
Bacteria
WASTEWATER
WASTEWATER
Bacteria
Batch Culture Continuous Culture
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Specific Growth Rate
(mg/L)ionconcentratsubstrateis
(mg/L)constantvelocity-half is
growthformodelMonod
s
s
m
s
K
s K
s
S
m
/2
dt
dX g r X
S K
XS
s
m
Ks
Substrate (food)- limited Condition
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Cell Growth and Substrate Utilization
New Cells
Inorganic and organicend products
rg= rate of bacterial growth, mg/(L. sec)
Y= maximum yield coefficient, mass of cells formed per unit mass
of BOD consumed, mg/mg
rsu = Substrate utilization rate, mg/(L. sec)
su g Yr r
For a given substrate (food) the quantity of new cells produced can
be defined with a mathematical relationship
Food
The yield of microorganism depends on (1) oxidation state of the
carbon source, (2) Degree of polymerization of the substrate, (3)
pathways of metabolism and (4) various environmental parameters
such as temperature, pH, pressure, etc.
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su g Yr r S K
XS r
s
m g
)( S K Y
XS r
s
m
su
Y k
m
k is defined to be the maximum rate of substrate utilization per unit
mass of microorganism
)( S K kXS r s
su
In a mixed system not all the cells are in log growth phase. Also, some
energy derived from the food is used for cell metabolism used formaintenance. Death and predation rates were not considered in the
above expression.
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Growth in Mixed Cultures
Growth curves for different species of microorganisms are different
from each other.
Most biological treatment processes are comprised of complex,
interrelated, mixed biological populations.
For a mixed population, the position and shape of a particular
growth pattern shall depend on the relative abundance of thedifferent species, food and nutrients available and also, on
environmental factors such as temperature, pH, availability of
oxygen, etc.
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Death and predation factors are often lumped together for ease of
design and calculation, without losing the accuracy.
Assumption: The decrease in cell mass caused by death and predation is
proportional to the concentration of the microorganism present. The
decrease in the number of microorganism is considered to be
endogenous decay. X k r d d
kd= endogenous decay coefficient, time-1
X= concentration of cells (microorganisms), mg/L
d g g r r r '
X k S K
XS r
d s
m
g
)(
'
rg’ = net rate of bacterial
growth
net specific bacterial growth rate = d s
m g
g k S K
S
X
r
)(
'
'
Observed Yield su
g
Obs r
r Y
'
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Bioreactors
The system in which a biochemical reaction take place is known as a
bioreactor. Bioreactors may contain live and dead microorganisms,
organic material, essential nutrients, and may be fed with external gasessuch as oxygen, natural or compressed air, or carbon dioxide depending
on the applications
Types of Reactors: a) Batch reactor, b) Completely mixed flow reactor
(CMFR) and c) Plug Flow Reactor (PFR)
c
Batch reactor: A vessel loaded with reactants and then
sealed, may or may not be mixed
CMFR: A fluid container with flow in and out.
Contents are instantly and completely mixed.
Concentration of species going out is assumed to be
equal to the concentration inside the container
PFR: Uniform velocity of fluid across the reactor, no axial
mixing , may or may not be any radial mixing,
concentration is not uniform, may vary along the length
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Reactor Mass Balances: Food and Microorganism
Completely Mixed Flow Reactor (CMFR)
Q Q, S, X
V, S, X
S0
Mass balance:
Rate of flow of
material into
the reactor=
Rate of flow of
material out of
the reactor
-Rate of
accumulation of
material+
Rate of
formation or
destruction of
material within
the reactor
X0
Microorganism balance:
dt
dX V
0. X Q X Q. V r g .'
Food (substrate) balance:
0.S Q
S Q.dt
dS
V V r su .
Suspended Growth Process:
microorganisms responsible for the
conversion of organic matter to gases
and cell tissue are maintained insuspension in the wastewater
R t M B l F d d Mi i
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Reactor Mass Balances: Food and Microorganism
Q Q, S, X
V, S, X
S0 X0At Steady State, there is no net
accumulation food or microorganism
with respect to time. The reactor keeps a
constant load of microorganism or food,
no change over time.
0dt
dX and 0
dt
dS
0. X Q X Q. V r g .' 0...
' X Q X QV r g
0
X
r
V
Q g '
X
X k S K
XS d
s
m
)(
QV
1d
s
m k S K
S
)(
d
s
m k
S K
S
)(
1
Hydraulic detention time
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Q Q, S, X
V, S, X
S0 X0
0dt
dS
V r S QS Q su... 0
Q
V r S S su.)( 0
.)(
)( 0S K
kXS S S s
)( S K
kXS r
s
su
At steady state,
)()( 0
S K S
X k S S
s
d
s
m k S K
S
)(
1
)(
1)
1(
S K
S k
sm
d
X k
S S k
m
d
0
1)
1( )1(
)( 0
d
m
k k
S S X
)1(
)( 0
d k
S S Y X
Task: Prove that 1)(
)1(
d
d s
k Yk
k K
S
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CMFR with RecycleQ, X 0 ,S 0
(Q + Qr )
V R
X
S
Qr X r S Qw , X r , S(Qr + Qw)
Qe , Xe , S
Clarifier X, S
X r , S
(Activated Sludge Process)
System
Boundary
Accumulation = Inflow - outflow + Net growth
dt
dX V R = 0QX - ][ eer w X Q X Q + )( ' g R r V
AERATION TANK
(REACTOR)
At Steady State,
0dt
dX eer wd
s
m
R
X Q X Q X k S K
XS V QX
)(0
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eer wd
s
m R X Q X Q X k
S K
XS V QX
)(0
X V
X Q X Qk
S K
S
R
eer wd
s
m
)(
00 X Assume,
d su
R
eer w k X
r Y
X V
X Q X Q
eer w
R
c X Q X Q
X V
Mean Cell Residence Time (MCRT)=
MCRT is defined as the mass of microorganisms in the reactor divided
by the mass of the microorganisms wasted per unit time (day). It
signifies the average time the microorganism spend inside the reactor. It
is also called sludge age or solids retention time (SRT).
su g Yr r
S K
XS r
s
m g
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CMFR with RecycleQ, X 0 ,S 0
(Q + Qr )
V R
X
S
Qr X r S Qw , X r , S(Qr + Qw)
Qe , Xe , S
Clarifier X, S
X r , S
(Activated Sludge Process)
System
Boundary
Accumulation = Inflow - outflow + Net growth
dt dS V R = 0QS - ][ S QS Q ew + su Rr V
AERATION TANK
(REACTOR)
At Steady
State,
0dt
dS QS S QQr V QS we su R )(0
S S
QV
S S r
R su
00
/
timeretentionHydraulicQ
V R
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d su
c
k X
r Y
1
d
c
k X
S S Y
01)1()( 0
cd
c
k S S Y X
S S
V Q
S S r
R
su
00
/
S S r su
0 su g Yr r S K
XS r
s
m g
S K
XS S S Y
s
m
0
)1(
1..
cd
c
s
m
k S K
XS X
1)(
)1(
d c
cd S
k Yk
k K S
Y k m
= maximum rate of
substrate utilization per unit
mass of microorganism
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d su
c
k X
r Y
1
X
S S
V
Q
X
S S
X
r U
r
su
00 .
Define a new term, specific utilization rate, U so that
d
c
k YU
1
Another important term Food-to-microorganism ratio, F/M, is defined as,
systemin theloadmicrobialTotal
timeof unit peravailablefoodTotal/ M F
X V
S Q
r
0. X
S
X
S
V
Q
r
00.
X is the concentration of microorganism in reactor. Often it is termed as Mixed Liquor
Suspended Solids (MLSS)
Efficiency of the Activated Sludge Process (ASP): 100*0
0
S
S S E
100**0
0
S
X
X
S S E
100*
/
1.
M F U Volumetric loading rate is
defined to be total amount
of organics loading perunit volume of the reactor.r V
QS 0
Important Variables and
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Important Variables and
relationships
The relationships
important for the design
and control of an activatedsludge process are:
)1(
)( 0
cd
c
k
S S Y X
1)(
)1(
d c
cd S
k Yk
k K S
d
c
k YU 1
X S M F
0/ 100*0
0
S S S E
100*/
1. M F
U E
eer w
Rc
X Q X Q
X V
Q
V r X
S S U
0
U=specific substrate utilization rate; E= efficiency; F/M = food to
microorganism ratio; X=microorganism concentration in the reactor or
Mixed Liquor Suspended Solids (MLSS); θ= hydraulic retention time (HRT);
θc= mean cell residence time (MCRT); Y =yield coefficient
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Operation of activated sludge treatment plant is regulated by 1) quantity of air supplied in
the aeration basin; 2) The rate of recirculation of activated sludge and 3) Amount of excess
sludge wasted from the system.
Sludge wasting is an important step to establish the desired concentration of MLSS, F/M
ratio and MCRT or mean cell residence time or sludge age.An important measurement for operational control is the settleability of the mixed liquor
as defined by sludge volume index (SVI). SVI is the volume in mL occupied by 1 g of
suspended solids after 30 minutes of settling.
(mg/L)MLSS
mg/g1000*(mL/L)liquormixedeunit volumSettlingfromVolumeSludgeSVI
SVI
1000*(mL/L)/VVs MLSS
(mL/g)
Start with 1L of
mixed liquor
Volume of settled
sludge = Vs
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If the rate of sludge return is less than the rate of accumulation of settled solids, the sludge
blanket in the final clarifier slowly rises until the suspended solids are carried out with
overflow.
If the rate of sludge return exceeds the rate of accumulation of settled solids, clear
treated water is drawn with the sludge, making it less concentrated by diluting it.
In Ideal case, the mass balance should follow the above diagram. By the time it settles
down so that a flow rate of Q R takes out all the sludge contained in it.
R Ree R X Q X Q X QQ )(
Neglecting any sludge wasting
R R R X Q X QQ )(0e X
X Q
QQ X
R
R R
)(
)/(*
)/(1000***
)(
g ml SVI V
g mg V
V
V MLSS
V
V X
Q
QQ X s
s s R
R R
SVI
1000*(mL/L)/VVs MLSS
)/(
10
)/(
6
g ml SVI Lmg X R
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Amount of microorganism wasted
New Cells (They will also
have some BODu)
Inorganic end products
Food
(BODU) c
In ASP, the cells are
recycled mostly in the
process; however, a part
of the active
microorganisms are
wasted
i.e. not all the BODu
in the influent
wastewater gets
stabilized or
degraded to inorganic
end products.
Total BODu destroyed = BODu of the influent wastewater destroyed
- BODu of the microorganism wasted
)(0
S S Q )(of demandOlBiochemica2 r w
X Q
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Amount of microorganism wasted
eer w
Rc
X Q X Q
X V
=0
r w
Rc
X Q
X V
c
Rr w
X V X Q
)1(
)( 0
cd
c
k
S S Y X
)1(
)(.* 0
cd
c
c
Rk
S S Y V
)1(
)(* 0
cd
R
k
S S Y V
)1()( 0
cd k
Y S S Q
obsY S S Q )( 0 cd
obs
k
Y Y
1
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Approximate chemical formula of a bacterial cell is C5H7NO2
energy NHO2H5CO5O NOHC 3222275
113 5X32
1 1.42
obs x Y S S Q P )( 0 Amount of sludge wasted per day Q is in cum/day
Oxygen demand of the wasted sludge is obs x Y S S Q P )(*42.142.1 0
Total Oxygen demand of the ASP process
=Total BODu destroyed
x P f
S S Q42.1
)( 0
S, S0 are in BOD5 and not BODu
cd
ob sk
Y Y
1
So, it has to be divided by
factor f to transform to BODuso that
u BOD
BOD
f 5
For BOD rate constant of value
0.23 per day (base e), f= 0.68
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Recommended Design Parameters for Activated Sludge
Process for Municipal Wastewater
Completely Mixed Type Aeration Tank
Parameter Design Values
Mixed Liquor Suspended Solids (MLSS), X (mg/L) 3000-4000
MLVSS/MLSS 0.8
F/M (kg BOD5/Kg MLSS/day) 0.3-0.5
HRT (θ), hours 4-6
MCRT or SRT or sludge age, (θc), days 5-8
Qr/Q, Sludge return ratio, recirculation ratio 0.25-0.5
E, (efficiency), % 85-95
Kg O2/kg of BOD5 removed 0.8-1.0
MLVSS = mixed liquor volatile suspended solids
k d l f d
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Design an aeration tank and suggest process control parameters of an activated
sludge process for treating 20,000 cum/day wastewater with influent BOD 250 mg/L.
Effluent BOD should be 20 mg/L. MLVSS to be maintained is 3000 mg/L. MCRT is 7
days. Yield Coefficient is 0.6 and endogenous death rate constant, kd =0.06/day, F/M
ratio = 0.4 /day. Assume that there is negligible suspended solid (microorganism) in
the effluent from the secondary clarifier. Sludge return ratio = 0.2
100*0
0
S
S S E
%92100*250
20250
100*/
1.
M F U E
100*4.0
1.92 U 368.0U
X
S S U
0
3000.
20250368.0
hours5day20833.0
Q
V r
cum41670.20833*cum/day000,20 QV r
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eer w
Rc
X Q X Q
X V
As per the problem
statement the secondary
clarifier have negligible SS
in the effluent
r w
Rc
X Q
X V
eer w
Rc
X Q X Q
X V
=0
Sludge return ratio = 0.2 2.0
Q
Qr cum/day000,4000,20*2.0*2.0 QQr
r wr ee R X QQ X Q X QQ )()( Microorganism balance in the clarifier
=0
r wr R X QQ X QQ )()(
c
Rr w
X V X Q
r w X Q )4000(3000*)400020000(
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r w X Q )4000(3000*)400020000(
c
Rr
X V X
*40003000*)400020000(
mg/L5.17553r X cum/day7.101
cr
Rw
X X V Q
we QQQ
cum/day4000r
Q
cum/day19900100000,20 we QQQ
cum/day20000Q cum4167r
V
Fi d h i f i d l d
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Find out the oxygen requirement for an activated sludge process
which operates at 95% efficiency and flowrate of 30,000 cum/day. The
influent BOD5 concentration is 250 mg/L. Mean cell residence time
(MCRT) is kept as 7days. The yield coefficient was found to be 0.5 kg
of biomass per kg of BOD5 utilized. Endogenous growth rate constantis 0.06 per day (kd)
100*
0
0
S
S S E
100*250
25095
S mg/L5.12S
cd
obsk
Y Y
1 7*06.01
5.0
7*06.01
5.0
352.0
ob s x Y S S Q P )( 0 kg/Day10*352.0*)5.12250(*10*000,30-63
Total Oxygen demand of the ASP process x P
f
S S Q42.1
)( 0
42.168.0
10*)5.12250(*10*000,30 63
x P
kg/day7969
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Diffused Aeration
37
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Aeration basin for activated sludge process
38
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Return sludge mixing with incoming wastewater
39
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Augurs lifting sludge coming from
clarifier outlet to be returned to
ti t d l d t t t 40