process design consideration
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WASTE WATER TREATMENT
PROCESS DESIGN CONSIDERATIONON ACTIVATED SLUDGE PROCESS
Drs. Subagiyo, MSi
2007
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PROCESS DESIGN CONSIDERATION
Selection of the reactor type
Loading criteria Sludge production
Oxygen requirment and transfer
Control filamentous organisms
Effluent characteristics
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LOADING CRITERIA
The 2 most commonly used parameters:
The food to microorganism ratio (F/M)
The mean cell residence time qc
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F/M
F/M = So / qX
F/M = food to microorganism ratioSo = influent BOD or COD concentration mg/L (g/m3)q = hydrolic detention time of the tank aeration = V/Q, dX = concentration of volatile suspended solid in the aeration tank,
mg/L (g/m3)
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F/M RATIO
Typical value for F/M vary from 0.05 to 1.0
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SPECIFIC UTILIZATION RATE
rsu So - S
U = - =X qX
Q So - S
=
Vr qX
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THE MEAN CELL-RESIDENCE TIME
Definition based on aeration tank volume
VrX
qc =QwXw + QeXe
Qc : mean cell-recident time based on the aeration tank volume, d
Vr : aeration tank volume, Mgal (m3)
X : concentration of VSS in the aeration tank, mg/L (g/m3)
Qw : waste sludge flowrate, Mgal/d (m3 /d)
Qe : treated effluent flowrate, Mgal/d (m3 /d)
Xe : concentration of VSS in the treated effluent, mg/L (g/m3)
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THE MEAN CELL-RESIDENCE TIME(U.S. customary units)
Definition based on total system volume
Xt 1
qct =(QwXw + QeXe) 8.34 lb/Mgal. (Mg/L)
qct : mean cell-residence time based on the total system
Xt : total mass of VSS in the system, including the solids in
the aeration tank, in the setling tank, and in the sludge-
return facilities
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THE MEAN CELL-RESIDENCE TIME(SI units)
Xt
qct =
QwXw + QeXe
qct : mean cell-residence time based on the total system
Xt : total mass of VSS in the system, including the solids in
the aeration tank, in the setling tank, and in the sludge-
return facilities
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SLUDGE PRODUCTION
It will affect the design of the sludge-
handling and disposal facilitiesnecessary for the excess (waste)sludge
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SLUDGE PRODUCTIONU.S customary units
Px = Y obs (So – S) x 8.34 lb/Mgal. (mg/L)
Px : net waste activated sludge produced each day,measured in term of VSS, lb/d (kg/d)
Yobs : observed yield, lb/lb (g/g)
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SLUDGE PRODUCTIONSI units
Px = Y obs (So – S) x (103 g/kg)-1
Px : net waste activated sludge produced each day,measured in term of VSS, lb/d (kg/d)
Yobs : observed yield, lb/lb (g/g)
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CONTROL OF FILAMENTOUS ORGANISMS
The most common operational problem in the
activated – sludge process
Proliferation of filamentous organisms in mixed
liquor result in porrly settling sludge
Causes the biological flocs in reactor to be
bulky and loosly packed
Filamentous bacteria, actinomycetes and
fungi
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CONTROL OF FILAMENTOUS ORGANISMS
The addition of chlorine or hydrogen peroxyde tothe return waste-activated sludge
Alteration of the DO concentration in the aerationtank
Increase F/M ratio
The addition of major nutrient (i.e. N & P) The addition of trace nutrient & growth factors
The use of selector
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SELECTOR
Control of the growth of filamentous organisms inthe complete-mix process have been achieved bymixing the return sludge with the incoming
wastewater in a small anoxic contact tank asselector
A separate compartment as the initial contact zoneof a biological reactor where the prymary effluent
and return activatedsludge are combined.
Contact time in the selector is relatively short, 10 – 30 min.
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EFFLUENT CHARACTERISTICS
Organic content is a major parameters
of effluent quality. Soluble biodegradable organics
Suspended organic material
Nonbiodegradable organics
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Soluble biodegradable organics
Organics that escape biological treatment Organic formed as intermediate productsin
biological degradation of the waste
Cellular componen (result of cell death orlysis)
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SUSPENDED ORGANIC MATERIAL
Biological solids produced during treatment
that escaoe separation in the final settlingtank
Colloidal organic solids in the plant influent
that escape treatment and separation
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NONDEGRADABLE ORGANICS
Those originally present
By-products of biological degradation
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PROCESS CONTROL
Control of the activated process is important
to maintain high level of treatment
performance under a wide range ofoperating condition.
The principal factors used in process control
Maintaining DO levels in aeration tanks Regulating the amount of return activated sludge
Controlling the waste activated sludge
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PROCESS CONTROL
The most commonly used parameters forcontrolling the act.process are : F/M ratio &mean cell-residence time, also MLSS.
RAS : important in maintaining the MLVSS
WAS: important in the controlling qc
OUR (oxygen uptake rate) : monitoring &controlling the activated-sludge process
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RETURN ACTIVATED-SLUDGE CONTROL
The purpose : to maintain a sufficient
concentration of activated sludge in the
aeration tank
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TECHNIQUES TO CALCULATE THE DESIRABLERETURN-SLUDGE FLOWRATE
Settleability
Sludge-blanket level control
Secondary clarifier mass balance
Aeration tank mass balance
Sludge quality
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Settleability
The return sludge-puming rate is set so that
the flowrate is approximately equal to the
percentage ratio of volume accupied bythe settleable solids from the aeration tank effluent to the volume of the clarified liquid
(supernatan) after settling for 30 min in a
1000 mL graduated cylinder
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example
If the settleable solids occupied a volume of
275 mL after 30 min of settling,
the percentage volume would be equal to 38% ( (275mL/725 mL) x 100 ).
If the plant flow were 46 Mgal/d (2 m
3
/sec), thereturn-sludge rate should be
0,38 x 46 Mgal/d = 17.5 Mgal/d (0.76 m3/sec)
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SLUDGE VOLUME INDEX (SVI)
Volume in mL occupied by one gram of activated-sludge mixed liquor solids, dry weight, after settlingfor 30 min in a 1000 mL graduated cylinder.
In practice, it is taken to be percentage volumeoccupied by the sludge in a mixed-liquor sampleafter 30 min of settling, Ov, divided by the SSconcentration of the mixed liquor expressed asapercentage, Pw.
The percentage of return sludge is
100/ ((100/PwSVI)-1)
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example
To maintain a mixed-liquor ss conc. of
0.3 % (3000 mg/L), the percentage of sludgethat must be returned when the SVI is 100 =100 /((100/0.3 x 100) – 1)
= 43 %
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Secondary clarifier mass balance
Accumulation = inflow – outflow0= X(Q+Qr)(8.34) - XrQr(8.34) + XrQ’w(8.34)
X : mixed-liquor SS, mg/L
Q : secondary influent flow, Mgal/d
Qr : return sludge flow, Mgal/d
Xr : return activated-sludge SS. Mg/L
Q’w : waste sludge flow, Mgal/d
8.34 : conversion factor (lb/Mgal. Mg/L)
Qr = (XQ – XrQ’w) / (Xr -X)
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Aeration tank mass balance
Accumulation = inflow – outflow
0 = XrQr (8,34) –
X(Q+Qr) (8.34)
X
Qr = Q
Xr-X