Deals with what is activated sludge, mechanisms and kinetics of treatment, design of activated sludge process, secondary clarifiers and their design and bulking sludge, raising sludge and foaming of ASP.

1. Activated Sludge Process
Dr. Akepati S. Reddy
School of Energy and Environment
Thapar University
Patiala (PUNJAB), INDIA

2. Activated Sludge Process
Most commonly used secondary treatment process
• Microbes, mainly aerobic heterotrophic bacteria, are involved
Designed to remove (soluble) biodegradable organic matter
• Removal of nutrients, TSS, pathogens and heavy metals is
coincidental
Usually clarified sewage (primary effluents) is treated
• Primary treatment is omitted in case of small flows and low TSS
sewage, and in hot climates (to avoid/control odour problems)
• SBR, oxidation ditches, aerated lagoons, contact-stabilization
process, etc. may not require primary treatment
Treatment involves conversion of soluble organic matter into
biological flocs and their removal as secondary sludge
Includes an aeration tank and a secondary sedimentation tank
• Aeration and mixing, and sludge recycling are additional
features

3. Grit
chamber
Primary
clarifier
Aeration
tank
Secondary
clarifier
Stabilization
tank
Sludge
drying beds
Sewage
Sump &
pumping
Bar
screen
Biogas
flaring unit
Dried sludge
for disposal
Exhaust gases
(CO2 and H2O)
Filtrate
to sewage sump
Clarified effluent
to sewage sump
Treated effluent
Raw sewage
Screenings
Biogas
if not flared
(supplied as fuel or emitted)
Grit
STP Incorporating ASP
Equalization
Tank
air
air
scum
ASP

4. Aeration
basin
Secondary
clarifierInfluent Effluent
Wasted activated sludge
Sludge recycling
Nutrients and
Alkalinity (if needed)
Air/oxygen for
aeration & mixing
Activated Sludge Process

5. Components of ASP
Aeration basin
• Wastewater comes in contact with active microbial biomass
for treatment
– Bioflocculation, biosoprtion and biooxidation occur
• Organic matter is transformed into biological flocs
– Suspended and colloidal solids become integral part of flocs
Aeration and mixing
• Aeration supplies enough oxygen for aerobic bio-oxidation of
organic matter
• Mixing keeps biological flocs suspended and ensures contact
between wastewater and microorganisms
• Two types of aeration/mixing systems: diffused and
mechanical aeration systems
– Diffused aeration (diffusers, piping and fittings, and blowers)
– Mechanical aeration systems - Surface aerators (fixed or floating
types) with or without draft tubes - Submerged turbine aerators
- Horizontal axis aerators (brush aerators)

6. Components of ASP
Secondary sedimentation tank
• To clarify the out-flowing aeration tank contents (mixed liquor)
• To separate and thicken the biological flocs from mixed liquor for
recycling or wasting
Sludge recycling
• Thickened sludge is returned back from secondary clarifier to the
aeration tank to maintain desired concentration of biological solids
• Includes pumps and necessary piping and fittings
Activated sludge wasting
• Sludge wasting is either as mixed liquor or as thickened sludge
Chemical feed systems
• Addition of nutrients and alkalinity may be required if the
wastewater is deficient in them – sewage is usually not deficient
– Urea and DAP are usually used as nutrients (phosphoric acid or
phosphate rock can also be used in place of DAP)
– Hydrated lime is dosed for alkalinity

7. Mechanisms of Treatment
Aerobic microorganisms (activated sludge), specially bacteria,
are responsible
• Suspended and colloidal solids of the wastewater becomes
integral part of biological sludge through bioflocculation
• Biological sludge is constituted of microorganisms, cell debris,
and suspended and colloidal solids of the influent
• Organic matter of wastewater is biosorbed (adsorbed and
absorbed) by microorganisms
• Adsorbed organic matter is solubilized through hydrolysis
• Simple soluble organic compounds are absorbed by microbes
as food
• Absorbed organic matter is bio-oxidized (partly respired &
rest is used in biological flocs - new microbial biomass -
synthesis)
• Involves biooxidation, biosynthesis and autooxidation
• Net synthesized biological flocs (excess sludge) is wasted

8. Soluble organic
matter
Nb soluble
organic matter
Nb. suspended
organic matter
Oxygen (1-1.42Y)
CO2, H2O, NH3,
Energy, etc.
New heterotrophic
Microbial biomass
Auto-oxidation
kd
CO2, H2O, NH3,
Energy, etc.
Carbonaceous BOD is the sum of oxygen utilized during biooxidation of the
organic matter and during autooxidation of the microbial biomass
Oxygen (1.42Kd)
Residual biomassBio-oxidation
B
io-synthesis
Y
Suspended
organic matter
Hydrolysis
Residual
biodegradable
organic matter
What happens to organic matter in Activated Sludge Process?
Bioflocculation and Biosorption are much faster than bio-oxidation
• Hydrolysis and bio-oxidation are slower processes
• Bio-oxidation requires O2 (DO - 0.5 to 1.5 mg/L)

9. Nutrient removal occurs through
• Ammonical-N from organic-N, nitrification and denitrification
• Assimilation of ammonical-N and conversion into organic-N
Nitrification
• Aerobic 2-step process (ammonia to nitrite and then to nitrate by
autotrophic bacteria
• Becomes significant if DO levels are higher (>2.0 mg/L) and oxygen
requirement is 4.57 g/g of NH3-N (3.43 to nitrite-N and 1.14 g to
nitrate-N)
• Demands alkalinity (7.14 g/g as CaCO3)
Denitrification (respiration where nitrate is electron acceptor)
• Reduction of NO3 by heterotrophic bacteria into N2O and N2
• Coupled with respiratory electron transport chain and demands
respiration of 4 g BOD per g of NO3
• 1 gram of O2 can be replaced by 2.86 g of nitrite or 1.71 g of nitrate
• Produces alkalinity (3.57 g (as CaCO3)/g nitrate denitrified)
• DO levels >0.1 or 0.2 mg/l are inhibitory
Mechanisms of Treatment

10. Mechanisms of Treatment
Phosphorus removal
• Phosphorus Accumulating Organisms (PAO) in an anaerobic –
aerobic system are involved
• Phosphorus is incorporated into sludge (as polyphosphate/
volutin granules) and removed through sludge wastage
• PAOs have 20-30% of the biomass as phosphorus
• PAOs form very dense, good settling flocs
• In the anaerobic tank of the system
• proliferation of PAOs occurs
• fermentation products (acetate) are assimilated and poly-
hydroxy-butyrate (PHB) is stored – concomitantly
polyphosphate is released as ortho phosphate
• In the aerobic tank
• PHB is oxidized and concomitantly phosphate of the effluent is
stored within the cell
• Stoichiometrically about 10 grams of bCOD is needed for the
removal of one gram of phosphate

11. Substrate Utilization Rate
Aeration
tank






+
−=
−
=
−
=
es
e
a
eiei
su
SK
Sq
x
SS
V
SSQ
r .max)()(
τ
V Xa
Q
Si
Q
Se
Se
qmax.
qmax./2
Ks
rsu is substrate or organic matter utilization rate (g/m3
.day)
qmax is maximum specific organic matter utilization rate (g/g microbial mass)
Xa is microbial biomass concentration (g/m3
)
Se is organic matter concentration (g/m3
) in the ASP
Ks is half-velocity constant (organic matter concentration in g/m3
at which organic
matter utilization rate is qmax./2 )
τ is hydraulic residence time (HRT)
q

12. Net Biomass Synthesis Rate
adsug xkYrr −=
d
kxYx
SK
Sq
r aa
es
e
g −





+
=
)(
.max
da
ei
gda
ei
g kx
YSS
rkx
V
YSSQ
r −
−
=−
−
=
τ
)()(
rg is net biomass production rate (g VSS/m3
.day)
Kd is endogenous decay coefficient (g VSS/g VSS. Day)
Y is yield coefficient
d
a
ei
k
x
YSS
SRT
−
−
=
τ.
).(1
d
es
e
k
SK
SqY
SRT
−
+
=
..1 .max

13. Oxygen Utilization Rate
adsuO xkrYr .42.1)42.11(2
+−=
d
kx
SK
Sq
xYr a
es
e
aO .42.1
)(
.
)42.11( .max
2
+





+
−=
da
ei
O kx
SS
Yr .42.1
)(
)42.11(2
+
−
−=
τ
gsuO rrr 42.12
−=
(1-1.42Y) is the fraction of utilized organic matter bio-oxidized
1.42kd is auto-oxidation rate in terms of oxygen or bCOD

14. qmax. (2-10 g of bCOD per g VSS day, 5 is typical)
Ks (10-60 mg/l of bCOD, 40 is typical)
Y (0.3 to 0.6 mg VSS per mg bCOD, 0.4 is typical)
kd (0.06 to 0.15 g VSS per g VSS.day, 0.1 is typical)
Values in parentheses are typical values for domestic sewage
Kinetic parameters values vary with the wastewater, with the
Microbial population and with Temperature
Kinetic parameter values can be determined from bench scale testing
or full-scale plant test results
Temperature correction to the kinetic parameter values is done by
ASP kinetics Parameters and typical
parameter values for the sewage
)20(
20
−
= T
T kk θ
θ is temperature activity coefficient
(typical value 1.02 to 1.25)
kT and k20 are k values at T°C and 20°C
respectively

15. Aeration tank
Se,Xa,V
Settling
tank
Q,Si,Xi
Qr,Xr,Se
Qw,Xr,Se
Qe or (Q-Qw)
Xe,Se
Aeration tank
Se,Xa,V
Settling
tank
Q,Si,Xi
Qr,Xr,Se
Qw,Xa,Se
Qe or (Q-Qw)
Xe,Se
Xi is considered negligible
All biodegradable suspended organic solids of influent are
hydrolyzed into soluble organic matter
Inorganic and non-biodegradable organic SS remain
unaffected and no new SS of these categories are formed
Only clarification & sludge thickening occurs in the clarifier

16. Treated effluent BODU (Se)
Use of this equation requires
– Primary variable SRT (assumed)
(typical values are 5 to 15 days)
– Ks, kd, qmax and Y are ASP kinetic parameters
Obtained from the following through solving for Se
Note that the Se is independent of influent bCOD (or BODu)
[ ]
( ) 1.
)(1
.max −−
+
=
d
ds
e
kYqSRT
SRTkK
S
d
es
e
k
SK
Sq
Y
SRT
−





+
= max1
)()(
)(
ratewastagesludgeorrategenerationsludgenet
systemtheofsludgetotal
SRT =

17. Active Biomass Concentration (xa)
Mixed Liquor Active Biomass Concentration
Use of this equation requires
– Primary variables SRT and τ (or HRT)
typical values are 4 to 12 hours
– ASP kinetics parameters Y and kd
– Si and Se are influent and effluent bCOD values
Obtained from the following basic equation
Here xa depends on kd, Y, SRT, τ and bCOD removal
( )
)(1 SRTk
YSSSRT
x
d
ei
a
+
−
=
τ
( )
d
a
ei
k
x
SSY
SRT
−
−
=
.
1
τ

18. Sludge Generation and Wastage Rates
Net biomass synthesis rate (NBSR):
Estimated by
Obtained through simplification of the following material balance
equation
)(1
)(.
SRTk
SSQY
NBSR
d
ei
+
−
=






−






=






rateionautooxidat
Biomass
ratesynthesis
biomassGross
ratesynthesis
biomassNet
daei kVxSSQYNBSR ..)(. −−=
( )
)(1 SRTk
YSSSRT
x
d
ei
a
+
−
=
τ
Here V is replaced by Q.τ
For xa the following equation is used

19. Sludge Generation and Wastage Rates
Secondary sludge generation rate is comprised of
– Net biomass synthesis rate
– Cell debris generation rate from biomass autooxidation
– Nonbiodegradable VSS contributed by the influent (Nb.VSS)
– Inorganic suspended solids contributed by the influent (In.SS)
GRSSInGRVSSNbCDGRNBSRSSGR .... +++=
)..(.. VSSNbQGRVSSNb =
)..(.. SSInQGRSSIn =
( )






+
−−=
−=
SRTk
SSQYfCDGR
NBSRratesynthesisGrossfCDGR
d
eid
d
.1
1
1)(
Here fd is the fraction of the auto-oxidized biomass left behind as
cell debris (usually taken as 0.15)

20. MLSS
x
GRSSInGRVSSNbCDGRNBSR
NBSR a
=
+++ ....
MLSS
MLVSS
GRSSInGRVSSNbCDGRNBSR
GRVSSNbCDGRNBSR
=
+++
++
....
..
can be obtained from
MLSS and MLVSS
Sludge Wastage – it can be
• From the return sludge line
– Lesser volume of sludge is wasted
– Control is difficult (may need measurement of MLSS and TSS
level in clarifier underflow)
• From the aeration tank in the form of mixed liquor
– Volume wasted is large
– Can be wasted either into a primary clarifier or a thickener
– Control is much easier (may need only TSSe measurement)
Sludge Generation and Wastage Rate

21. Sludge wasting rate
• Determined on the basis of SRT
– Due consideration is given to sludge washout (TSSe in the
clarified effluent)
• Depends on secondary sludge generation rate (SSGR) minus
secondary sludge washout rate (SWOR)
SSWR = SSGR – SWOR Where SWOR is Q.TSSe
Volumetric sludge wastage rate
SSWR/MLSSu (when wasted from the secondary clarifier
underflow)
SSWR/MLSSa (when wasted from the aeration tank)
Observed SRT = (V.MLSSa)/SSWR
It is greater than the SRT chosen as primary variable
TSS of the clarified secondary effluent influences its value
At TSS = 0, observed SRT is equal to primary variable SRT
Sludge Generation and Wastage Rates

22. Determined by writing material balance around secondary clarifier
• Mass balance for secondary clarifier
• Assuming Xe as negligible and taking QwXr as VXa/SRT and taking V
as Qτ one can find Qr as
Determined by writing material balance around the aeration basin
• Assuming new biomass growth and influent biomass (Xi)
concentration as negligible, material balance for aeration tank is
Determined by the sludge settlability characteristic (SVI)
eerwrrra XQXQXQQQX ++=+ )(
ar
a
r
XX
SRT
QX
Q
−






−
=
τ
1
)( rarr QQXXQ +=
ar
a
r
XX
QX
Q
−
=
Sludge Recycling
1
100
100
−
=
SVIP
r
w
Pw is MLSS as % (3000 mg/L is 0.3%)
SVI is in mL/g
r is sludge recycle ratio RQ
Q
r =

23. Oxygen Demand Rate
Here ‘n’ is oxygen equivalence of microbial biomass(1.42!)
The oxygen demanded is supplied by
Surface (floating or fixed) aerators
Diffused aeration systems (introduce oxygen/air into liquid)
Turbine mixers can disperse introduced air bubbles
Hydraulic shear devices can reduce bubble size
Suppliers of aeration systems indicate oxygen transfer rates of
their equipment at standard conditions (SOTE/SOTR)
– These rates require correction to actual operating conditions
(AOTE/AOTR)






−






=






CDGRplusNBSR
ofequivalentOxygen
substrateloadedof
equivalentOxygen
demand
Oxygen
( ) [ ]CDGRNBSRnSSQdemandO ei +−−=2

24. Actual Oxygen Transfer Efficiency/Rate
• AOTR is actual oxygen transfer rate under field conditions – it
is influenced by
– Salinity-surface tension of the wastewater (β)
– Operating temperature of the wastewater
– Atmospheric pressure (related to altitude)
– Average depth of aeration (diffused aeration system)
– Operating DO of the aeration tank
– Oxygen transfer coefficient of wastewater compared to that of
clean tap water (α)
– Degree of fouling of the diffusers (diffused aeration system)
• SOTR is standard oxygen transfer rate in tap water at 20°C
and zero dissolved oxygen level
• Applicable even for oxygen transfer efficiencies
( ) F
C
CC
SOTEorSOTRAOTRorAOTE T
s
LTHs
..024.1
. 20
20,
α
β −







 −
=

25. Actual Oxygen Transfer Rate or Efficiency
β is salinity – surface tension factor
• Taken as saturation DO ratio of wastewater to clean water
• Typical value is 0.92 to 0.98 (0.95 is commonly used)
α is oxygen transfer correction factor for the wastewater
• Typical range for diffused aeration systems is 0.4-0.8
• Typical range for mechanical aerators is 0.6-1.2
F is fouling factor - accounts for both internal and external
fouling of diffusers
• Impurities of compressed air cause internal fouling
• Biological slimes and inorganic precipitants cause external
fouling
• Typical value is 0.65 to 0.9

26. Actual Oxygen Transfer Rate
Cs
_
,T,H is average saturation DO of clean water at operating temp.
and altitude at mid-water depth (aerator–surface)
• For surface aerators
Can be obtained from literature (for the atmospheric pressure at
the altitude in question – Annexure C of MetCalf-Eddy)
• For diffused aerators it can be obtained by
Ot is volume % O2 in the air leaving the aeration basin (typically 18-
20%)
HTsHTs CC ,,,, =






+=
212
1
.
,,,,
t
Hatm
d
HTsHTs
O
P
P
CC
( )
( ) 





+
−×
−=
T
H
P
P
atm
Hatm
15.2738314
097.2881.9
exp
0,
,

27. Air Requirements
Air is also required for the mixing of aeration tank contents
Typical air requirement for mixing is 0.01 to 0.02 m3
/m3
.min.
Air required for mixing and for oxygenation, whichever is larger
is used as design air requirement
Air required is expressed in kg/hr. and Nm3
/hr
Actual temperature of the air depends on the level of
compression
Ambient temperature + pressure (in kg/cm2
gauge) X 10°C!
{ }






×






=






airthein
fractionoxygen
efficiencytransfer
oxygenActual
demandOxygen
required
Air

28. Nutrient Requirements
Inflow of nitrogen
Influent may have TKN (Organic-N + Ammonical-N) and Nitrate-N
(Nitrate + Nitrite)
Nutrient addition (in the form of Urea and DAP)
Fate of nitrogen in the ASP
Organic-N is converted into Ammonical-N
Ammonical-N can nitrified into Nitrate-N
Nitrate-N can be denitrified and lost in the gaseous from (as N2O and N2)
Ammonical-N and Nitrate-N can be assimilation by active biomass and
stored within as Organic-N
Outflow of nitrogen
Loss in the treated effluent either as TKN or as Nitrate-N or as both
Loss as Organic-N in wasted activated sludge

29. Nutrient Requirements
Nitrate-N in the influent is negligible (influent mainly has TKN)
Nitrogen in the treated effluent can be Ammonical-N or Nitrate-N or
Organic-N (in the TSSe)
Nitrogen in the wasted activated sludge is 12.23% - obtained from
empirical formula of the activated sludge (C60H87O23N12P)
Denitrification loss of nitrogen can be significant if the ASP is designed
for nitrification and denitrification to occur
When concentration is <0.3 mg/L nitrogen is believed to be limiting for
the biooxidation removal of substrate






−






+






+






=
luent
theinN
ationdenitrific
throughlostN
sludgewasted
theinN
effluent
theinN
trequiremenN
inf
( )NNitrateTKNQxQ
MLSS
x
TSSQtrequiremenN uw
a
e +−+





+= 1223.01223.03.0

30. Phosphorus requirement can be assessed in a manner similar to
the nitrogen requirement by
N and P required can also be conservatively estimated as
Here bCOD is in g/m3
Y is yield coefficient (0.4!)
Nutrient requirement can also be expressed as the required
bCOD:N:P ratio of the influent
Nutrient Requirements
)3.0.1223.0( iii NNitrateTKNYbCODQtrequiremenN −−+=
)3.0.0226.0( ii PTotalYbCODQtrequiremenP −+=
2.1:2.5:100:: =PNbCOD
( )iuw
a
e PTotalQxQ
MLSS
x
TSSQtrequiremenP −+





+= 02263.00226.03.0

31. Alkalinity Requirements
• 70-80 mg/L as CaCO3 for maintaining the pH at 6.8 to 7.4
• Nitrification if occurring requires 7.07 g as CaCO3 per g of NH3-N
nitrified
• Denitrification if occurring produces 3.57 g as CaCO3 per g of nitrate
reduced
Treated effluent quality
• Characterized by soluble bCOD, TSS and VSS, and nutrients
• Soluble bCOD for SRT >4days is 2 to 4 mg/L
• Ammonical nitrogen and total phosphorus (soluble form) are >0.1
and >0.3 mg/L respectively
• For properly functioning secondary clarifier in case of mixed liquor
solids with good settling characteristics TSS is 5-15 mg/L
Others Aspects of ASP Design

32. Total bCOD of the effluent
MLSS
x
TSSS a
ee ××+ 42.1
QHRTV .=
Aeration tank volume
Vx
QS
M
F
a
i
=
VMLVSS
QS
M
F i
.
=
Food to microorganisms (F/M) ratio
In terms of active biomass
In terms of MLVSS
V
QS
loadingBOD i
=
BOD loading
Other Aspects of ASP Design

33. Center-feed circular tanks with side wall liquid depth of 3.7 to 6
m and radius of < 5 times liquid depth are used
Includes
– Inlet section or central well
• Size is 30-35% of tank diameter
• It is separated from the sludge settling zone by a cylindrical baffle
• It is meant to dissipate the influent energy, to evenly distribute
flow and to promote flocculation
– Sludge settling zone
– Sludge thickening and storage zone
– Peripheral overflow weir and collection trough
• Baffles are often provided to deflect density currents and avoid
scum overflow (scum baffles)
Has a central rotating mechanism to scrap, transport and
remove the thickened sludge (and also the floating scum)
– Sludge is removed directly from tank bottom by suction orifices
Secondary Clarifier

35. Design of Secondary Clarifier
Very similar to primary sedimentation tank
• Rather than just clarification both clarification and sludge
thickening occur
• Sludge blanket is maintained for thickening to occur and hence
depth is >3.7 m
• Larger central well, density currents, relatively lower weir
loading rates
Area required for clarification and area required for thickening
are found out and the larger of the two is used
Design approaches for the secondary clarifiers
– Talmadge and Fitch method - uses data derived from a single
batch settling test
– Solids flux method - uses data obtained from a series settling
tests conducted at different solids concentration
Secondary clarifier is also designed on the basis of SVI and ZSV

36. Secondary Clarifier: Talmadge and Fitch
method
Final overflow rate for a secondary
clarifier is selected based on the
consideration of
– Area for clarification
– Area for thickening
– Rate of sludge withdrawal
Data from a single settling test is
used for finding both area
required for thickening and for
clarification and greater of the
two is considered for design
Area required for clarification is
usually lesser than the area
required for thickening

37. Area required for thickening
• Tu corresponds to Hu and obtained through
• Co is initial TSS and Ho column height
• Cu is underflow sludge concentration
Critical concentration controlling sludge handling capability
– Draw tangents to initial and final legs of settling curve
– Bisect the angle of intersection and extend to settling curve to get
Cc
Find tu (time at which sludge concentration is Cu)
• Draw tangent through Cc
• Locate Hu on y-axis, extend horizontal line to the tangent through
Cc - draw vertical from the intersection to obtain Tu
o
u
t
H
Qt
A =
u
oo
u
C
CH
H =
Secondary Clarifier:
Talmadge and Fitch method

38. Secondary Clarifier:
Talmadge and Fitch method
Area for clarification
– Here Qc is clarification rate
– V is interface subsidence velocity
Interface subsidence velocity
• Slope of the tangent on the initial leg of the settling curve
is taken as subsidence velocity
Clarification rate
• Taken as proportional to the liquid volume above Hc
and computed as
– Here Hc is critical sludge depth
– Q is flow rate of mixed liquor into the clarifier
v
Q
A c
c =
o
c
c
H
HH
QQ
−
= 0

39. Secondary Clarifier: Solids flux method
Area required for thickening
depends on the limiting solids
flux that can be transported to
the bottom of the settling tank
Data obtained from a series of
column settling tests conducted
at different solids concentration
is used
Solids flux depends on the
characteristics of the sludge
(relationship between sludge
concentration and settling rate
and solids flux)

40. Downward flux of solids in a settling tank occurs due
– gravity settling
– bulk transport from sludge withdrawal
– Here SFg is solids flux due to gravity
– SFu is solids flux by bulk transport
Solids flux due to gravity
– Ci is concentration of solids at the point in question
– Vi is settling velocity of the solids at Ci concentration
– Vi of sludge at different concentrations is obtained from multiple
settling tests - Slope of the initial portion of the curve is Vi
Secondary Clarifier: Solids flux method
ugt SFSFSF +=
iig VCSF =

41. Solids flux by bulk transport
– Ub is bulk underflow velocity
– Qu is underflow rate of sludge
– A cross sectional area of the sludge
– Flux by bulk transport linearly increases with increasing withdrawal
rate
Total flux increases initially, then drops to limiting solids flux (SFL)and
then increases with increasing withdrawal rate
Secondary Clarifier: Solids flux method
A
QC
UCSF ui
biu ==

42. Alternative graphical method for limiting solids flux (SFL)
• Uses only the gravity flux curve
• Decide the underflow sludge concentration and draw tangent to
gravity flux curve through Cu on X-axis and extend to Y-axis
• Point of intersection on Y-axis gives SFL
Secondary Clarifier: Solids flux method

43. Secondary Clarifier: Solids flux method
Area for thickening
• Area required for thickening will that area at which actual solids
is lower than equal to limiting solids flux (SFL)
– If solids loading is greater than limiting solids flux then solids will
build up in the settling basin and ultimately overflow
• Area required for thickening
• For a desired underflow concentration one can increase or
decrease the slope of the underflow flux line
( )
L
u
SF
CQQ
A 0+
=
Q is overflow
Qu is underflow
SFL is limiting solids flux

44. Settling and thickening characteristics of the mixed liquor
measured by either SVI or ZSV can be used as basis
SVI below 100 is desired and above 150 typically indicates
filamentous growth
Surface over flow rate for a secondary clarifier is related to zone
settling velocity as shown below
ZSV (Vi) can be estimated by
Here Vi is zone settling velocity (SVI)
SF is safety factor and taken as 1.75 to 2.5
Vmax is maximum zone settling velocity taken as 7 m/h
K is a constant with value 600 l/mg for ML with SVI 150
X if MLSS concentration
Design of Secondary clarifier on the basis of
SVI and ZSV
SF
V
rateoverflowSurface i
=
xKVVi )exp(max −=

45. MLSS, ZSV and SVI/DSVI are related
Here x is MLSS concentration in g/l
DSVI and SVI in ml/g
Fluctuations in wastewater and return sludge flow rates and MLSS
concentration should be considered in the design
– Safety factor used is meant for this purpose
Solids loading rate is a limiting parameter and affects
effluent concentration of TSS
– Effluent quality remains unaffected over a wide range of surface
overflow rates (upto 3-4 m/h)
xSVIVi )001586.01646.0(871.1)(ln +−=
xDSVIVi )002555.0103.0(028.2)(ln +−=
Design of Secondary clarifier on the basis of
SVI and ZSV

46. Side wall liquid depth can be as low as 3.5 m for large clarifiers and
as high as 6 m for smaller clarifiers
– Deeper clarifiers have greater flexibility of operation and larger
margin of safety
Tank inlet section or central well
– Jetting of influent (cause for density currents) should be avoided
through dissipate influent energy
– Distribution of flow should be even in horizontal and vertical
directions and should not disturb the sludge blanket
– Design of central well should promote flocculation
– Cylindrical baffle of diameter 30-35% of the tank diameter can be
used as central well
– Bottom of the feed well should end well above the sludge blanket
interface
Other information for the design of
Secondary Clarifiers

47. Operational Problems of ASP
Common problems encountered in operating the ASP
• Bulking sludge
• Rising sludge
• Nocardia foam
Bulking sludge
• Causes high suspended solids in the effluent
Flocs do not compact and settle well and sludge blanket depth increases
(beyond typical 10 to 30 cm)
• Results in poor treatment performance
Maintaining desired level of MLSS/MLVSS becomes difficult, effluent
has suspended BOD, higher recycle rates reduce wastewater’s HRT
Two types of bulking: Filamentous and Viscous bulking

48. Filamentous bulking
• Filaments normally protrude out of the sludge floc
• Surface area to mass ratio increases and sludge attains poor settling
properties
Viscous bulking
• Caused by excessive amount of extracellular hydrophilic
biopolymer
• Makes the sludge highly water retentive (hydrous bulking)
Bulking Sludge

49. Factors causing bulking
– Wastewater characteristics, like, readily biodegradable organic matter
and fermentation products, H2S and reduced sulfur compounds (septic
water), nutrient deficiency and low pH
– Flow variations and variations in pH
– Design limitations, like complete mix reactor conditions, limited air
supply, poor mixing, short circuiting, defective sludge collection and
removal and limited return sludge pumping capacity
– Operational issues, like, low DO, insufficient nutrients, longer SRT
and subsequent low F/M, insufficient soluble BOD (for these
filamentous organisms are very competitive), internal plant
overloading (recycle loads of centrate and filtrate)
Nutrient limiting systems and very high loading of wastewater with
high levels of readily biodegradable COD can cause viscous bulking
Bulking Sludge

50. Control of bulking may require investigation on
– Wastewater characteristics
– Process loading
– Return and waste sludge pumping rates
– Internal plant overloading
– Clarifier operation
Investigation is usually started with microscopic examination of mixed
liquor
Bulking Sludge

51. Solutions for bulking
– Decreasing SRT or operating the aeration equipment at full
capacity can take care of bulking from limiting DO
• DO should be >2 mg/l under normal loading conditions
– Selector processes (aerobic, anoxic and anaerobic) in place of
complete mix systems can be a solution for bulking from longer
SRT and low F/M ratios
– Internal plant overloading can be avoided through recycling
centrate and filtrate during the periods of minimal hydraulic and
organic loading
– Not retaining the sludge for more than 30 minutes can avoid
septic conditions and subsequent bulking
Bulking Sludge

52. Bulking can be temporarily controlled by Cl2 and H2O2
– 0.002-0.008 kg per day of Cl2 per kg of MLVSS for 5-10 hr
HRT systems
– Chlorination can produce turbid effluent and kill nitrifiers
– Trihalomethanes and other compounds with potential health and
environmental effects can be formed
– Dose of H2O2 depends on extent of filamentous development
Bulking Sludge

53. Differentiated from bulking sludge by presence of small gas bubbles in
the sludge
Common in systems with conditions favourable for nitrification
Nitrification is the common cause
• Nitrification in the aeration basins produces nitrite and nitrate
• Denitrification in the clarifiers converts produces nitrogen gas
• Trapping of nitrogen gas makes the sludge buoyant
Solutions may include
• Reduced sludge detention in the clarifier (increasing the speed of sludge
collection and withdrawal)
• Reduced mixed liquor flow to the clarifier (decreases sludge depth)
• Decrease SRT and/or aeration for controlling nitrification
• Post-aeration anoxic process prevents denitrification in clarifiers
Rising Sludge

54. Usually associated with Nocardia and Microthrix parvicella
– Hydrophobic cell surfaces allow attachment of bacteria to and
stabilization of air bubbles to cause foaming (0.5 to 1.0 m thick)
The foaming can go beyond the ASP and get into aerobic and anaerobic
sludge digesters
Control measures
– Avoid foam trapping aeration basins (baffles with flow under can trap
foam in the basin)
– Reduce oil and grease (Nocardia and Microthrix are usually associated
with these) flow into the aeration basin
– Avoid recycling of skimmings of clarifiers to aeration basins
– Use of selectors can discourage foaming
– Addition of small concentrations of cationic polymers and chlorine
spray over the surface of foam can also reduce foaming
Foaming

55. Selector Processes
A small tank or a series of small tanks are used for mixing the
incoming wastewater with the return sludge under aerobic or
anoxic/anaerobic conditions
• Controls filamentous bulking and improves sludge settling
characteristics
• High rbCOD F/M ratio discourages filamentous growth but
encourages floc forming non-filamentous bacterial growth
Selector process designs are two types
• kinetic or high F/M selectors
– Higher substrate concentrations result in faster substrate uptake by floc
forming bacteria
– High DO (6 -8 mg/L) is needed for maintaining aerobic floc
– Recommended F/M ratios are 12, 6 and 3 per day COD F/M ratios for
a 3 tank selector
– too high F/M ratios, >8 BOD/day ) can cause viscous bulking

56. Selector Processes
Metabolic or anoxic or anaerobic processes selectors
• Improved sludge settling characteristics and minimal filamentous
bacteria are observed with the biological nutrient removal processes
– Filamentous bacteria can not use nitrate or nitrite as electron acceptor
under anoxic conditions
– Filamentous bacteria do not store polyphosphates and hence can not
consume acetate under anaerobic conditions
• Anoxic or anaerobic metabolic conditions are used
– Anaerobic selector can be used before the aeration tank (phosphorus
removal can occur)
– If nitrification is used, then anoxic selectors can be used
• For high F/M anoxic/anerobic selectors SVI of mixed liquor can be
as low as 65-90 mL/g (common SVI is 100-120 mL/g)