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Secondary Treatment
Dr. Akepati S. Reddy
School of Energy and Environment
Thapar University, Patiala
Punjab, INDIA
Secondary Treatment
• Biological treatment for the removal of biodegradable
organic matter from the wastewater
– Mostly by aerobic and facultative microbiological treatment
• Involves removal of biodegradable organic matter by design
– Coincidental removal of pathogens (MPN count) and nutrients
(nitrogen and phosphorus) occurs
• Divisible into three types or categories
– Suspended growth types: ASP and its modifications, MBR, SBR
(denitrification), aerated lagoons, oxidation ponds, oxidation
ditches
– Attached growth types: TFs, RBC, SAFF, FAB, MBBR
– Others: Facultative ponds, constructed wetlands and vegetated
ponds
Activated Sludge Process
(ASP)
ASP and its Components
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
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
Aeration
basin
Secondary
clarifierInfluent Effluent
Wasted activated sludge
Sludge recycling
Nutrients and
Alkalinity (if needed)
Air/oxygen for
aeration & mixing
Activated Sludge Process
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)
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
Mechanisms of Treatment
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
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 biomass
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)
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
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
ASP kinetics and Kinetic
parameters
Substrate Utilization Rate
Aeration
tank












es
e
a
eiei
su
SK
Sq
x
SS
V
SSQ
r .max)()(

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
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 





)()(
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



.
).(1
d
es
e
k
SK
SqY
SRT



..1 .max
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




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
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 
 is temperature activity coefficient
(typical value 1.02 to 1.25)
kT and k20 are k values at T°C and 20°C
respectively
ASP design
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
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 
Active Biomass Concentration (xa)
Mixed Liquor Active Biomass Concentration
Use of this equation requires
– Primary variables SRT and  (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,  and bCOD removal
 
)(1 SRTk
YSSSRT
x
d
ei
a




 
d
a
ei
k
x
SSY
SRT



.
1

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




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



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



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







rateionautooxidat
Biomass
ratesynthesis
biomassGross
ratesynthesis
biomassNet
daei kVxSSQYNBSR ..)(. 
 
)(1 SRTk
YSSSRT
x
d
ei
a




Here V is replaced by Q.
For xa the following equation is used
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)
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
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
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 Q 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










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 
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)

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


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
CDGRplusNBSR
ofequivalentOxygen
substrateloadedof
equivalentOxygen
demand
Oxygen
   CDGRNBSRnSSQdemandO ei 2
Actual Oxygen Transfer Efficiency/Rate
• AOTR is actual oxygen transfer rate under field conditions – it
is influenced by
– Salinity-surface tension of the wastewater ()
– 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 ()
– 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,

 







 

Actual Oxygen Transfer Rate or Efficiency
 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)
 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
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,
,
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!
 
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airthein
fractionoxygen
efficiencytransfer
oxygenActual
demandOxygen
required
Air
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
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

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luent
theinN
ationdenitrific
throughlostN
sludgewasted
theinN
effluent
theinN
trequiremenN
inf
 NNitrateTKNQxQ
MLSS
x
TSSQtrequiremenN uw
a
e 





 1223.01223.03.0
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
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
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
Secondary Clarifier
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
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
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
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
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
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)
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 
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 
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
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
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 
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
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
Weir placement and loading
• In larger clarifiers circular overflow weir with trough can be
placed at 2/3rd to 3/4th radial distance from the center
– For smaller clarifiers it can at the perimeter
– A baffle can be provided to deflect density currents away from the
overflow weir and avoid scum overflow
– Up-flow velocity in the vicinity of weir should be 3.5-7 m/hr
• Weir loading rates should be < 375 m3/m.day
– Should be <250 m3/m.day if located in density current upturn
zone
– Should be <125 m3/m.day for average flow and <250 m3/m.day for
maximum flow in smaller tanks
Scum removal: not a problem in secondary clarifiers and may be
needed when primary clarifiers are not used
– Removed scum should not be taken back for treatment
Other information for the design of
Secondary Clarifiers
Monitoring, Operation, and
Control of ASP
Operation and Control of ASP
• Nutrient dosing
– Monitor the influent (flow rate, BOD, TKN and T-P)
– Estimate nutrient requirement and dose
• Aeration
– Monitor DO level in the aeration tank
– Use variable speed drives to regulate aeration (affinity laws)
• Sludge recycling
– Monitor sludge blanket depth in the secondary clarifier
– Maintain the sludge blanket depth through regulating sludge
recycling
• Sludge wastage from the underflow
– MLSS and underflow sludge concentration
– Maintain MLSS through controlled wastage of sludge
• Sludge wastage from the aeration tank
– TSSe in the clarified effluent and estimation apparent SRT
– Estimation and wasting of mixed liquor
Performance evaluation of ASP
• Monitor the influent for
• TSS, VSS and biodegradable VSS
• BOD/COD
• TKN, TP and Alkalinity
• Flow rate, temperature and pH
• Monitor the treated effluent for
• Soluble and total BOD/COD
• TSSe
• TKN and TP
• Monitoring wasted sludge
• Wastage rate
• TSS, VSS and biodegradable VSS
• Nitrogen and Phosphorus levels
• Monitoring of aeration tank
• MLSS and MLVSS and biodegrable MLVSS
• DO levels
• Microscopic examination of the sludge
Common problems encountered: Bulking sludge; Rising sludge and
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
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
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
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
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
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
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
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
ASP Process Control
Principal aspects of monitoring, operation and control
• Monitoring of the ASP:
• Influent: TSS and VSS, TKN, Total-P, BOD/COD (total and
soluble), alkalinity, pH and Temperature
• Mixed liquor: MLSS and MLVSS, DO
• Effluent: TSS, VSS, Total nitrogen, Total-P, Soluble BOD/COD
• Maintaining DO level in the aeration tank
• Regulating activated sludge recycling rate
• Control sludge blanket level in the secondary clarifier
• Controlling wastage of the activated sludge
• Assessment and dosing of nutrients and alkalinity
• Tackling rinsing sludge and bulking sludge problems
• Limiting DO levels (<0.5 mg/L!) can result in filamentous sludge
(bulking sludge problem) and affect settlability of sludge
• Desired DO is 1mg/L (0.5 to 2.0 mg/L) in all the areas of the aeration
basin
– Higher DO results in nitrification (>2 mg/L favourable) in the aeration
tank, and denitrification (raising sludge problem) in the secondary settling
tank – DO level below 0.1 - 0.2 mg/L in the secondary settling tank can
facilitate denitrification
– > 4 mg/L DO may not improve operations (nitrification becomes zero
order reaction) but prove costly
• Monitoring of DO in the aeration tank (simultaneously at different
depths and at different locations)
• Monitoring of redox potential and DO level in the sludge blanket of
the secondary settling tank
Monitoring and Maintaining DO
Matching oxygen supply with demand
– Clogged filters and fouled diffusers affect supply
– Providing filter upstream to blowers and their frequent cleaning
can enhance air supply and can minimize internal fouling of
diffusers
Organic overloading and higher SRT can increase O2 demand
– Decreasing SRT to match with the increased organic overloading
can control oxygen demand
Non-uniform mixing and localized DO deficiencies
– DO monitoring in the aeration basin can identify the problem
– Can be caused by burst diffusers
– Lower MLSS, anoxic/anaerobic conditions, filamentous growth,
denitrification and efficiency drop can occur
Monitoring and Maintaining DO
• Sludge washout in the clarified effluent should be prevented
– Maintain the sludge blanket below the effluent weir
• Return sludge pump should be of ample capacity (100 to
150% of the average design sludge flow)
– Hydraulic overloading of ASP leads to solids overloading of
clarifier
increases sludge blanket depth and necessitates pumping of
return sludge at higher rate and lower consistency
– Depending on settling characteristics return sludge
concentration may vary between 4000-12000 mg/L
necessitates better control over return sludge pumping rates
• Flow management through equalizing the influent can avoid
the hydraulic overloads
Regulating sludge recycling
Return sludge flow rate should be determined by sludge blanket
level control
• Maintain the sludge blanket below the effluent weir for
avoiding washout
– Optimum depth is 0.3 to 0.9 m
– Variations in flow and strength (bCOD and TSS), sludge settling
properties, and sludge withdrawal rates affect the level
• Sludge blanket levels can be detected by
– Withdrawing samples using air lift pumps, gravity flow tubes,
portable sampling pumps and core samples
– Sludge-supernatant interface detectors
• Regulation of sludge blanket level requires considerable
operator attention
Regulating sludge recycling
Sludge wasting can be
• From the return sludge line
– Lesser volume of sludge is wasted
– Control is difficult (require monitoring TSS in the mixed
liquor, in the clarified effluent and in the underflow sludge)
• From the aeration tank in the form mixed liquor
– Volume wasted is large (may require a thickener)
– Can be wasted either into a primary clarifier or a thickener
– Control is much easier (may need only TSSe measurement)
Sludge wasting is determined on the basis of SRT
– Due consideration should be given to sludge washout
Sludge wasting
Nutrient and chemical feed control
Lower pH and nutrient deficiency can cause filamentous bulking
and affect the treatment efficiency
• bCOD:N:P should be favourably maintained
– Measure TKN, nitrate and total phosphorus levels in the influent
- compare with desired bCOD:N:P ratioand decide nutrient dose
– Urea and/or DAP can be dosed to adjust nutrient deficiencies
(to cut the cost powdered phosphate rock or phosphoric acid
can also be used in place of DAP)
• pH of the influent should adjusted and pH of the aeration
basin contents should maintained favourable
– Measure the influent alkalinity and pH
– If pH is lower adjust it (neutralize fermentation products of
septic influent) with MOL
– If sufficient alkalinity is not available add MOL (70-80 mg/L as
CaCO3 may be required)
Secondary clarifier control
Raising sludge, density currents and sludge washout
• Nitrified mixed liquor loading of the clarifier
• Longer retention of sludge in the clarifier
• Higher temperature of mixed liquor inducing density currents
• Design defects in the secondary clarifier
Solutions to the problems may include
• Design corrections such as
– Providing density current deflectors and/or scum baffles
– Ensuring even distribution of the ML and uniform overflow
• Proper collection from all over the clarifier and continuous
recycling of sludge
Secondary clarifier control
Increasing sludge blanket height and sludge washout
• Solids overloading from the hydraulic overloading of ASP
• Poor settling & thickening properties of the bulking sludge
• Insufficient sludge recycling
Solutions
• Small dose of polymers can improve sludge settling properties
• Microscopic examination of the sludge and measurement of SVI
and sludge blanket depth can be useful
• Regulate blanket depth through altering sludge recycling rate
• Use SVI to decide on sludge recycling rate
• Minimize loading by
– Wasting activated sludge directly from aeration tank
– Avoid internal hydraulic loads during peak hydraulic loads
Start up of an ASP
• Seeding of the aeration basin initially on a regular basis may
be needed to ensure acclimation
• Start with a HRT equal to designed SRT and gradually decrease
towards designed HRT while maintaining the sludge recycling
rate at the designed level and ensuring gradual buildup of
MLSS
• When wasting is from the sludge recycle line
• Start sludge wasting only after the actual MLSS reached the
designed MLSS
• Gradually increase the sludge wastage rate until designed SRT is
achieved while ensuring that the MLSS is at the designed level
• When wasting is from the aeration tank
• Start sludge wasting once the HRT reaches the designed HRT
Safety considerations
Physical hazards: trips, falls and drowning
Chemical hazards: associated with the handling of alkali, urea, DAP,
Chlorine, hydrogen peroxide, etc.
Biological hazards: air borne pathogens from aeration, contact with
wastewater containing pathogens
Noise hazards: blowers and drives
Electrical and mechanical hazards
Confined space hazards: secondary clarifier under-drain system
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
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)
Monitoring
Monitoring including sampling frequency and parameters to be
analyzed should be need based
Monitoring may be required for
– Carrying out treatability studies and design of the ASP
– Facilitating operation and control of the ASP
– Assessing compliance with the requirements (achievement of set
and design efficiencies)
– Performance evaluation of the ASP
Monitoring involves sampling and analysis of samples of
– Influent wastewater of the ASP
– Aeration tank contents
– Mixed liquor being fed to the secondary clarifier
– Clarified secondary effluent (secondary clarifier overflows)
– Return sludge
Monitoring
Monitoring can be online (appropriate for process control !)
– Monitoring involves collection of samples and analysis in
– Industrial units own ETP or central laboratory (for routine
parameters)
– Outside laboratory (sometimes 3rd party laboratory) for advanced
analysis (for non-routine parameters requiring sophisticated
instruments)
3rd party laboratory is depended on specially for compliance
assessment/ monitoring
Sampling location should judiciously chosen
Monitoring
Method of sampling chosen should ensure collection of representative
samples
– Usually grab sampling can serve the purpose
– Occationally composite (flow proportionated?) sampling may be
required
Frequency of sampling and parameters to be analysed for should be
decided on the basis of the monitoring objective/purpose
Typical sampling locations and comprehensive list of parameters for
analysis for an ASP are identified here
Sampling at all the locations, sampling by any specific method of
analysis, and analysis for all the identified parameters may not
always be required
Sampling frequency can be at fixed intervals and whenever a need
arises
Streams to be monitored and parameter
Influent (including internal recycle flows): Assess both quantity
and characteristics
– Flow rate and variations (if needed the flow should be managed)
– Internal recycle flows
– Organic matter concentration – BOD, bCOD (BOD kinetics) and
COD
– Nutrients – TKN (ammonical and organic nitrogen), nitrate plus
nitrite nitrogen, and total phosphorus
– Suspended solids: TSS, VSS, nbVSS (indirect method)
– Total alkalinity, Temperature and pH
Aeration basin contents:
– Temperature, pH and Dissolved oxygen
– Oxygen uptake rate and specific oxygen uptake rate (Toxicity
assessment)
Monitoring of ASP
• pH in the aeration tank to check whether enough alkalinity
is present
• Nutrients in the treated effluent – TKN, Nitrate-N and
Total-P
• DO in the aeration tank (sample should not come in
contact with air and biological flocs should be deactivated)
• MLSS and MLVSS, TSS in the clarified secondary effluent
and SVI of mixed liquor
• Visual and microscopic examination of microbial flocs
Mixed liquor
– MLSS and MLVSS
– Nitrate and nitrite nitrogen
– SVI and ZSV
– Microscopic examination of biological flocs
Return sludge
– MLSS or consistency
Clarified secondary effluent
– Soluble BOD, total BOD, COD
– Nitrate plus nitrite nitrogen and TKN (ammonical and organic
nitrogen separately)
– Total phosphorus and TSS
Streams to be monitored and parameter
Design of the ASP
Characterization and quantification of the influent to be treated
– Flow rate and variations (average flow and peaking factor) –
flow equalization may be needed to dampen variations
– Organic matter concentration – BOD, bCOD (BOD kinetics) and
COD
– Nutrients – TKN (ammonical and organic nitrogen), nitrate plus
nitrite nitrogen, and total phosphorus
– Suspended solids: TSS, nbVSS (indirect method)
– Total alkalinity
– Temperature (summer critical and winter critical temperature of
the influent)
Ambient conditions, like, summer and winter critical temp.,
altitude and atmospheric pressure, may be needed
Operation and Control of the ASP
Chemical dose requirements
– Flow rate, BOD, TKN (even nitrate plus nitrite –N), Total –P,
and Alkalinity of the influent
– Residual TKN (and nitrate plus nitrite –N) and total –P of treated
effluent
– Frequency of monitoring can be weekly once or lesser
Aeration system
– Suction pressure down stream to the air filter, compressed air
pressure and air flow rate (online monitoring)
• May indicate filter clogging, diffuser fouling and bursting of
diffuser membrane
– DO level in the aeration basin (may be as a part of investigation
of bulking sludge)
Operation and Control of the ASP
Regulation of the return sludge system
– Sludge blanket depth in the clarifier (continuous monitoring may be
needed)
– Consistency of the return sludge and MLSS in the mixed liquor
– SVI of the mixed liquor
– Influent flow rate
Sludge wasting system
– Consistency of the clarifier underflow and MLSS of the mixed liquor
– TSS in the clarified effluent of the clarifier
Presence of toxic/inhibitory subtances
– OUR and SOUR of the mixed liquor in the aeration
Operation and Control of the ASP
Secondary clarifier control
• Influent flow rate (to assess hydraulic overloading)
• Nitrate, temperature, SVI and ZSV of mixed liquor
– (to indicate raising sludge, density currents, and settling properties of
mixed liquor solids)
• Sludge blanket depth
• TSS level and turbidity of the clarified effluent
– Straggler floc: fluffy floc in the clarified effluent – associated with low
SRT operation – often coexist with white foam
– Pinpoint floc: pin-floc in the clarified effluent – associated with high
SRT operation – coexist with quickly settling floc and often with darker
foam
– Surface ashing: condition of lighter pin-floc floating to the surface and
spreading out in the clarifier
Analytical Methods
Flow meaurement (online measurement) by
– Pumping rate and pump runtime recording
– Treated effluent flow rate by flow meters such as V- or
rectangular notches, parshall flume, etc.
BOD, bCOD and COD
– BOD: BOD bottle method, head-space BOD method,
respirometric technique, or by BOD sensors
– bCOD: through BOD kinetic experiments
– COD: by closed or open reflux methods
• Use BOD – COD – bCOD relationship for the operation and
control of ASP
Analytical Methods
Nutrients
• TKN (organic and ammonical –N) by kjeldahl apparatus and either
titrimetry or colorimetry
• Nitrite and nitrate –N by using cadmium reduction column and
colorimetry
Suspended solids (TSS, VSS, nbVSS, MLSS and MLVSS)
• Filtration, gravimetry and ashing for MLSS and MLVSS
• TS – TDS difference technique for TSS (and ashing for VSS)
• Change in VSS over sufficiently long time through aeration by
autooxidation can be basis for nbVSS measurement
SVI for mixed liquor: volume in mL occupied by one gram of
mixed liquor solids after 30 minutes settling
ZSV: subsidence velocity in m/hr. of the sludge blanket interface
in the settling column)
Analytical Methods
• Oxygen uptake rate measurement by recording DO depletion of the
mixed liquor over a few minutes
• Specific oxygen uptake rate from OUR by dividing with MLVSS or
active biomass concentration
• Microscopic examination of biological flocs of mixed liquor or of
secondary clarifier for filamentous growth and for microscopic air
bubbles
• pH by pH meter (or indicator strip!) and alkalinity (in mg/L as
CaCO3) by titrimetry
• Temperature (of ambient air, influent and mixed liquor)
• DO (of the aeration basin contents) by DO meter with submersible
long leed probe
Data recording and analysis
All monitoring data needs recording in log books
– Data may be weekly/monthly/seasonally analysed for trends and
performance reports may be generated
All incidents may be recorded and the recording can include the
investigatory monitoring being carried out and corrective and
preventive actions taken
– Hydraulic or organic overloading
– Loading of toxic or inhibitory substances
– Bulking sludge, raising sludge, and foaming problems
– Critical machinery failures
Compliance Assessment
BOD and TSS (and even flow rate) are needed to be monitored
– Comparison with statutory requirements may be needed
– BOD is contributed by
• Residual sBOD
• Biodegradable fraction of the TSS (MLVSS to MLSS ratio!)
If operated for nutrient removal then TKN and nitrate and nitrite –N
and total –P may also be needed
– Nutrients are also present in the TSS
ASP Performance Evaluation
ASP performance evaluation may involve
• Measurement of actual performance of the ASP and comparison
with the designed performance
– May concentrate mainly on
• Treatment efficiencies (BOD removal and nutrient removal)
• Sludge generation rates and MLVSS/MLSS ratio
• Oxygen consumption rates
• Back calculation of ASP kinetic parameters and comparison with
values used in the design and improvement of equations used in the
ASP performance assessment
• Performance evaluation of the secondary clarifier
ASP Variants
Complete mix reactor (CFSTR)
Most used in India – simple design - suitable for all types of
aeration equipment
Uniform and low levels of substrate, and uniform MLSS and
constant oxygen demand throughout the basin
Resistant to shock loads and toxic loads
Hydraulic and organic load variations are dampened better
Toxic discharges are mitigated through greater dilution
Filamentous bulking from exposure of recycled sludge to relatively
low levels of substrate
A pre-contacting zone (of 15 min. HRT) can avoid this!
Typical design and operational conditions
SRT: 3-15 days MLSS: 1500 to 4000 mg/L
F/M ratio: 0.2 to 0.6/day BOD loading: 0.3 to 1.6 kg/m3.day
HRT: 3 to 5 hours Sludge recycle ratio: 0.25 to 1.0
Plug flow reactor
Long narrow aeration tanks (aspect ratio >10) with plugflow regime
True plug flow does not exist – extent of longitudinal mixing
depends on the type of aeration system used
Degree of longitudinal mixing is described by Dispersion Number
(ND) which is defined as D/(UL) or Dt/L2
D – Coefficient of axial dispersion (m2/sec.) – for diffused
aeration system it increases by a factor of 2 with air flow
increase from 1.2 to 6 m3/m3.hr
U – mean velocity of flow (m/Sec.)
L – length of the tank
t – HRT of the tank (L/U) for Q+QR flow
For a good plug flow condition ND value is <0.1
A plug flow reactor is equivalent to a series of complete mix
reactors
Substrate concentration varies along the reactor length
O2 utilization is highest at the inlet end and decreases towards lowest
at the outlet end
Plug flow reactor
Affected by toxic or inhibiting organics (problematic for industrial
wastewater with toxic constituents)
A baffled inlet section can ensure better sorption in case of readily
degradable wastewaters
A separate inlet zone (15% of total volume) with only mixing but no
aeration can facilitate denitrification of recycled sludge
If designed for nitrification, an anoxic zone at the outlet end can
bring about denitrification
Design and operation are relatively more complicated – matching
oxygen demand and supply is difficult
Early designs had uniform air application throughout the tank
length but modern designs have tapered aeration
Typical design values
SRT: 3-15 days HRT: 4 to 8 hours
BOD loading: 0.3 to 0.7 kg/m3.day F/M ratio: 0.2 to 0.4/day
Sludge recycling ratio: 0.25 - 0.75 MLSS: 1000 to 3000 mg/l
Plug flow
Aeration tank
Wasted sludge
Recycledsludge
Clarifier
Effluent
Influent
Plug flow reactor: Step Feed
Plug flow reactor with wastewater introduction at 3 or 4 feed points
(equalizes F/M ratio)
MLSS is initially highest (5000 to 9000 mg/L), and decreases with
each of the feed points
Establishes more uniform oxygen demand
Flexible operation
Wet weather flows can be bypassed to the last pass
Adaptable to many operating schemes (anoxic/aerobic processes)
If needed can also be operated in contact stabilization mode by
feeding only at the last feed point
More complicated design and complex operation
Typical design and operation values
SRT: 3-15 days MLSS: 1500-4000 mg/l
BOD loading: 0.7-1.0 kg/m3.day F/M ratio: 0.2 to 0.4/day
Sludge recycling ratio: 0.25 to 0.75 HRT: 3-5 hours
Plug flow
Aeration tank
Effluent
Influent
Recycled sludge
Clarifier
Contact Stabilization process
Has two separate tanks or compartments one for wastewater
treatment and the other for sludge stabilization
Requires smaller aeration volume and good for low solubility index
wastewaters
Wet weather flows can be handled without loss of MLSS
Has little or no nitrification capacity and operation is somewhat
complicated
Typical design and operation values
Contact time: 30 - 60 min. Stabilization time: 2 - 4 hours
SRT: 5-10 days Sludge recycling ratio: 0.5 to 1.5
F/M ratio: 0.2-0.6 /day Volumetric loading: 1-1.3 kg/m3day
MLSS: 1000-3000 mg/L (for contact tank) & 6000-10000 (for
stabilization tank)
Contact tankStabilization tank Effluent
Clarifier
Wasted sludge
Influent
Recycled sludge
Staged Reactor Systems
Consist of usually 2 complete mix reactors in series
• Employed for nitrogen removal (or just nitrification) and for
phosphorus removal
System for nitrogen removal
• Uses pre or post anoxic reactor
• Aerobic reactor is maintained at higher (>2 mg/L) DO level
• Post anoxic often requires dosing of organic substrates
System for nitrification
• 2 stages: Stage-1 for BOD removal and stage-2 for nitrification
• Each stage has a clarifier of its own
• A portion of wastewater is directly taken into stage-2
• Stage-2 is operated at a longer SRT
System for the phosphorus removal
• Two stage system with a single clarifier after stage-2
• Stage-1 is anaerobic where as stage-2 is aerobic
• Wasted sludge (phosphate accumulating organisms) contains the
removed phosphorus in the form of polyphosphates
Aeration
reactor
Nitrification
reactor
Influent Effluent
Wasted sludge
clarifier
Return sludge
clarifier
Influent bypass
Return sludge
Wasted sludge
Treatment for BOD removal and Nitrification
Anaerobic
reactor
Aeration
reactor
Influent Effluent
Wasted sludge
clarifier
Return sludge
Aeration
reactor
Aeration
reactor
Digester
Stabilized
sludge
Supernatant
Return sludge
Influent Effluent
Return sludge
clarifier
Phosphorus removal
Pure Oxygen Activated Sludge Process
A series of well-mixed covered aeration tanks with co-current gas-
liquid contact
Influent, recycled sludge & O2 are introduced at stage-1 (O2 can
also be mixed with the influent under pressure)
Restricted exhaust is allowed from the last stage (O2 in the
exhaust is ~50% and O2 utilization rate ~90%)
Disadvantages
More complicated equipment and complex installation, operation and
maintenance
Peak flows can disrupt operation by sludge washout
Has limited capacity for nitrification
Nocardia foaming is possible
Designed for
DO >6 mg/L F/M ratio 0.6-1/day
SRT: 1-4 days MLSS 2000-9000 mg/L
BOD loading: 1.3-3.2 kg/m3.day HRT: 1-3 hours
Sludge recycling ratio: 0.25-0.5
Membrane Bioreactor
Membrane process (micro and ultra filtration) replaces the
secondary clarifier
• Membrane process helps to maintain higher SRT against low HRT
- allows 10-20 day SRT and 10-15 g/L MLSS
• Two configurations, internal (submerged) bioreactor and external
bioreactor, are in use
• Hollow fiber and flat sheet membranes are usually used
Problems associated with MBR are
• High membrane cost and high energy requirements for
maintaining high trans-membrane pressure
• Membrane fouling rapidly deteriorates flux rates across the
membrane and necessitates frequent membrane cleaning and
replacement
• Aeration (2-phase flow) helps in controlling the fouling through
keeping the solids in suspension and scouring the membrane
surface
SBR: Sequencing Batch Reactor
A modification to the ASP transforming a continuous process into
a batch process
Usually preferred when the quantity of wastewater is lesser and
highly variable
All the treatment steps (aeration and clarification) are brought
about in the same tank sequentially in cycles of filling, aeration,
settling, and decanting supernatant
Excess activated sludge is wasted after decanting prior to the
starting of cycle of treatment (quantity and frequency of
wastage is decided on the basis SRT to be maintained)
The processes and operations are accomplished as timed
sequences
– 1. Fill: 3 hr 2. React: 2-20 3. Settle: 0.5-1hr
– 4. Decant: 0.5-1 hr X. Idle
Even nitrification, denitrification and even sludge stabilization can
also be accommodated within
– Just mixing without aeration during the fill stage ensures anoxic
conditions needed for denitrification
Complicated process control and require higher maintenance skills
for the equipment used
Batch discharge may necessitate equalization for down stream
processing of the effluent
Typical design and operational parameters
SRT: 10-30 days F/M ratio: 0.04-0.1/day
BOD loading: 0.1-0.3 kg/m3.day MLSS: 2000-5000 mg/L
SBR modifications
• Batch decant reactor, intermittent extended aeration system:
– Treatment include continuous filling and sequential reaction,
settling and decanting
– A pre-react (baffled) chamber facilitates continuous feeding without
disturbing the settling/decanting operations
• Cyclic activated sludge system:
– Continuous wastewater feeding, but batch removal of effluent
– Reactor has three baffled zones of 1:2:20 volumes and is fed
continuously but the effluent is removed in batches
– Mixed liquor is recycled from 3rd zone to 1st zone of the reactor
Sequencing Batch Reactor (SBR)
Aerated lagoons, oxidation ponds
and oxidation ditches
Extended Aeration Process: Counter-
current aeration system
A circular tank with revolving bridge is used
Air diffusers mounted at the bottom of the revolving bridge
supply oxygen
Turning off air but revolving the bridge keeps the tank
contents in suspension and facilitate denitrification
Typical design and operational parameters
SRT: 10-30 days F/M ratio: 0.04 to 0.1/day
HRT: 15-40 hours BOD loading: 0.1-0.3 kg/m3.day
MLSS: 2000-4000 mg/L Sludge recycle ratio: 0.25-0.75
Oxygen transfer efficiency is higher but diffuser fouling can
be problem (fine screening of wastewater can prevent)
Complicated operation requiring good operator skills
Down time for maintenance is relatively higher
Extended Aeration: Other
modifications
Orbal process
A modified oxidation ditch using a series of concentric
channels of depth upto 4.3 m
Wastewater enters the outer channel and flows towards
the center before entering an internal/external clarifier
Nitrification and denitrification are facilitated by regulating
aeration rates
Biolac process
Earthen tanks of 2.4-4.6 m depth with submerged
aeration and with either internal or external clarifiers
Fine bubble diffusers attached to floating aeration chains
move across the basin by air released from diffusers
Use of timers to cycle air flow through each aeration
chain facilitates nitrification and denitrification
Extended Aeration Process
Well stabilized and low bio-solids sludge is generated – the
sludge is mainly of cell debris and sludge contributed by
the influent
Primary clarification is usually not used
Considered suitable for smaller flows
Aeration tanks are larger, and oxygen demand and aeration
energy requirement are higher
Aeration equipment design is controlled by mixing needs
(mostly not by oxygen demand)
Sensitive to hydraulic overloads (clarifier can be overloaded
by solids) and insensitive to concentration shock loads
Typical design and operational parameters
F/M ratio is 0.04-0.10/day BOD loading: 0.1-0.3 kg/m3.day
SRT: 20-40 days MLSS: 2000-5000 g/m3
HRT: 20-30 hours Sludge recycling ratio: 0.5 to 1.5
Extended Aeration: Oxidation Ditch
Ring or oval shaped loop reactor system with unidirectional
flow (velocity: 0.25-0.3 m/sec. and cycling time: 5-15 min.)
Brush type/surface type mechanical aerators power
horizontal flow and bring about aeration/mixing
Screened wastewater is mixed with recycled sludge and
allowed into the tank of 20-30 hour HRT
Intra-channel clarifiers can be used (for secondary clarifiers)
Advantages
Highly reliable process and simple operation
Amenable for both BOD and nitrogen removal
Uses less energy and adaptable for nutrient removal
Disadvantages
Space requirement is higher
Plant capacity expansion is more difficult
Low F/M bulking is possible
EffluentClarifier
Wasted sludge
Oxidation ditch
Mixed liquor
Recycled sludge
Influent
Trickling Filters
Trickling filters
• Most conventional technology and relatively less used
• Circular or rectangular non-submerged fixed film bioreactors
• Includes
– Rock or plastic packing as a medium for bio-film development
– Wastewater dosing or application system
– Under drain system
– Structure containing packing
– Secondary settling unit
• Clarified or fine screened primary effluent is applied on the
packing and allowed to trickle through
• The bio-film is alternatively exposed to wastewater and air
• Wastewater made to flows as a thin film over the bio-film
• Treated effluent from TF is passed through a secondary clarifier
and let out as treated secondary effluent
• Portion of the clarified effluent may be recycled to TF for
– Diluting the strength of incoming wastewater
– Maintaining enough wetting of the bio-film during minimal or no
flow conditions
Trickling filters
Wastewater
Trickling filter
Primary
clarifier
Trickling
filter
Secondary
clarifier
Treated
effluent
To sludge
Handling & disposal
Recirculation
sludge
Trickling Filter
In some wire mesh screen is placed over the top
Requires less energy and easy to operate
Have more potential for odors and quality of treated effluent
is low –causes are
Inadequate ventilation
Poor clarifier design
Inadequate protection from cold temperatures
Dosing operations
Biofilm development may not be uniform in the trickling
filter
Liquid may not uniformly flow over the entire packing
surface (wetting efficiency)
Packing
Low cost material with high specific surface area, high enough
porosity and high durability is ideal
• Rock (rounded river rock, crushed stone or high quality
granite), blast furnace slag, or plastic can be used
Corrugated plastic sheet packing
– High hydraulic capacity, high porosity, and resistance to plugging
(for better air circulation and bio-film sloughing)
– Most used and two types: cross flow and vertical flow
– Packing depth is 4 to 12 m (6 to 6.5 m is typical), hence require
less land area
– Superior to stone packing for higher organic loading rates
Rock packing
– High density limits depth – 0.9 to 2.5 m (1.8 m is typical)
– Size is 75 to 100 mm (95% of the material)
Wastewater dosing/application system
Two types of wastewater dosing/distribution systems:
Rotary distributors & Fixed Nozzle Distribution Systems
Rotary distributors
• Has 2 or more hollow arms mounted on a central pivot
– Arms have nozzles and extend across the TF diameter
– Spacing between nozzles decreases center to perimeter (ensures
uniform distribution of wastewater)
• Dynamic reaction of the discharged wastewater from nozzles
revolves the distribution arm
– Rotational speed varies with flow
– Alternatively electrical motor can also be used
• Clearance between distributor arm and filter packing top is 150
to 225 mm
• Typical head loss in the distributor is 0.6 to 1.5 m
• Rotational speeds of distributors
– In the past these were 0.5 to 2 rpm – but now reduced for better
filter performance
– Change from conventional 1 to 5 min/rev. for rock filters to dosing
every 30-55 min. once improved performance (reduced bio-film
thickness & odour observed)
– Can be decided by
N = (1+R)q / (A.DR)
R, recycle ratio q, influent flow rate
A, no. of distributor arms DR, dosing rate
Wastewater dosing/application system
• Dosing volumes
– Higher dosing volumes are reported to improve wetting efficiency,
agitation, flushing and wash away of fly eggs, and resulted in
thinner bio-film
– Daily intermittent high flushing doses are shown to control bio-
film thickness and solids inventory
Fixed nozzle distribution system
– In case of square or rectangular filters fixed flat spray nozzles are
used
– Used for shallow rock filters
– Flat spray pattern nozzles are used (at periphery half spray nozzles
are used)
– Water pressure is varied systematically in order to spray
wastewater first at maximum distance and then at decreasing
distance
Wastewater dosing/application system
Under-drain system
Meant to catch filtered wastewater and solids and convey to
secondary clarifier
• Should facilitate ventilation, easy inspection and flushing if
needed
– Drains are open at both the ends and open into a circumferential
channel
Floor and drains of the filter should be strong enough to support
packing, slime growth and wastewater
– Rock filters use pre-cast blocks of vitrified clay or fiber glass
grating laid on reinforced concrete sub-floor
– Plastic packing filters use beams over columns
Free draining out of filtered wastewater should be ensured
– 1 in 5 bottom slope is provided towards the outlet
– under-drains are designed for flow velocity >0.6 m/sec. at average
flow
– Beams are provided with channels on their top
Ventilation and air flow
Natural draft is the primary means for providing the needed
air flow
• Often inadequate – forced ventilation by low pressure fans can
be solution – FD or ID fans can be used
• Driving force is temperature difference between ambient air and
wastewater
• During periods of no temperature difference natural draft may
not be there
• Cooler wastewater forces downward air flow through the filter
and warmer wastewater forces upwards air flow
• Upward air flow not desirable - supply of oxygen is lower where
demand is higher and odours can be a problem
For better air flow
• Drains should not flow beyond half full
• Open area over the top of the under-drains should be >15%
• Each 23m2 filter area should have 1 m2 area of ventilation
manholes or vent stacks
Ventilation and air flow
Secondary clarifier
Settling is meant for the removal of suspended flocs and
discharging the clarified sewage –
– Sludge is not recycled – but can be returned to primary
clarifier
– Part of the clarified effluent may be recycled for reducing
strength or for keeping wet of the bio-film
• Sludge is relatively easily settlable
Settling tanks with side wall depth of 3.6 to 3.7 and overflow
rate 1m/hr for average flow and 2 m/hr for peak flow are
used.
With proper design and operation of a secondary clarifier
clarified effluent has <20 mg/l of TSS
Trickling filters
Classified as
• Low or standard rate filters
– Produce effluent of consistent quality though influent is of varying
strength
– Top 0.6 to 1.2 m of the filter has appreciable bio-film and lower
portion may have nitrifying bacteria
– Odours and filter flies may be the problems
• Intermediate and high rate filters
– Flow is usually continuous
– Recirculation of effluent allows higher organic loadings
– May be a single or two stage process
– High rates filters usually use plastic packing
• Roughing filters
– Much higher organic loadings (>1.6 kg/m2.day) are used
– Hydraulic loading can be upto 190 m3/m2.day
– Used to treat wastewater prior to secondary treatment (require low
energy than ASP – 2 to 4 kg BOD per kWh for TF against 1.2 to
2.4 kg BOD per kWh for ASP)
2-stage filters
• High strength wastewaters prefer 2-stage filter with intermediate
clarifier
• Preferred when nitrification is required
– Nitrification demands low organic loading (<30 mg/l) – complete
nitrification demands <15 mg/l of organic load
• Separate TF, after secondary treatment, can also be used for
nitrification
Trickling filters
Biological community of trickling filter
Biological community of trickling filter includes
– Aerobic, anaerobic and facultative bacteria
– Fungi, algae and protozoans
– Worms, insect larva and snails
Bio-film on the medium surface is mainly composed of aerobic,
facultative and anaerobic bacteria
– Lower reaches of the filter can have nitrifying bacteria
– At lower effluent pH, fungi play important role
– Algae may grow in the upper reaches of the filter
Problems caused by biological community
– Protozoa, worms, insect larva and snails feed on biological films
and increase effluent turbidity
– Snails are troublesome in the TFs used for nitrification
– Algae can cause filter clogging and odors problem
– Fungal growth can also clog the filter
Sloughing of bio-films
Bio-film can be up to 10 mm thick – in the top 0.1 to 0.2 mm
aerobic degradation occurs
• Organic loading increases thickness of the film
• Aerobic environment and substrate can not penetrate inner
depths of the bio-film
• In case of thick films, due to anaerobic environment and
substrate limiting conditions and consequent endogenous
respiration, bio-film looses its clinging ability
• Under hydraulic loading sloughing off of bio-film occurs
• Mechanisms of sloughing for rock packing is apparently
different from that of plastic packing
– In plastic packing sloughing off is a small scale process and occurs
from hydrodynamic shear
– In rock packing large scale spring sloughing occurs mainly due to
insect larva activity – affects total BOD of the effluent (can be
higher than that of the influent)
Trickling Filter: design
Design is based on empirical relationships derived from
pilot plant studies and full-scale plant experiences
Following parameters are related with treatment efficiency
and used as design and operating parameters
– Volumetric organic loading
– Unit area loading
– Hydraulic application rates
Treatment efficiency: >90% at <0.5 kg/m3.day BOD
loading; and <60% at >3.5 kg/m3.day BOD loading
Nitrification:
– For effective nitrification BOD loading must be very little
– Nitrification rates can be 0.5 to >3 g/m3.day
– Efficiencies depend on the packing surface area and on the
specific NH4-N loading rate
– Higher temperature and better wetting efficiencies can enhance
the nitrification efficiency
Empirical formula for BOD removal for 1st stage TF is
E1 is BOD removal efficiency at 20C
W1 is BOD loading kg/day
V is volume of filter packing (in m3)
F is recirculation factor; Defined as
Here R is recycle ratio and its value can be 0 to 2.0
VF
W
E
1
1
4432.01
1


 2
10
1
1
R
R
F



Empirical formulae for BOD removal
for rock filter media
Empirical formulae for BOD removal
for rock filter media
BOD removal efficiency at 20°C in the 2nd stage TF
W2 is BOD loading to 2nd stage TF in kg/m3
BOD removal efficiency is temperature dependent and
temperature correction can be made by
VF
W
E
E
2
1
2
1
4432.0
1
1



20
20 )035.1( 
 T
T EE
Empirical formulae for plastic packing
Contact time of wastewater with bio-film depends on the
filter depth and the hydraulic application rate
 Wastewater contact time (in min.)
q, hydraulic loading rate in L/m2.min.
Q, wastewater flow rate (L/min.)
A, cross sectional area of the filter in m2
c, constant for the packing used
D, depth of the packing in m (typical is 6.1 -6.7 m)
n, hydraulic constant for packing
Depends on temperature and specific surface area
Taken as 0.67 at 20C and as 0.5 for a packing of 90 m2/m3
specific surface area
n
q
Dc

A
Q
q 
Change in BOD concentration with time is
k is taken as 0.69/day at 20C
It incorporates the constant ‘C’ (constant for the packing)
Its value is temperature dependent
kS
dt
dS

n
q
kD
kte
S
S 

 expexp
0
20
20 035.1 
 T
T kk
Empirical formulae for plastic packing
Modified Velz equation for the effluent
BOD for the plastic packing
R is recycle ratio
 is temperature correction coefficient (1.035)
So is influent BOD and Se is effluent BOD, mg/L
As is specific surface area (m2/m3)
D is depth of the filter (m)
‘q is hydraulic application rate (L/m2.sec.)
recommended value is >0.5L/m2.sec.
‘n’ is constant characteristic of the packing used (depends
on temperature and specific surface area (taken as 0.67)
K20 is rate constant at 20°C
 
  
R
Rq
DAK
R
S
S
n
S
Te









 
1
exp1
20
20
0

Modified Velz equation
K value
Its value is 0.059 to 0.351
for domestic wastewater it is 0.21
it requires adjustment for both packing depth and influent BOD
K1 is K value for D1 = 6.1 m and S1 = 150 mg/l of BOD
For influent BOD of 400 to 500 mg/l oxygen transfer can
become limiting in trickling filters
Loading of soluble bCOD >3.3 kg/m3.day can also limit oxygen
5.0
2
1
5.0
2
1
12 












S
S
D
D
KK
Natural air draft through filter can be determined by
Dair, draft in mm of water column
Z, height of the filter in meters
Tc, cold temperature in K
Th, hot temperature in K
Air flow velocity through the filter is proportional to the natural air
draft
Pressure drop across filter is related to flow velocity by
total head loss in kPa (kPa=100 mm water column) - 101.325 kPa
is equal to 1 atmos or 10132 mm water column;)
V is superficial air flow velocity (m/sec.)
Np is filter tower resistance factor in terms of velocity heads
Ventilation and air flow
Z
TT
D
hc
air 




  11353





g
VNP pT 2
2
P
Np is estimated as
Np is number of velocity heads
D is depth of packing (m)
L/A is liquid mass loading rate (kg/m2.hr)
A is filter cross-section area (m2)
Total head loss through the filter (Npt) is usually taken as 1.3
to 1.5 times to Np
2.0 times for rock packing of 45 m2/m3 specific surface area
1.6 times for plastic random packing with 100 m2/m3 specific surface
area
For plastic packing with cross flow conditions the multiplying factor is
1.3 when specific surface area is 100 m2/m3 and 1.6 for the
specific surface area of 140 m2/m3 packing
 A
L
p DN
)1036.1( 5
exp33.10



Ventilation and air flow
Air flow requirements
Oxygen required per kg of BOD loaded to the TF is
Ro, kg of oxygen needed per kg of BOD loaded
Lb, BOD loading rate kg/m3.day (typical: 0.3-1.0 kg/m3.day)
If nitrification is also considered then
Nox/BOD, ratio of influent nitrogen oxidized to influent BOD
72 m3 of air at 20°C, at assumed O2 transfer efficiency of 5%,
can supply one kg of O2 to the TF – transfer efficiency is
taken as 2.5% in case of nitrification
– Oxygen content of air 0.279 kg/m3 at 20°C
bb -0.17L-9L
o exp1.2exp0.8R 
BOD
NO
4.6exp1.2exp0.8R x0.17L-9L-
o
bb

Air flow requirements
Air flow (in m3/day) required (AR) for a TF is
For temperature and pressure different form 20C and one
atmos, air flow rate is
If temperature is >20C air requirement will increase -
solubility of oxygen decreases with increasing temperature
Recommended air flow is 0.3 m3/m2.min.
a
A
20T
P
760
293
T273
ARAR







100
20-T
1ARAR A
T
'
T
00QSR72AR 
Q, flow rate of influent in m3/day
S0 is influent BOD in kg/m3
Nitrification
Used either as a combined system or as a tertiary treatment
Influent BOD and DO within the TF impact nitrification rates
– Maximum rates occur when sBOD is <5mg/l, rates are
inhibited at >10 mg/l and rates are insignificant at >30 mg/l
– For rock media filters for BOD loading of <0.08 kg/m3.day,
nitrification efficiencies are >90% and for the loading of 0.22
kg/m3.day it is 50%
– For >90% nitrification surface loading of NH3-N of <2.4
g/m2.day is needed (biofilm surface)
Linear relationship exists between specific nitrification rate
and BOD/TKN ratio and can be shown by
– Here RN is specific nitrification rate (in g/m2.day)
44.0
82.0








TKN
BOD
RN
Nitrification
Packing design, temperature, oxygen availability and ammonia
loading rate all affect nitrification rates
• DO concentration has very great effect
• Follows zero order reaction (against NH4-N) for most portion of
the packing (for NH4-N in the > 5-7 mg/L range)
• Rates decline from top with depth may be from the decrease of
bio-film growth, from predation (grazing by snails!) and from
changed wetting efficiency
• Rates can be 1 to 3 g/m2.day (bio-film) and may decline by 20-
50% with decrease in temperature from 20 to 10°C
For higher nitrification rates
– NH4-N should be >5 mg/L
– hydraulic loading rates should also higher
– Specific surface area of 100 m2/m3 may be appropriate
Periodic floods can minimize the predation problem
For determining the volume of packing and the hydraulic
application rate, the following empirical equation is used
– Here RN is surface nitrification rate for Z depth of TF and T
operating temperature
– N is NH4-N concentration in the bulk liquid
– r is an empirical constant
– KN is N conc. at RN=RN.Max./2 – its expected value is 1.5 mg/l
– RN.Max. is in g.N/m2.day – its value is 1.1 to 2.9 g.N/m2.day -
temperature correction is considered not needed for 10-25C range
– beyond this correction is done by
rZ
N
MaxNTZN
NK
N
RR 







 exp..,),(
10
)10.(.).(. 045.1 
 T
MaxNTMaxN RR
Nitrification
Rotating Biological Contactors
Rotating biological contactor (RBC)
• Series of closely spaced polystyrene/polyvinyl chloride
circular disks on horizontal shaft constitute RBC unit
• Standard RBC unit includes
– 3.5 m dia disks with total disk area of 9300 m2/unit to support
microbial film
– A shaft of 8.23 m length (of this 7.23 m is occupied by disks)
• RBC unit is placed in a 45 m3 capacity tank
– Shaft orientation is either perpendicular to or parallel to the
wastewater flow
• RBC unit is usually provided with an enclosure
– Prevents algal growth
– Discs are protected from sunlight (UV light)
– Prevents heat loss and exposure to cold weather
RBC
Treatment process (sec. or advanced level)
• Treatment is for BOD removal, or nitrification, or both, or
for pretreatment of higher strength industrial effluent
• Wastewater clarified in primary clarifier or fine screened is
fed to the reactor
• 40% of disc surface is submerged in wastewater
maintained in a 45 m3 tank
• Disk surface is alternatively brought in contact with
wastewater and atmosphere by rotating at 1 to 1.6 rpm
rate either mechanical or pneumatically
• Treatment occurs through bio-sorption of organic matter of
wastewater into bio-film and aerobic biooxidation of the
sorbed matter when bio-film is exposed to atmospheric air
• Bio-film as it thickens and looses its ability to cling to disc
surface due to hydrodynamic shears sloughs off
• Treated effluent needs secondary clarification
• A number of RBC units may be operated in series to
form a process train
– Exploit the benefits of staged biological reactor design –
facilitates maintaining different conditions in different stages
– For reliability two or more parallel flow trains are employed
– 2-4 units in series are used for BOD removal
–  6 units are used for combined BOD removal & nitrification
– To avoid overloading on initial stages, stepped feed or
tapered systems are opted
– RBC units decrease as one moves to higher stages
• RBC units are of low, medium and high density types
– Low density or standard type units are used for initial stages
– Medium and high density units (11000 and 16,700 m2 area)
are used in the mid and final stages
RBC
Simple to operate and involves low energy costs
Performance is related to specific surface loading of
BOD and/or NH4-N
– For first stage it is 12-20 g/m2.day of soluble BOD (as total
BOD it is 24-30 g/m2.day)
– For nitrification maximum loading rate is 1.5 g/m2.day
Associated with odor and bio-film sloughing problems
– Occurs when oxygen demand exceeds supply
– Sulfur oxidizing bacteria form tenacious whitish film and
prevent sloughing off
Structural failure of shafts, disks and disk support
systems can occur
– Excessive bio-film growth and sloughing problems cause it
RBC
RBC design considerations
Principal elements of RBC
• Disc material and configuration
– HDPE of different configurations or corrugation patterns
– Corrugation increases available surface area and enhances
structural stability
• Shaft
– Shape is square, round or octagonal
– Steel shafts coated for protection against corrosion of 13-30
mm thickness are used
• Tankage
– Requirement is 0.0049 m3/m2 film area
– Typical side wall depth is 1.5 m
– At 0.08 m3/m2 day hydraulic loading rate HRT is 1.44 hrs
Drive system
– Mechanical or pneumatic drives are used for shaft rotation
– Mechanical drive capacity is 3.7 or 5.6 kW per unit
– Deep plastic cups are attached to the perimeter of the disks
and compressed air is released into the cups for rotation
– Air requirement is 5.3 m3/min for standard density shaft and
7.6 m3/min for high density shaft
Enclosures
– Segmented fiberglass reinforced plastic enclosures are used
RBC design considerations
RBC design
First stage RBC disk area is determined by using 12-20
g/m2.day sBOD loading
Disk area of subsequent stages is found by second order
model by Opatken
Sn is soluble BOD concentration in mg/l
As is disk surface area for stage-n (in m2)
Q is flow rate in m3/day
Here sBOD/BOD is taken as
0.5 for secondary clarified effluent
0.5 to 0.75 for primary clarified effluent
)/(00974.02
)/(00974.0411 1
QA
SQA
S
s
ns
n


 
For nitrification stages area required is found by using
maximum nitrification rate (rn.max) as 1.5 g/m2.day
– Applicable if sBOD of wastewater is <10 to 15 mg/l -
otherwise rn.max should be corrected by:
Here Frx is fraction of nitrification rate possible
sBOD is soluble BOD loading in g/m2.day
sBODFrx  1.00.1
RBC design
SAFF, FAB and MBBR
reactors
Facultative Ponds
Waste Stabilization Ponds
• Shallow, manmade basins comprising of one or more series of
anaerobic, facultative and maturation ponds
• Used to treat domestic or municipal wastewater to
– Remove biodegradable organic matter, BOD (by >90%)
– Remove pathogens (bacteria and viruses by 4-6 log units, and
protozoan cysts and helminth eggs by upto 100%)
– Remove nutrients (Nitrogen by 70-90% and Phosphorus by 30-45%)
and sufficiently clarify the wastewater
• If properly designed and operated, can give the effluent of
– Filtered BOD <25 mg/L
– TSS <150 mg/L
– Nematode eggs <1/L
– Fecal coliform count <1000/100 mL
Waste Stabilization Ponds (WSP)
• Represent sustainable natural effluent treatment systems
– Uses solar energy and do not require electricity
– Do not use any electromechanical equipment
• Low cost, low energy, and low maintenance systems, and do not
require skilled manpower
– Construction involves earth moving, pond lining and pond
embankment protection, and pond inlets and outlets and construction of
screens and grit chambers
– Operation and maintenance requirements are minimal (repair of
embankments, cutting embankment grass, removing scum and
vegetation, keeping both inlet and outlet clear, etc.) and requires only
unskilled but carefully supervised labour
– When compared with trickling filters, aerated lagoons, oxidation
ditches, and ASP, WSP are cheapest and even land cost may not be
acting against WSP
Waste Stabilization Ponds (WSP)
• Can be easily scaled down to small scale applications
• Robust systems (withstand organic & hydraulic shocks and
copes up well with heavy metals upto < 60 mg/L)
• Principal requirements are sufficient land, and soil with low
coefficient of permeability (<10-7)
• Suited to tropical and sub-tropical countries, like india – sun
light and temp. (high throughout) are favourable
– Inexpensive land, restricted foreign currency availability and shortage
of skilled manpower favour the use
• Also produce fish
Waste Stabilization Ponds (WSP)
Disadvantages
• Requires more land (2-5 m2/capita)
– 1-2 day HRT for anaerobic pond and 3-6 day HRT for facultative pond
– Require 25 day HRT in 5 pond WSP in hot climates to produce the
quality fit for restricted irrigation
– Require 10 day HRT in 2 pond system for producing the quality fit for
restricted irrigation
• Potential odour and mosquito nuisance specially from
anaerobic ponds
• High algal content in the treated effluent
• High evaporation losses of water specially in facultative and
maturation ponds
• Adverse environmental impacts may include ground water
pollution
• Anaerobic ponds represent primary treatment
– sludge stabilization is add on feature
– patogen removal (helminth eggs) is coincidental
• Facultative ponds represent secondary treatment
– coincidental removal of nutrients and pathogens
• Maturation ponds represent tertiary treatment
– used to remove pathogens (fecal bacteria)
– nutrient removal is coincidental
Waste stabilization Ponds (WSP)
Preliminary Treatment
Unless very small WSP systems must include both screening and
grit removal facilities
– Hygienic disposal of screenings and grit is needed (haulage to
sanitary landfills or on-site burial in trenches)
– All wastewater should be pass through screening and degritting
Provisions may be made for flow measurement and recording
both upstream & downstream to WSP system
Provisions may be made for
– Diverting the flow beyond 6 times to dry weather flow into
stormwater and receiving water course
– Allowing a maximum of 3 times to dry weather flow into
anaerobic ponds and diverting rest into facultative ponds
– Bypassing the anaerobic pond
Effluent limits to be complied with
EU’s requirements
• Filtered (non-algal) BOD and COD: 25 mg/L and 125 mg/L
respectively
• Suspended solids: 150 mg/L
• Total nitrogen and total phosphorus for avoiding
eutrophication: 15 mg/L and 2 mg/L respectively
– If population is >1,00,000 then total-N and total-P should be 10
mg/L and 1 mg/L respectively
Effluent limits to be complied with
Indian limits
– BOD (non-filtered): 30 mg/L
– Suspended solids: 100 mg/L
– Total-N: 100 mg/L
– Total ammonical-N: 50 mg/L
– Free ammonical-N: 5 mg/L
– Sulfide – 2 mg/L
– pH 5.5 to 9.0
Effluent limits to be complied with
Discharge into surface or ground water!
WHO guidelines of 1989 for restricted crop irrigation:
• 105 E. coli per 100 ml
• Human intestinal nematode eggs 1 per liter
• If children under 15 years are exposed (playing or working in the
irrigated field) then 0.1 eggs/L
• Intestinal nematodes include
• Ascaris lubricoides
• Trichuris trichiura (human whipworm)
• Ancylostoma duodanale and
• Necator americanus (hookworms)
Effluent limits to be complied with
WHO guidelines of 1989 for unrestricted crop irrigation:
• 1000 E.coli per 100 ml
• human intestinal nematode eggs 1 per liter – if children are
eating the food crops uncooked then 0.1 eggs/L
Restricted irrigation: irrigation of all crops except salads and
vegetables eaten uncooked
WHO guidelines of 1989 for aquacultural use of effluent
– 104 E.coli per 100 ml in the fish pond water
– ‘0’/L of detectable human trematode eggs in the effluent
Human trematodes include
– Schistosoma sp.
– Clonorchis sinensis
– Fasciolopsis buski
• For fish (Carp and Tilapia)/aquatic vegetable culturing effluent
from facultative ponds can be used
• For restricted irrigation systems with only anaerobic and
facultative ponds can be sufficient
• Maturation ponds are required for producing the effluent
suitable for unrestricted irrigation or for effluent discharge into
bathing water
• Fish ponds can be loaded on the basis of nitrogen load (4 kg-
N/ha.day)
• Free NH3 in ponds > 0.5 mg/L can prove toxic
What is required for complying with the
requirements?
Facultative Ponds
2 types: primary and secondary
– Primary ponds received screened & degritted wastewater
– Secondary ponds receive effluent from anaerobic ponds
Typical depth is 1 – 2 m and HRT is 5 to 30 days
Properly designed facultative pond has
• Aerobic top layer all through the day and night
– Diurnal variation in DO concentration is experienced – oxypause
(depth beyond which DO is zero) show vertical movement
– Presence of aerobic layer reduces methane and H2S emissions –
remove odours
• Anaerobic bottom layer never in direct contact with
atmosphere
Facultative Ponds
Filtered effluent BOD is 20 to 60 mg/L (TSS level in the effluent
is 30 to 150 mg/L)
• Algae also contributes both BOD (80% of the algae is
biodegradable) and TSS
– Effluent take off or removal from top 50 cm layer can result in large
fluctuations in effluent quality
• Algae mostly settles to bottom and anaerobically biooxidized
Facultative Ponds
Ponds look dark green in colour due to algae
– Healthy ponds have 500-2000 µg/L of chlorophyll-a
– Can also look red or pink due to the presence of anaerobic purple
sulfide oxidizing photosynthetic bacteria (slight BOD5 overloading can
cause it)
Diurnal variations of DO & pH can be high due to high photosynthetic
activity
– DO as high as 20 mg/L and pH >9.4 are possible
– High DO and pH are important for fecal bacteria and viruses removal
BOD5 removal in primary facultative ponds is about 70% on unfiltered
basis and 90% on filtered basis
– Filtering removes algae and hence higher efficiency
– In Europian Union the WSP effluent should achieve 25 mg/L of BOD5
in the filtered effluent
Facultative Ponds: Oxygen Balance
Can be shown by
1. Q(Cin-Cout): Net oxygen entry with the wastewater
2. AK(Csat-C): Reaeration at the pond surface
3. : Net photosynthetic contribution:
photosynthetic O2 generation – DO consumption in algal
respiration and in algal biomass decomposition
4. : Nitrification demand of oxygen
(Ammonical-N to Nitrate-N) - aN can be taken as 4.5
5. : DO consumption for the bCOD removal - aB
can be taken as 1.5
          BODBODBNNNdodresphotosatin CCQaCCQarrrCCKACCQ
dt
dC
V inin

 BODBODB CCQa in

 inNNN CCQa 
 dodresphoto rrr 
Facultative Ponds: Surface Reaeration
Surface reaeration occurs by the combined effect of molecular
diffusion and vertical mixing of pond by wind
– rain fall also increases mixing plus it carries DO
Mass transfer coefficient K for zero wind conditions can be
estimated by
At 20C temp. & 0 mg/L DO, reaeration is 0.13 to 0.62 g/m2.day
For every 3 m/s raise in wind speed the reaeration rate increases
by a multiplying factor of 2.5
h
DU
K 
D is molecular diffusivity of oxygen in water
h is water depth
U is water speed – may be 3-40 m/day
Facultative Ponds: Algal Growth and
Photosynthetic O2 Production
Algal growth (biomass yield Y) can be estimated by
Algal concentration depends on organic loading and temperature
and may range between 500-2000 µg/L as chlorophyll-A
Photosynthetic oxygen production can be stoichiometrically
related to algal growth by
Oxygen production occurs mostly in the top 50 cm of water - In
the absence wind mixing concentrated band of algae moves up
and down through the top 50 cm
21645180106
3
4322
2
309
1690106 OPNOHCLightPONOOHCO  
h
S
Y

3.1
Y is net algal biomass yield
S is average visible radiation
 is light conversion efficiency
h is specific chemical energy of algal biomass – in
tropical areas under clear skies 172 cal/m2.day
Treatment Mechanisms:Pathogen destruction/
removal
Fecal coliform removal
• Natural decay or disinfection
• Occurs by a combination of processes via complex interaction of
various adverse environmental factors
• Aquatic environment
• Algal activity and photo-oxidation
• Adsorption to particles and subsequent sedimentation and grazing by
protozoa etc. also contributes but very little
• In anaerobic ponds sedimentation of solids is the major
contributor
• In facultative ponds and maturation ponds the removal is
influenced by
– Time and temperature
– High pH (>9.0)
– High light intensity and high DO
Treatment Mechanisms:Pathogen destruction/
removal
• Algal growth and photosynthesis
– Bactericidal effect - excretion of anti-bacterial substances
– High DO enhances the photo-oxidation process
• Raising pH above a critical level (>9.5) - Carbonate and bicarbonate ions
provide CO2 to algae and leave hydroxide ions raising pH to >9.0
• Photo-oxidation by incident solar radiation through sensitizer
molecules, like, singlet oxygen, superoxide, hydrogen peroxide,
hydroxyl radicals, etc.
– Solar radiation, pH and DO have synergistic effect
DNA damage by UV radiation (UV-B: 290-320 nm)
– UV radiation attenuates with depth (16 to 46 attenuations per meter)
and becomes ineffective beyond the upper few cm
– at higher pH (>8.5) even longer wavelengths are effective
• Vertical mixing (increases the decay)
• Starvation due to lack of nutrients or carbon source
Viruses
– Apparently removed by adsorption on to settlable solids and
consequent sedimentation
Helminth eggs & protozoan cysts
– Removed by sedimentation
– Most removal takes place anaerobic and facultative ponds
Treatment Mechanisms:Pathogen destruction/
removal
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02d secondary treatment

  • 1. Secondary Treatment Dr. Akepati S. Reddy School of Energy and Environment Thapar University, Patiala Punjab, INDIA
  • 2. Secondary Treatment • Biological treatment for the removal of biodegradable organic matter from the wastewater – Mostly by aerobic and facultative microbiological treatment • Involves removal of biodegradable organic matter by design – Coincidental removal of pathogens (MPN count) and nutrients (nitrogen and phosphorus) occurs • Divisible into three types or categories – Suspended growth types: ASP and its modifications, MBR, SBR (denitrification), aerated lagoons, oxidation ponds, oxidation ditches – Attached growth types: TFs, RBC, SAFF, FAB, MBBR – Others: Facultative ponds, constructed wetlands and vegetated ponds
  • 4. ASP and its Components
  • 5. 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
  • 6. 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
  • 7. Aeration basin Secondary clarifierInfluent Effluent Wasted activated sludge Sludge recycling Nutrients and Alkalinity (if needed) Air/oxygen for aeration & mixing Activated Sludge Process
  • 8. 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)
  • 9. 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
  • 11. 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
  • 12. 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 biomass 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)
  • 13. 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
  • 14. 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
  • 15. ASP kinetics and Kinetic parameters
  • 16. Substrate Utilization Rate Aeration tank             es e a eiei su SK Sq x SS V SSQ r .max)()(  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
  • 17. 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       )()( 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    . ).(1 d es e k SK SqY SRT    ..1 .max
  • 18. 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     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
  • 19. 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   is temperature activity coefficient (typical value 1.02 to 1.25) kT and k20 are k values at T°C and 20°C respectively
  • 21. 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
  • 22. 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 
  • 23. Active Biomass Concentration (xa) Mixed Liquor Active Biomass Concentration Use of this equation requires – Primary variables SRT and  (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,  and bCOD removal   )(1 SRTk YSSSRT x d ei a       d a ei k x SSY SRT    . 1 
  • 24. 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     Here V is replaced by Q. For xa the following equation is used
  • 25. 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)
  • 26. 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
  • 27. 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
  • 28. 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 Q 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           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 
  • 29. 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
  • 30. Actual Oxygen Transfer Efficiency/Rate • AOTR is actual oxygen transfer rate under field conditions – it is influenced by – Salinity-surface tension of the wastewater () – 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 () – 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,             
  • 31. Actual Oxygen Transfer Rate or Efficiency  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)  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
  • 32. 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, ,
  • 33. 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
  • 34. 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
  • 35. 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
  • 36. 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
  • 37. 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
  • 38. 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
  • 40. 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
  • 41.
  • 42. 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
  • 43. 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
  • 44. 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
  • 45. 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
  • 46. 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)
  • 47. 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 
  • 48. 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 
  • 49. 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
  • 50. 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
  • 51. 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 
  • 52. 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
  • 53. 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
  • 54. Weir placement and loading • In larger clarifiers circular overflow weir with trough can be placed at 2/3rd to 3/4th radial distance from the center – For smaller clarifiers it can at the perimeter – A baffle can be provided to deflect density currents away from the overflow weir and avoid scum overflow – Up-flow velocity in the vicinity of weir should be 3.5-7 m/hr • Weir loading rates should be < 375 m3/m.day – Should be <250 m3/m.day if located in density current upturn zone – Should be <125 m3/m.day for average flow and <250 m3/m.day for maximum flow in smaller tanks Scum removal: not a problem in secondary clarifiers and may be needed when primary clarifiers are not used – Removed scum should not be taken back for treatment Other information for the design of Secondary Clarifiers
  • 56. Operation and Control of ASP • Nutrient dosing – Monitor the influent (flow rate, BOD, TKN and T-P) – Estimate nutrient requirement and dose • Aeration – Monitor DO level in the aeration tank – Use variable speed drives to regulate aeration (affinity laws) • Sludge recycling – Monitor sludge blanket depth in the secondary clarifier – Maintain the sludge blanket depth through regulating sludge recycling • Sludge wastage from the underflow – MLSS and underflow sludge concentration – Maintain MLSS through controlled wastage of sludge • Sludge wastage from the aeration tank – TSSe in the clarified effluent and estimation apparent SRT – Estimation and wasting of mixed liquor
  • 57. Performance evaluation of ASP • Monitor the influent for • TSS, VSS and biodegradable VSS • BOD/COD • TKN, TP and Alkalinity • Flow rate, temperature and pH • Monitor the treated effluent for • Soluble and total BOD/COD • TSSe • TKN and TP • Monitoring wasted sludge • Wastage rate • TSS, VSS and biodegradable VSS • Nitrogen and Phosphorus levels • Monitoring of aeration tank • MLSS and MLVSS and biodegrable MLVSS • DO levels • Microscopic examination of the sludge
  • 58. Common problems encountered: Bulking sludge; Rising sludge and 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 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
  • 59. 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
  • 60. 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
  • 61. 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
  • 62. 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
  • 63. 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
  • 64. 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
  • 65. ASP Process Control Principal aspects of monitoring, operation and control • Monitoring of the ASP: • Influent: TSS and VSS, TKN, Total-P, BOD/COD (total and soluble), alkalinity, pH and Temperature • Mixed liquor: MLSS and MLVSS, DO • Effluent: TSS, VSS, Total nitrogen, Total-P, Soluble BOD/COD • Maintaining DO level in the aeration tank • Regulating activated sludge recycling rate • Control sludge blanket level in the secondary clarifier • Controlling wastage of the activated sludge • Assessment and dosing of nutrients and alkalinity • Tackling rinsing sludge and bulking sludge problems
  • 66. • Limiting DO levels (<0.5 mg/L!) can result in filamentous sludge (bulking sludge problem) and affect settlability of sludge • Desired DO is 1mg/L (0.5 to 2.0 mg/L) in all the areas of the aeration basin – Higher DO results in nitrification (>2 mg/L favourable) in the aeration tank, and denitrification (raising sludge problem) in the secondary settling tank – DO level below 0.1 - 0.2 mg/L in the secondary settling tank can facilitate denitrification – > 4 mg/L DO may not improve operations (nitrification becomes zero order reaction) but prove costly • Monitoring of DO in the aeration tank (simultaneously at different depths and at different locations) • Monitoring of redox potential and DO level in the sludge blanket of the secondary settling tank Monitoring and Maintaining DO
  • 67. Matching oxygen supply with demand – Clogged filters and fouled diffusers affect supply – Providing filter upstream to blowers and their frequent cleaning can enhance air supply and can minimize internal fouling of diffusers Organic overloading and higher SRT can increase O2 demand – Decreasing SRT to match with the increased organic overloading can control oxygen demand Non-uniform mixing and localized DO deficiencies – DO monitoring in the aeration basin can identify the problem – Can be caused by burst diffusers – Lower MLSS, anoxic/anaerobic conditions, filamentous growth, denitrification and efficiency drop can occur Monitoring and Maintaining DO
  • 68. • Sludge washout in the clarified effluent should be prevented – Maintain the sludge blanket below the effluent weir • Return sludge pump should be of ample capacity (100 to 150% of the average design sludge flow) – Hydraulic overloading of ASP leads to solids overloading of clarifier increases sludge blanket depth and necessitates pumping of return sludge at higher rate and lower consistency – Depending on settling characteristics return sludge concentration may vary between 4000-12000 mg/L necessitates better control over return sludge pumping rates • Flow management through equalizing the influent can avoid the hydraulic overloads Regulating sludge recycling
  • 69. Return sludge flow rate should be determined by sludge blanket level control • Maintain the sludge blanket below the effluent weir for avoiding washout – Optimum depth is 0.3 to 0.9 m – Variations in flow and strength (bCOD and TSS), sludge settling properties, and sludge withdrawal rates affect the level • Sludge blanket levels can be detected by – Withdrawing samples using air lift pumps, gravity flow tubes, portable sampling pumps and core samples – Sludge-supernatant interface detectors • Regulation of sludge blanket level requires considerable operator attention Regulating sludge recycling
  • 70. Sludge wasting can be • From the return sludge line – Lesser volume of sludge is wasted – Control is difficult (require monitoring TSS in the mixed liquor, in the clarified effluent and in the underflow sludge) • From the aeration tank in the form mixed liquor – Volume wasted is large (may require a thickener) – Can be wasted either into a primary clarifier or a thickener – Control is much easier (may need only TSSe measurement) Sludge wasting is determined on the basis of SRT – Due consideration should be given to sludge washout Sludge wasting
  • 71. Nutrient and chemical feed control Lower pH and nutrient deficiency can cause filamentous bulking and affect the treatment efficiency • bCOD:N:P should be favourably maintained – Measure TKN, nitrate and total phosphorus levels in the influent - compare with desired bCOD:N:P ratioand decide nutrient dose – Urea and/or DAP can be dosed to adjust nutrient deficiencies (to cut the cost powdered phosphate rock or phosphoric acid can also be used in place of DAP) • pH of the influent should adjusted and pH of the aeration basin contents should maintained favourable – Measure the influent alkalinity and pH – If pH is lower adjust it (neutralize fermentation products of septic influent) with MOL – If sufficient alkalinity is not available add MOL (70-80 mg/L as CaCO3 may be required)
  • 72. Secondary clarifier control Raising sludge, density currents and sludge washout • Nitrified mixed liquor loading of the clarifier • Longer retention of sludge in the clarifier • Higher temperature of mixed liquor inducing density currents • Design defects in the secondary clarifier Solutions to the problems may include • Design corrections such as – Providing density current deflectors and/or scum baffles – Ensuring even distribution of the ML and uniform overflow • Proper collection from all over the clarifier and continuous recycling of sludge
  • 73. Secondary clarifier control Increasing sludge blanket height and sludge washout • Solids overloading from the hydraulic overloading of ASP • Poor settling & thickening properties of the bulking sludge • Insufficient sludge recycling Solutions • Small dose of polymers can improve sludge settling properties • Microscopic examination of the sludge and measurement of SVI and sludge blanket depth can be useful • Regulate blanket depth through altering sludge recycling rate • Use SVI to decide on sludge recycling rate • Minimize loading by – Wasting activated sludge directly from aeration tank – Avoid internal hydraulic loads during peak hydraulic loads
  • 74. Start up of an ASP • Seeding of the aeration basin initially on a regular basis may be needed to ensure acclimation • Start with a HRT equal to designed SRT and gradually decrease towards designed HRT while maintaining the sludge recycling rate at the designed level and ensuring gradual buildup of MLSS • When wasting is from the sludge recycle line • Start sludge wasting only after the actual MLSS reached the designed MLSS • Gradually increase the sludge wastage rate until designed SRT is achieved while ensuring that the MLSS is at the designed level • When wasting is from the aeration tank • Start sludge wasting once the HRT reaches the designed HRT
  • 75. Safety considerations Physical hazards: trips, falls and drowning Chemical hazards: associated with the handling of alkali, urea, DAP, Chlorine, hydrogen peroxide, etc. Biological hazards: air borne pathogens from aeration, contact with wastewater containing pathogens Noise hazards: blowers and drives Electrical and mechanical hazards Confined space hazards: secondary clarifier under-drain system
  • 76. 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
  • 77. 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)
  • 78. Monitoring Monitoring including sampling frequency and parameters to be analyzed should be need based Monitoring may be required for – Carrying out treatability studies and design of the ASP – Facilitating operation and control of the ASP – Assessing compliance with the requirements (achievement of set and design efficiencies) – Performance evaluation of the ASP Monitoring involves sampling and analysis of samples of – Influent wastewater of the ASP – Aeration tank contents – Mixed liquor being fed to the secondary clarifier – Clarified secondary effluent (secondary clarifier overflows) – Return sludge
  • 79. Monitoring Monitoring can be online (appropriate for process control !) – Monitoring involves collection of samples and analysis in – Industrial units own ETP or central laboratory (for routine parameters) – Outside laboratory (sometimes 3rd party laboratory) for advanced analysis (for non-routine parameters requiring sophisticated instruments) 3rd party laboratory is depended on specially for compliance assessment/ monitoring Sampling location should judiciously chosen
  • 80. Monitoring Method of sampling chosen should ensure collection of representative samples – Usually grab sampling can serve the purpose – Occationally composite (flow proportionated?) sampling may be required Frequency of sampling and parameters to be analysed for should be decided on the basis of the monitoring objective/purpose Typical sampling locations and comprehensive list of parameters for analysis for an ASP are identified here Sampling at all the locations, sampling by any specific method of analysis, and analysis for all the identified parameters may not always be required Sampling frequency can be at fixed intervals and whenever a need arises
  • 81. Streams to be monitored and parameter Influent (including internal recycle flows): Assess both quantity and characteristics – Flow rate and variations (if needed the flow should be managed) – Internal recycle flows – Organic matter concentration – BOD, bCOD (BOD kinetics) and COD – Nutrients – TKN (ammonical and organic nitrogen), nitrate plus nitrite nitrogen, and total phosphorus – Suspended solids: TSS, VSS, nbVSS (indirect method) – Total alkalinity, Temperature and pH Aeration basin contents: – Temperature, pH and Dissolved oxygen – Oxygen uptake rate and specific oxygen uptake rate (Toxicity assessment)
  • 82. Monitoring of ASP • pH in the aeration tank to check whether enough alkalinity is present • Nutrients in the treated effluent – TKN, Nitrate-N and Total-P • DO in the aeration tank (sample should not come in contact with air and biological flocs should be deactivated) • MLSS and MLVSS, TSS in the clarified secondary effluent and SVI of mixed liquor • Visual and microscopic examination of microbial flocs
  • 83. Mixed liquor – MLSS and MLVSS – Nitrate and nitrite nitrogen – SVI and ZSV – Microscopic examination of biological flocs Return sludge – MLSS or consistency Clarified secondary effluent – Soluble BOD, total BOD, COD – Nitrate plus nitrite nitrogen and TKN (ammonical and organic nitrogen separately) – Total phosphorus and TSS Streams to be monitored and parameter
  • 84. Design of the ASP Characterization and quantification of the influent to be treated – Flow rate and variations (average flow and peaking factor) – flow equalization may be needed to dampen variations – Organic matter concentration – BOD, bCOD (BOD kinetics) and COD – Nutrients – TKN (ammonical and organic nitrogen), nitrate plus nitrite nitrogen, and total phosphorus – Suspended solids: TSS, nbVSS (indirect method) – Total alkalinity – Temperature (summer critical and winter critical temperature of the influent) Ambient conditions, like, summer and winter critical temp., altitude and atmospheric pressure, may be needed
  • 85. Operation and Control of the ASP Chemical dose requirements – Flow rate, BOD, TKN (even nitrate plus nitrite –N), Total –P, and Alkalinity of the influent – Residual TKN (and nitrate plus nitrite –N) and total –P of treated effluent – Frequency of monitoring can be weekly once or lesser Aeration system – Suction pressure down stream to the air filter, compressed air pressure and air flow rate (online monitoring) • May indicate filter clogging, diffuser fouling and bursting of diffuser membrane – DO level in the aeration basin (may be as a part of investigation of bulking sludge)
  • 86. Operation and Control of the ASP Regulation of the return sludge system – Sludge blanket depth in the clarifier (continuous monitoring may be needed) – Consistency of the return sludge and MLSS in the mixed liquor – SVI of the mixed liquor – Influent flow rate Sludge wasting system – Consistency of the clarifier underflow and MLSS of the mixed liquor – TSS in the clarified effluent of the clarifier Presence of toxic/inhibitory subtances – OUR and SOUR of the mixed liquor in the aeration
  • 87. Operation and Control of the ASP Secondary clarifier control • Influent flow rate (to assess hydraulic overloading) • Nitrate, temperature, SVI and ZSV of mixed liquor – (to indicate raising sludge, density currents, and settling properties of mixed liquor solids) • Sludge blanket depth • TSS level and turbidity of the clarified effluent – Straggler floc: fluffy floc in the clarified effluent – associated with low SRT operation – often coexist with white foam – Pinpoint floc: pin-floc in the clarified effluent – associated with high SRT operation – coexist with quickly settling floc and often with darker foam – Surface ashing: condition of lighter pin-floc floating to the surface and spreading out in the clarifier
  • 88. Analytical Methods Flow meaurement (online measurement) by – Pumping rate and pump runtime recording – Treated effluent flow rate by flow meters such as V- or rectangular notches, parshall flume, etc. BOD, bCOD and COD – BOD: BOD bottle method, head-space BOD method, respirometric technique, or by BOD sensors – bCOD: through BOD kinetic experiments – COD: by closed or open reflux methods • Use BOD – COD – bCOD relationship for the operation and control of ASP
  • 89. Analytical Methods Nutrients • TKN (organic and ammonical –N) by kjeldahl apparatus and either titrimetry or colorimetry • Nitrite and nitrate –N by using cadmium reduction column and colorimetry Suspended solids (TSS, VSS, nbVSS, MLSS and MLVSS) • Filtration, gravimetry and ashing for MLSS and MLVSS • TS – TDS difference technique for TSS (and ashing for VSS) • Change in VSS over sufficiently long time through aeration by autooxidation can be basis for nbVSS measurement SVI for mixed liquor: volume in mL occupied by one gram of mixed liquor solids after 30 minutes settling ZSV: subsidence velocity in m/hr. of the sludge blanket interface in the settling column)
  • 90. Analytical Methods • Oxygen uptake rate measurement by recording DO depletion of the mixed liquor over a few minutes • Specific oxygen uptake rate from OUR by dividing with MLVSS or active biomass concentration • Microscopic examination of biological flocs of mixed liquor or of secondary clarifier for filamentous growth and for microscopic air bubbles • pH by pH meter (or indicator strip!) and alkalinity (in mg/L as CaCO3) by titrimetry • Temperature (of ambient air, influent and mixed liquor) • DO (of the aeration basin contents) by DO meter with submersible long leed probe
  • 91. Data recording and analysis All monitoring data needs recording in log books – Data may be weekly/monthly/seasonally analysed for trends and performance reports may be generated All incidents may be recorded and the recording can include the investigatory monitoring being carried out and corrective and preventive actions taken – Hydraulic or organic overloading – Loading of toxic or inhibitory substances – Bulking sludge, raising sludge, and foaming problems – Critical machinery failures
  • 92. Compliance Assessment BOD and TSS (and even flow rate) are needed to be monitored – Comparison with statutory requirements may be needed – BOD is contributed by • Residual sBOD • Biodegradable fraction of the TSS (MLVSS to MLSS ratio!) If operated for nutrient removal then TKN and nitrate and nitrite –N and total –P may also be needed – Nutrients are also present in the TSS
  • 93. ASP Performance Evaluation ASP performance evaluation may involve • Measurement of actual performance of the ASP and comparison with the designed performance – May concentrate mainly on • Treatment efficiencies (BOD removal and nutrient removal) • Sludge generation rates and MLVSS/MLSS ratio • Oxygen consumption rates • Back calculation of ASP kinetic parameters and comparison with values used in the design and improvement of equations used in the ASP performance assessment • Performance evaluation of the secondary clarifier
  • 95. Complete mix reactor (CFSTR) Most used in India – simple design - suitable for all types of aeration equipment Uniform and low levels of substrate, and uniform MLSS and constant oxygen demand throughout the basin Resistant to shock loads and toxic loads Hydraulic and organic load variations are dampened better Toxic discharges are mitigated through greater dilution Filamentous bulking from exposure of recycled sludge to relatively low levels of substrate A pre-contacting zone (of 15 min. HRT) can avoid this! Typical design and operational conditions SRT: 3-15 days MLSS: 1500 to 4000 mg/L F/M ratio: 0.2 to 0.6/day BOD loading: 0.3 to 1.6 kg/m3.day HRT: 3 to 5 hours Sludge recycle ratio: 0.25 to 1.0
  • 96. Plug flow reactor Long narrow aeration tanks (aspect ratio >10) with plugflow regime True plug flow does not exist – extent of longitudinal mixing depends on the type of aeration system used Degree of longitudinal mixing is described by Dispersion Number (ND) which is defined as D/(UL) or Dt/L2 D – Coefficient of axial dispersion (m2/sec.) – for diffused aeration system it increases by a factor of 2 with air flow increase from 1.2 to 6 m3/m3.hr U – mean velocity of flow (m/Sec.) L – length of the tank t – HRT of the tank (L/U) for Q+QR flow For a good plug flow condition ND value is <0.1 A plug flow reactor is equivalent to a series of complete mix reactors Substrate concentration varies along the reactor length O2 utilization is highest at the inlet end and decreases towards lowest at the outlet end
  • 97. Plug flow reactor Affected by toxic or inhibiting organics (problematic for industrial wastewater with toxic constituents) A baffled inlet section can ensure better sorption in case of readily degradable wastewaters A separate inlet zone (15% of total volume) with only mixing but no aeration can facilitate denitrification of recycled sludge If designed for nitrification, an anoxic zone at the outlet end can bring about denitrification Design and operation are relatively more complicated – matching oxygen demand and supply is difficult Early designs had uniform air application throughout the tank length but modern designs have tapered aeration Typical design values SRT: 3-15 days HRT: 4 to 8 hours BOD loading: 0.3 to 0.7 kg/m3.day F/M ratio: 0.2 to 0.4/day Sludge recycling ratio: 0.25 - 0.75 MLSS: 1000 to 3000 mg/l
  • 98. Plug flow Aeration tank Wasted sludge Recycledsludge Clarifier Effluent Influent
  • 99. Plug flow reactor: Step Feed Plug flow reactor with wastewater introduction at 3 or 4 feed points (equalizes F/M ratio) MLSS is initially highest (5000 to 9000 mg/L), and decreases with each of the feed points Establishes more uniform oxygen demand Flexible operation Wet weather flows can be bypassed to the last pass Adaptable to many operating schemes (anoxic/aerobic processes) If needed can also be operated in contact stabilization mode by feeding only at the last feed point More complicated design and complex operation Typical design and operation values SRT: 3-15 days MLSS: 1500-4000 mg/l BOD loading: 0.7-1.0 kg/m3.day F/M ratio: 0.2 to 0.4/day Sludge recycling ratio: 0.25 to 0.75 HRT: 3-5 hours
  • 101. Contact Stabilization process Has two separate tanks or compartments one for wastewater treatment and the other for sludge stabilization Requires smaller aeration volume and good for low solubility index wastewaters Wet weather flows can be handled without loss of MLSS Has little or no nitrification capacity and operation is somewhat complicated Typical design and operation values Contact time: 30 - 60 min. Stabilization time: 2 - 4 hours SRT: 5-10 days Sludge recycling ratio: 0.5 to 1.5 F/M ratio: 0.2-0.6 /day Volumetric loading: 1-1.3 kg/m3day MLSS: 1000-3000 mg/L (for contact tank) & 6000-10000 (for stabilization tank)
  • 102. Contact tankStabilization tank Effluent Clarifier Wasted sludge Influent Recycled sludge
  • 103. Staged Reactor Systems Consist of usually 2 complete mix reactors in series • Employed for nitrogen removal (or just nitrification) and for phosphorus removal System for nitrogen removal • Uses pre or post anoxic reactor • Aerobic reactor is maintained at higher (>2 mg/L) DO level • Post anoxic often requires dosing of organic substrates System for nitrification • 2 stages: Stage-1 for BOD removal and stage-2 for nitrification • Each stage has a clarifier of its own • A portion of wastewater is directly taken into stage-2 • Stage-2 is operated at a longer SRT System for the phosphorus removal • Two stage system with a single clarifier after stage-2 • Stage-1 is anaerobic where as stage-2 is aerobic • Wasted sludge (phosphate accumulating organisms) contains the removed phosphorus in the form of polyphosphates
  • 104. Aeration reactor Nitrification reactor Influent Effluent Wasted sludge clarifier Return sludge clarifier Influent bypass Return sludge Wasted sludge Treatment for BOD removal and Nitrification
  • 105. Anaerobic reactor Aeration reactor Influent Effluent Wasted sludge clarifier Return sludge Aeration reactor Aeration reactor Digester Stabilized sludge Supernatant Return sludge Influent Effluent Return sludge clarifier Phosphorus removal
  • 106. Pure Oxygen Activated Sludge Process A series of well-mixed covered aeration tanks with co-current gas- liquid contact Influent, recycled sludge & O2 are introduced at stage-1 (O2 can also be mixed with the influent under pressure) Restricted exhaust is allowed from the last stage (O2 in the exhaust is ~50% and O2 utilization rate ~90%) Disadvantages More complicated equipment and complex installation, operation and maintenance Peak flows can disrupt operation by sludge washout Has limited capacity for nitrification Nocardia foaming is possible Designed for DO >6 mg/L F/M ratio 0.6-1/day SRT: 1-4 days MLSS 2000-9000 mg/L BOD loading: 1.3-3.2 kg/m3.day HRT: 1-3 hours Sludge recycling ratio: 0.25-0.5
  • 107. Membrane Bioreactor Membrane process (micro and ultra filtration) replaces the secondary clarifier • Membrane process helps to maintain higher SRT against low HRT - allows 10-20 day SRT and 10-15 g/L MLSS • Two configurations, internal (submerged) bioreactor and external bioreactor, are in use • Hollow fiber and flat sheet membranes are usually used Problems associated with MBR are • High membrane cost and high energy requirements for maintaining high trans-membrane pressure • Membrane fouling rapidly deteriorates flux rates across the membrane and necessitates frequent membrane cleaning and replacement • Aeration (2-phase flow) helps in controlling the fouling through keeping the solids in suspension and scouring the membrane surface
  • 108.
  • 109. SBR: Sequencing Batch Reactor A modification to the ASP transforming a continuous process into a batch process Usually preferred when the quantity of wastewater is lesser and highly variable All the treatment steps (aeration and clarification) are brought about in the same tank sequentially in cycles of filling, aeration, settling, and decanting supernatant Excess activated sludge is wasted after decanting prior to the starting of cycle of treatment (quantity and frequency of wastage is decided on the basis SRT to be maintained) The processes and operations are accomplished as timed sequences – 1. Fill: 3 hr 2. React: 2-20 3. Settle: 0.5-1hr – 4. Decant: 0.5-1 hr X. Idle Even nitrification, denitrification and even sludge stabilization can also be accommodated within – Just mixing without aeration during the fill stage ensures anoxic conditions needed for denitrification
  • 110. Complicated process control and require higher maintenance skills for the equipment used Batch discharge may necessitate equalization for down stream processing of the effluent Typical design and operational parameters SRT: 10-30 days F/M ratio: 0.04-0.1/day BOD loading: 0.1-0.3 kg/m3.day MLSS: 2000-5000 mg/L SBR modifications • Batch decant reactor, intermittent extended aeration system: – Treatment include continuous filling and sequential reaction, settling and decanting – A pre-react (baffled) chamber facilitates continuous feeding without disturbing the settling/decanting operations • Cyclic activated sludge system: – Continuous wastewater feeding, but batch removal of effluent – Reactor has three baffled zones of 1:2:20 volumes and is fed continuously but the effluent is removed in batches – Mixed liquor is recycled from 3rd zone to 1st zone of the reactor Sequencing Batch Reactor (SBR)
  • 111. Aerated lagoons, oxidation ponds and oxidation ditches
  • 112. Extended Aeration Process: Counter- current aeration system A circular tank with revolving bridge is used Air diffusers mounted at the bottom of the revolving bridge supply oxygen Turning off air but revolving the bridge keeps the tank contents in suspension and facilitate denitrification Typical design and operational parameters SRT: 10-30 days F/M ratio: 0.04 to 0.1/day HRT: 15-40 hours BOD loading: 0.1-0.3 kg/m3.day MLSS: 2000-4000 mg/L Sludge recycle ratio: 0.25-0.75 Oxygen transfer efficiency is higher but diffuser fouling can be problem (fine screening of wastewater can prevent) Complicated operation requiring good operator skills Down time for maintenance is relatively higher
  • 113. Extended Aeration: Other modifications Orbal process A modified oxidation ditch using a series of concentric channels of depth upto 4.3 m Wastewater enters the outer channel and flows towards the center before entering an internal/external clarifier Nitrification and denitrification are facilitated by regulating aeration rates Biolac process Earthen tanks of 2.4-4.6 m depth with submerged aeration and with either internal or external clarifiers Fine bubble diffusers attached to floating aeration chains move across the basin by air released from diffusers Use of timers to cycle air flow through each aeration chain facilitates nitrification and denitrification
  • 114. Extended Aeration Process Well stabilized and low bio-solids sludge is generated – the sludge is mainly of cell debris and sludge contributed by the influent Primary clarification is usually not used Considered suitable for smaller flows Aeration tanks are larger, and oxygen demand and aeration energy requirement are higher Aeration equipment design is controlled by mixing needs (mostly not by oxygen demand) Sensitive to hydraulic overloads (clarifier can be overloaded by solids) and insensitive to concentration shock loads Typical design and operational parameters F/M ratio is 0.04-0.10/day BOD loading: 0.1-0.3 kg/m3.day SRT: 20-40 days MLSS: 2000-5000 g/m3 HRT: 20-30 hours Sludge recycling ratio: 0.5 to 1.5
  • 115. Extended Aeration: Oxidation Ditch Ring or oval shaped loop reactor system with unidirectional flow (velocity: 0.25-0.3 m/sec. and cycling time: 5-15 min.) Brush type/surface type mechanical aerators power horizontal flow and bring about aeration/mixing Screened wastewater is mixed with recycled sludge and allowed into the tank of 20-30 hour HRT Intra-channel clarifiers can be used (for secondary clarifiers) Advantages Highly reliable process and simple operation Amenable for both BOD and nitrogen removal Uses less energy and adaptable for nutrient removal Disadvantages Space requirement is higher Plant capacity expansion is more difficult Low F/M bulking is possible
  • 116. EffluentClarifier Wasted sludge Oxidation ditch Mixed liquor Recycled sludge Influent
  • 118. Trickling filters • Most conventional technology and relatively less used • Circular or rectangular non-submerged fixed film bioreactors • Includes – Rock or plastic packing as a medium for bio-film development – Wastewater dosing or application system – Under drain system – Structure containing packing – Secondary settling unit • Clarified or fine screened primary effluent is applied on the packing and allowed to trickle through
  • 119. • The bio-film is alternatively exposed to wastewater and air • Wastewater made to flows as a thin film over the bio-film • Treated effluent from TF is passed through a secondary clarifier and let out as treated secondary effluent • Portion of the clarified effluent may be recycled to TF for – Diluting the strength of incoming wastewater – Maintaining enough wetting of the bio-film during minimal or no flow conditions Trickling filters
  • 121.
  • 122.
  • 123. Trickling Filter In some wire mesh screen is placed over the top Requires less energy and easy to operate Have more potential for odors and quality of treated effluent is low –causes are Inadequate ventilation Poor clarifier design Inadequate protection from cold temperatures Dosing operations Biofilm development may not be uniform in the trickling filter Liquid may not uniformly flow over the entire packing surface (wetting efficiency)
  • 124. Packing Low cost material with high specific surface area, high enough porosity and high durability is ideal • Rock (rounded river rock, crushed stone or high quality granite), blast furnace slag, or plastic can be used Corrugated plastic sheet packing – High hydraulic capacity, high porosity, and resistance to plugging (for better air circulation and bio-film sloughing) – Most used and two types: cross flow and vertical flow – Packing depth is 4 to 12 m (6 to 6.5 m is typical), hence require less land area – Superior to stone packing for higher organic loading rates Rock packing – High density limits depth – 0.9 to 2.5 m (1.8 m is typical) – Size is 75 to 100 mm (95% of the material)
  • 125. Wastewater dosing/application system Two types of wastewater dosing/distribution systems: Rotary distributors & Fixed Nozzle Distribution Systems Rotary distributors • Has 2 or more hollow arms mounted on a central pivot – Arms have nozzles and extend across the TF diameter – Spacing between nozzles decreases center to perimeter (ensures uniform distribution of wastewater) • Dynamic reaction of the discharged wastewater from nozzles revolves the distribution arm – Rotational speed varies with flow – Alternatively electrical motor can also be used
  • 126. • Clearance between distributor arm and filter packing top is 150 to 225 mm • Typical head loss in the distributor is 0.6 to 1.5 m • Rotational speeds of distributors – In the past these were 0.5 to 2 rpm – but now reduced for better filter performance – Change from conventional 1 to 5 min/rev. for rock filters to dosing every 30-55 min. once improved performance (reduced bio-film thickness & odour observed) – Can be decided by N = (1+R)q / (A.DR) R, recycle ratio q, influent flow rate A, no. of distributor arms DR, dosing rate Wastewater dosing/application system
  • 127. • Dosing volumes – Higher dosing volumes are reported to improve wetting efficiency, agitation, flushing and wash away of fly eggs, and resulted in thinner bio-film – Daily intermittent high flushing doses are shown to control bio- film thickness and solids inventory Fixed nozzle distribution system – In case of square or rectangular filters fixed flat spray nozzles are used – Used for shallow rock filters – Flat spray pattern nozzles are used (at periphery half spray nozzles are used) – Water pressure is varied systematically in order to spray wastewater first at maximum distance and then at decreasing distance Wastewater dosing/application system
  • 128. Under-drain system Meant to catch filtered wastewater and solids and convey to secondary clarifier • Should facilitate ventilation, easy inspection and flushing if needed – Drains are open at both the ends and open into a circumferential channel Floor and drains of the filter should be strong enough to support packing, slime growth and wastewater – Rock filters use pre-cast blocks of vitrified clay or fiber glass grating laid on reinforced concrete sub-floor – Plastic packing filters use beams over columns Free draining out of filtered wastewater should be ensured – 1 in 5 bottom slope is provided towards the outlet – under-drains are designed for flow velocity >0.6 m/sec. at average flow – Beams are provided with channels on their top
  • 129. Ventilation and air flow Natural draft is the primary means for providing the needed air flow • Often inadequate – forced ventilation by low pressure fans can be solution – FD or ID fans can be used • Driving force is temperature difference between ambient air and wastewater • During periods of no temperature difference natural draft may not be there • Cooler wastewater forces downward air flow through the filter and warmer wastewater forces upwards air flow • Upward air flow not desirable - supply of oxygen is lower where demand is higher and odours can be a problem
  • 130. For better air flow • Drains should not flow beyond half full • Open area over the top of the under-drains should be >15% • Each 23m2 filter area should have 1 m2 area of ventilation manholes or vent stacks Ventilation and air flow
  • 131. Secondary clarifier Settling is meant for the removal of suspended flocs and discharging the clarified sewage – – Sludge is not recycled – but can be returned to primary clarifier – Part of the clarified effluent may be recycled for reducing strength or for keeping wet of the bio-film • Sludge is relatively easily settlable Settling tanks with side wall depth of 3.6 to 3.7 and overflow rate 1m/hr for average flow and 2 m/hr for peak flow are used. With proper design and operation of a secondary clarifier clarified effluent has <20 mg/l of TSS
  • 132. Trickling filters Classified as • Low or standard rate filters – Produce effluent of consistent quality though influent is of varying strength – Top 0.6 to 1.2 m of the filter has appreciable bio-film and lower portion may have nitrifying bacteria – Odours and filter flies may be the problems • Intermediate and high rate filters – Flow is usually continuous – Recirculation of effluent allows higher organic loadings – May be a single or two stage process – High rates filters usually use plastic packing
  • 133. • Roughing filters – Much higher organic loadings (>1.6 kg/m2.day) are used – Hydraulic loading can be upto 190 m3/m2.day – Used to treat wastewater prior to secondary treatment (require low energy than ASP – 2 to 4 kg BOD per kWh for TF against 1.2 to 2.4 kg BOD per kWh for ASP) 2-stage filters • High strength wastewaters prefer 2-stage filter with intermediate clarifier • Preferred when nitrification is required – Nitrification demands low organic loading (<30 mg/l) – complete nitrification demands <15 mg/l of organic load • Separate TF, after secondary treatment, can also be used for nitrification Trickling filters
  • 134.
  • 135. Biological community of trickling filter Biological community of trickling filter includes – Aerobic, anaerobic and facultative bacteria – Fungi, algae and protozoans – Worms, insect larva and snails Bio-film on the medium surface is mainly composed of aerobic, facultative and anaerobic bacteria – Lower reaches of the filter can have nitrifying bacteria – At lower effluent pH, fungi play important role – Algae may grow in the upper reaches of the filter Problems caused by biological community – Protozoa, worms, insect larva and snails feed on biological films and increase effluent turbidity – Snails are troublesome in the TFs used for nitrification – Algae can cause filter clogging and odors problem – Fungal growth can also clog the filter
  • 136. Sloughing of bio-films Bio-film can be up to 10 mm thick – in the top 0.1 to 0.2 mm aerobic degradation occurs • Organic loading increases thickness of the film • Aerobic environment and substrate can not penetrate inner depths of the bio-film • In case of thick films, due to anaerobic environment and substrate limiting conditions and consequent endogenous respiration, bio-film looses its clinging ability • Under hydraulic loading sloughing off of bio-film occurs • Mechanisms of sloughing for rock packing is apparently different from that of plastic packing – In plastic packing sloughing off is a small scale process and occurs from hydrodynamic shear – In rock packing large scale spring sloughing occurs mainly due to insect larva activity – affects total BOD of the effluent (can be higher than that of the influent)
  • 137. Trickling Filter: design Design is based on empirical relationships derived from pilot plant studies and full-scale plant experiences Following parameters are related with treatment efficiency and used as design and operating parameters – Volumetric organic loading – Unit area loading – Hydraulic application rates Treatment efficiency: >90% at <0.5 kg/m3.day BOD loading; and <60% at >3.5 kg/m3.day BOD loading Nitrification: – For effective nitrification BOD loading must be very little – Nitrification rates can be 0.5 to >3 g/m3.day – Efficiencies depend on the packing surface area and on the specific NH4-N loading rate – Higher temperature and better wetting efficiencies can enhance the nitrification efficiency
  • 138. Empirical formula for BOD removal for 1st stage TF is E1 is BOD removal efficiency at 20C W1 is BOD loading kg/day V is volume of filter packing (in m3) F is recirculation factor; Defined as Here R is recycle ratio and its value can be 0 to 2.0 VF W E 1 1 4432.01 1    2 10 1 1 R R F    Empirical formulae for BOD removal for rock filter media
  • 139. Empirical formulae for BOD removal for rock filter media BOD removal efficiency at 20°C in the 2nd stage TF W2 is BOD loading to 2nd stage TF in kg/m3 BOD removal efficiency is temperature dependent and temperature correction can be made by VF W E E 2 1 2 1 4432.0 1 1    20 20 )035.1(   T T EE
  • 140. Empirical formulae for plastic packing Contact time of wastewater with bio-film depends on the filter depth and the hydraulic application rate  Wastewater contact time (in min.) q, hydraulic loading rate in L/m2.min. Q, wastewater flow rate (L/min.) A, cross sectional area of the filter in m2 c, constant for the packing used D, depth of the packing in m (typical is 6.1 -6.7 m) n, hydraulic constant for packing Depends on temperature and specific surface area Taken as 0.67 at 20C and as 0.5 for a packing of 90 m2/m3 specific surface area n q Dc  A Q q 
  • 141. Change in BOD concentration with time is k is taken as 0.69/day at 20C It incorporates the constant ‘C’ (constant for the packing) Its value is temperature dependent kS dt dS  n q kD kte S S    expexp 0 20 20 035.1   T T kk Empirical formulae for plastic packing
  • 142. Modified Velz equation for the effluent BOD for the plastic packing R is recycle ratio  is temperature correction coefficient (1.035) So is influent BOD and Se is effluent BOD, mg/L As is specific surface area (m2/m3) D is depth of the filter (m) ‘q is hydraulic application rate (L/m2.sec.) recommended value is >0.5L/m2.sec. ‘n’ is constant characteristic of the packing used (depends on temperature and specific surface area (taken as 0.67) K20 is rate constant at 20°C      R Rq DAK R S S n S Te            1 exp1 20 20 0 
  • 143. Modified Velz equation K value Its value is 0.059 to 0.351 for domestic wastewater it is 0.21 it requires adjustment for both packing depth and influent BOD K1 is K value for D1 = 6.1 m and S1 = 150 mg/l of BOD For influent BOD of 400 to 500 mg/l oxygen transfer can become limiting in trickling filters Loading of soluble bCOD >3.3 kg/m3.day can also limit oxygen 5.0 2 1 5.0 2 1 12              S S D D KK
  • 144. Natural air draft through filter can be determined by Dair, draft in mm of water column Z, height of the filter in meters Tc, cold temperature in K Th, hot temperature in K Air flow velocity through the filter is proportional to the natural air draft Pressure drop across filter is related to flow velocity by total head loss in kPa (kPa=100 mm water column) - 101.325 kPa is equal to 1 atmos or 10132 mm water column;) V is superficial air flow velocity (m/sec.) Np is filter tower resistance factor in terms of velocity heads Ventilation and air flow Z TT D hc air        11353      g VNP pT 2 2 P
  • 145. Np is estimated as Np is number of velocity heads D is depth of packing (m) L/A is liquid mass loading rate (kg/m2.hr) A is filter cross-section area (m2) Total head loss through the filter (Npt) is usually taken as 1.3 to 1.5 times to Np 2.0 times for rock packing of 45 m2/m3 specific surface area 1.6 times for plastic random packing with 100 m2/m3 specific surface area For plastic packing with cross flow conditions the multiplying factor is 1.3 when specific surface area is 100 m2/m3 and 1.6 for the specific surface area of 140 m2/m3 packing  A L p DN )1036.1( 5 exp33.10    Ventilation and air flow
  • 146. Air flow requirements Oxygen required per kg of BOD loaded to the TF is Ro, kg of oxygen needed per kg of BOD loaded Lb, BOD loading rate kg/m3.day (typical: 0.3-1.0 kg/m3.day) If nitrification is also considered then Nox/BOD, ratio of influent nitrogen oxidized to influent BOD 72 m3 of air at 20°C, at assumed O2 transfer efficiency of 5%, can supply one kg of O2 to the TF – transfer efficiency is taken as 2.5% in case of nitrification – Oxygen content of air 0.279 kg/m3 at 20°C bb -0.17L-9L o exp1.2exp0.8R  BOD NO 4.6exp1.2exp0.8R x0.17L-9L- o bb 
  • 147. Air flow requirements Air flow (in m3/day) required (AR) for a TF is For temperature and pressure different form 20C and one atmos, air flow rate is If temperature is >20C air requirement will increase - solubility of oxygen decreases with increasing temperature Recommended air flow is 0.3 m3/m2.min. a A 20T P 760 293 T273 ARAR        100 20-T 1ARAR A T ' T 00QSR72AR  Q, flow rate of influent in m3/day S0 is influent BOD in kg/m3
  • 148. Nitrification Used either as a combined system or as a tertiary treatment Influent BOD and DO within the TF impact nitrification rates – Maximum rates occur when sBOD is <5mg/l, rates are inhibited at >10 mg/l and rates are insignificant at >30 mg/l – For rock media filters for BOD loading of <0.08 kg/m3.day, nitrification efficiencies are >90% and for the loading of 0.22 kg/m3.day it is 50% – For >90% nitrification surface loading of NH3-N of <2.4 g/m2.day is needed (biofilm surface) Linear relationship exists between specific nitrification rate and BOD/TKN ratio and can be shown by – Here RN is specific nitrification rate (in g/m2.day) 44.0 82.0         TKN BOD RN
  • 149. Nitrification Packing design, temperature, oxygen availability and ammonia loading rate all affect nitrification rates • DO concentration has very great effect • Follows zero order reaction (against NH4-N) for most portion of the packing (for NH4-N in the > 5-7 mg/L range) • Rates decline from top with depth may be from the decrease of bio-film growth, from predation (grazing by snails!) and from changed wetting efficiency • Rates can be 1 to 3 g/m2.day (bio-film) and may decline by 20- 50% with decrease in temperature from 20 to 10°C For higher nitrification rates – NH4-N should be >5 mg/L – hydraulic loading rates should also higher – Specific surface area of 100 m2/m3 may be appropriate Periodic floods can minimize the predation problem
  • 150. For determining the volume of packing and the hydraulic application rate, the following empirical equation is used – Here RN is surface nitrification rate for Z depth of TF and T operating temperature – N is NH4-N concentration in the bulk liquid – r is an empirical constant – KN is N conc. at RN=RN.Max./2 – its expected value is 1.5 mg/l – RN.Max. is in g.N/m2.day – its value is 1.1 to 2.9 g.N/m2.day - temperature correction is considered not needed for 10-25C range – beyond this correction is done by rZ N MaxNTZN NK N RR          exp..,),( 10 )10.(.).(. 045.1   T MaxNTMaxN RR Nitrification
  • 152. Rotating biological contactor (RBC) • Series of closely spaced polystyrene/polyvinyl chloride circular disks on horizontal shaft constitute RBC unit • Standard RBC unit includes – 3.5 m dia disks with total disk area of 9300 m2/unit to support microbial film – A shaft of 8.23 m length (of this 7.23 m is occupied by disks) • RBC unit is placed in a 45 m3 capacity tank – Shaft orientation is either perpendicular to or parallel to the wastewater flow • RBC unit is usually provided with an enclosure – Prevents algal growth – Discs are protected from sunlight (UV light) – Prevents heat loss and exposure to cold weather
  • 153.
  • 154. RBC Treatment process (sec. or advanced level) • Treatment is for BOD removal, or nitrification, or both, or for pretreatment of higher strength industrial effluent • Wastewater clarified in primary clarifier or fine screened is fed to the reactor • 40% of disc surface is submerged in wastewater maintained in a 45 m3 tank • Disk surface is alternatively brought in contact with wastewater and atmosphere by rotating at 1 to 1.6 rpm rate either mechanical or pneumatically • Treatment occurs through bio-sorption of organic matter of wastewater into bio-film and aerobic biooxidation of the sorbed matter when bio-film is exposed to atmospheric air • Bio-film as it thickens and looses its ability to cling to disc surface due to hydrodynamic shears sloughs off • Treated effluent needs secondary clarification
  • 155. • A number of RBC units may be operated in series to form a process train – Exploit the benefits of staged biological reactor design – facilitates maintaining different conditions in different stages – For reliability two or more parallel flow trains are employed – 2-4 units in series are used for BOD removal –  6 units are used for combined BOD removal & nitrification – To avoid overloading on initial stages, stepped feed or tapered systems are opted – RBC units decrease as one moves to higher stages • RBC units are of low, medium and high density types – Low density or standard type units are used for initial stages – Medium and high density units (11000 and 16,700 m2 area) are used in the mid and final stages RBC
  • 156. Simple to operate and involves low energy costs Performance is related to specific surface loading of BOD and/or NH4-N – For first stage it is 12-20 g/m2.day of soluble BOD (as total BOD it is 24-30 g/m2.day) – For nitrification maximum loading rate is 1.5 g/m2.day Associated with odor and bio-film sloughing problems – Occurs when oxygen demand exceeds supply – Sulfur oxidizing bacteria form tenacious whitish film and prevent sloughing off Structural failure of shafts, disks and disk support systems can occur – Excessive bio-film growth and sloughing problems cause it RBC
  • 157. RBC design considerations Principal elements of RBC • Disc material and configuration – HDPE of different configurations or corrugation patterns – Corrugation increases available surface area and enhances structural stability • Shaft – Shape is square, round or octagonal – Steel shafts coated for protection against corrosion of 13-30 mm thickness are used • Tankage – Requirement is 0.0049 m3/m2 film area – Typical side wall depth is 1.5 m – At 0.08 m3/m2 day hydraulic loading rate HRT is 1.44 hrs
  • 158. Drive system – Mechanical or pneumatic drives are used for shaft rotation – Mechanical drive capacity is 3.7 or 5.6 kW per unit – Deep plastic cups are attached to the perimeter of the disks and compressed air is released into the cups for rotation – Air requirement is 5.3 m3/min for standard density shaft and 7.6 m3/min for high density shaft Enclosures – Segmented fiberglass reinforced plastic enclosures are used RBC design considerations
  • 159. RBC design First stage RBC disk area is determined by using 12-20 g/m2.day sBOD loading Disk area of subsequent stages is found by second order model by Opatken Sn is soluble BOD concentration in mg/l As is disk surface area for stage-n (in m2) Q is flow rate in m3/day Here sBOD/BOD is taken as 0.5 for secondary clarified effluent 0.5 to 0.75 for primary clarified effluent )/(00974.02 )/(00974.0411 1 QA SQA S s ns n    
  • 160. For nitrification stages area required is found by using maximum nitrification rate (rn.max) as 1.5 g/m2.day – Applicable if sBOD of wastewater is <10 to 15 mg/l - otherwise rn.max should be corrected by: Here Frx is fraction of nitrification rate possible sBOD is soluble BOD loading in g/m2.day sBODFrx  1.00.1 RBC design
  • 161. SAFF, FAB and MBBR reactors
  • 163. Waste Stabilization Ponds • Shallow, manmade basins comprising of one or more series of anaerobic, facultative and maturation ponds • Used to treat domestic or municipal wastewater to – Remove biodegradable organic matter, BOD (by >90%) – Remove pathogens (bacteria and viruses by 4-6 log units, and protozoan cysts and helminth eggs by upto 100%) – Remove nutrients (Nitrogen by 70-90% and Phosphorus by 30-45%) and sufficiently clarify the wastewater • If properly designed and operated, can give the effluent of – Filtered BOD <25 mg/L – TSS <150 mg/L – Nematode eggs <1/L – Fecal coliform count <1000/100 mL
  • 164. Waste Stabilization Ponds (WSP) • Represent sustainable natural effluent treatment systems – Uses solar energy and do not require electricity – Do not use any electromechanical equipment • Low cost, low energy, and low maintenance systems, and do not require skilled manpower – Construction involves earth moving, pond lining and pond embankment protection, and pond inlets and outlets and construction of screens and grit chambers – Operation and maintenance requirements are minimal (repair of embankments, cutting embankment grass, removing scum and vegetation, keeping both inlet and outlet clear, etc.) and requires only unskilled but carefully supervised labour – When compared with trickling filters, aerated lagoons, oxidation ditches, and ASP, WSP are cheapest and even land cost may not be acting against WSP
  • 165. Waste Stabilization Ponds (WSP) • Can be easily scaled down to small scale applications • Robust systems (withstand organic & hydraulic shocks and copes up well with heavy metals upto < 60 mg/L) • Principal requirements are sufficient land, and soil with low coefficient of permeability (<10-7) • Suited to tropical and sub-tropical countries, like india – sun light and temp. (high throughout) are favourable – Inexpensive land, restricted foreign currency availability and shortage of skilled manpower favour the use • Also produce fish
  • 166. Waste Stabilization Ponds (WSP) Disadvantages • Requires more land (2-5 m2/capita) – 1-2 day HRT for anaerobic pond and 3-6 day HRT for facultative pond – Require 25 day HRT in 5 pond WSP in hot climates to produce the quality fit for restricted irrigation – Require 10 day HRT in 2 pond system for producing the quality fit for restricted irrigation • Potential odour and mosquito nuisance specially from anaerobic ponds • High algal content in the treated effluent • High evaporation losses of water specially in facultative and maturation ponds • Adverse environmental impacts may include ground water pollution
  • 167. • Anaerobic ponds represent primary treatment – sludge stabilization is add on feature – patogen removal (helminth eggs) is coincidental • Facultative ponds represent secondary treatment – coincidental removal of nutrients and pathogens • Maturation ponds represent tertiary treatment – used to remove pathogens (fecal bacteria) – nutrient removal is coincidental Waste stabilization Ponds (WSP)
  • 168.
  • 169. Preliminary Treatment Unless very small WSP systems must include both screening and grit removal facilities – Hygienic disposal of screenings and grit is needed (haulage to sanitary landfills or on-site burial in trenches) – All wastewater should be pass through screening and degritting Provisions may be made for flow measurement and recording both upstream & downstream to WSP system Provisions may be made for – Diverting the flow beyond 6 times to dry weather flow into stormwater and receiving water course – Allowing a maximum of 3 times to dry weather flow into anaerobic ponds and diverting rest into facultative ponds – Bypassing the anaerobic pond
  • 170. Effluent limits to be complied with EU’s requirements • Filtered (non-algal) BOD and COD: 25 mg/L and 125 mg/L respectively • Suspended solids: 150 mg/L • Total nitrogen and total phosphorus for avoiding eutrophication: 15 mg/L and 2 mg/L respectively – If population is >1,00,000 then total-N and total-P should be 10 mg/L and 1 mg/L respectively
  • 171. Effluent limits to be complied with Indian limits – BOD (non-filtered): 30 mg/L – Suspended solids: 100 mg/L – Total-N: 100 mg/L – Total ammonical-N: 50 mg/L – Free ammonical-N: 5 mg/L – Sulfide – 2 mg/L – pH 5.5 to 9.0
  • 172. Effluent limits to be complied with Discharge into surface or ground water! WHO guidelines of 1989 for restricted crop irrigation: • 105 E. coli per 100 ml • Human intestinal nematode eggs 1 per liter • If children under 15 years are exposed (playing or working in the irrigated field) then 0.1 eggs/L • Intestinal nematodes include • Ascaris lubricoides • Trichuris trichiura (human whipworm) • Ancylostoma duodanale and • Necator americanus (hookworms)
  • 173. Effluent limits to be complied with WHO guidelines of 1989 for unrestricted crop irrigation: • 1000 E.coli per 100 ml • human intestinal nematode eggs 1 per liter – if children are eating the food crops uncooked then 0.1 eggs/L Restricted irrigation: irrigation of all crops except salads and vegetables eaten uncooked WHO guidelines of 1989 for aquacultural use of effluent – 104 E.coli per 100 ml in the fish pond water – ‘0’/L of detectable human trematode eggs in the effluent Human trematodes include – Schistosoma sp. – Clonorchis sinensis – Fasciolopsis buski
  • 174. • For fish (Carp and Tilapia)/aquatic vegetable culturing effluent from facultative ponds can be used • For restricted irrigation systems with only anaerobic and facultative ponds can be sufficient • Maturation ponds are required for producing the effluent suitable for unrestricted irrigation or for effluent discharge into bathing water • Fish ponds can be loaded on the basis of nitrogen load (4 kg- N/ha.day) • Free NH3 in ponds > 0.5 mg/L can prove toxic What is required for complying with the requirements?
  • 175. Facultative Ponds 2 types: primary and secondary – Primary ponds received screened & degritted wastewater – Secondary ponds receive effluent from anaerobic ponds Typical depth is 1 – 2 m and HRT is 5 to 30 days Properly designed facultative pond has • Aerobic top layer all through the day and night – Diurnal variation in DO concentration is experienced – oxypause (depth beyond which DO is zero) show vertical movement – Presence of aerobic layer reduces methane and H2S emissions – remove odours • Anaerobic bottom layer never in direct contact with atmosphere
  • 176.
  • 177.
  • 178.
  • 179. Facultative Ponds Filtered effluent BOD is 20 to 60 mg/L (TSS level in the effluent is 30 to 150 mg/L) • Algae also contributes both BOD (80% of the algae is biodegradable) and TSS – Effluent take off or removal from top 50 cm layer can result in large fluctuations in effluent quality • Algae mostly settles to bottom and anaerobically biooxidized
  • 180. Facultative Ponds Ponds look dark green in colour due to algae – Healthy ponds have 500-2000 µg/L of chlorophyll-a – Can also look red or pink due to the presence of anaerobic purple sulfide oxidizing photosynthetic bacteria (slight BOD5 overloading can cause it) Diurnal variations of DO & pH can be high due to high photosynthetic activity – DO as high as 20 mg/L and pH >9.4 are possible – High DO and pH are important for fecal bacteria and viruses removal BOD5 removal in primary facultative ponds is about 70% on unfiltered basis and 90% on filtered basis – Filtering removes algae and hence higher efficiency – In Europian Union the WSP effluent should achieve 25 mg/L of BOD5 in the filtered effluent
  • 181. Facultative Ponds: Oxygen Balance Can be shown by 1. Q(Cin-Cout): Net oxygen entry with the wastewater 2. AK(Csat-C): Reaeration at the pond surface 3. : Net photosynthetic contribution: photosynthetic O2 generation – DO consumption in algal respiration and in algal biomass decomposition 4. : Nitrification demand of oxygen (Ammonical-N to Nitrate-N) - aN can be taken as 4.5 5. : DO consumption for the bCOD removal - aB can be taken as 1.5           BODBODBNNNdodresphotosatin CCQaCCQarrrCCKACCQ dt dC V inin   BODBODB CCQa in   inNNN CCQa   dodresphoto rrr 
  • 182. Facultative Ponds: Surface Reaeration Surface reaeration occurs by the combined effect of molecular diffusion and vertical mixing of pond by wind – rain fall also increases mixing plus it carries DO Mass transfer coefficient K for zero wind conditions can be estimated by At 20C temp. & 0 mg/L DO, reaeration is 0.13 to 0.62 g/m2.day For every 3 m/s raise in wind speed the reaeration rate increases by a multiplying factor of 2.5 h DU K  D is molecular diffusivity of oxygen in water h is water depth U is water speed – may be 3-40 m/day
  • 183. Facultative Ponds: Algal Growth and Photosynthetic O2 Production Algal growth (biomass yield Y) can be estimated by Algal concentration depends on organic loading and temperature and may range between 500-2000 µg/L as chlorophyll-A Photosynthetic oxygen production can be stoichiometrically related to algal growth by Oxygen production occurs mostly in the top 50 cm of water - In the absence wind mixing concentrated band of algae moves up and down through the top 50 cm 21645180106 3 4322 2 309 1690106 OPNOHCLightPONOOHCO   h S Y  3.1 Y is net algal biomass yield S is average visible radiation  is light conversion efficiency h is specific chemical energy of algal biomass – in tropical areas under clear skies 172 cal/m2.day
  • 184. Treatment Mechanisms:Pathogen destruction/ removal Fecal coliform removal • Natural decay or disinfection • Occurs by a combination of processes via complex interaction of various adverse environmental factors • Aquatic environment • Algal activity and photo-oxidation • Adsorption to particles and subsequent sedimentation and grazing by protozoa etc. also contributes but very little • In anaerobic ponds sedimentation of solids is the major contributor • In facultative ponds and maturation ponds the removal is influenced by – Time and temperature – High pH (>9.0) – High light intensity and high DO
  • 185. Treatment Mechanisms:Pathogen destruction/ removal • Algal growth and photosynthesis – Bactericidal effect - excretion of anti-bacterial substances – High DO enhances the photo-oxidation process • Raising pH above a critical level (>9.5) - Carbonate and bicarbonate ions provide CO2 to algae and leave hydroxide ions raising pH to >9.0 • Photo-oxidation by incident solar radiation through sensitizer molecules, like, singlet oxygen, superoxide, hydrogen peroxide, hydroxyl radicals, etc. – Solar radiation, pH and DO have synergistic effect
  • 186. DNA damage by UV radiation (UV-B: 290-320 nm) – UV radiation attenuates with depth (16 to 46 attenuations per meter) and becomes ineffective beyond the upper few cm – at higher pH (>8.5) even longer wavelengths are effective • Vertical mixing (increases the decay) • Starvation due to lack of nutrients or carbon source Viruses – Apparently removed by adsorption on to settlable solids and consequent sedimentation Helminth eggs & protozoan cysts – Removed by sedimentation – Most removal takes place anaerobic and facultative ponds Treatment Mechanisms:Pathogen destruction/ removal