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8/3/2019 Activated Sludge Design
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Design of an activated sludge
plant with recycle
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Definitions
Q: flowC: concentrationX: particulatesS: solubleM: massSludge recycle rate: R = Q4/Q1
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Fractionation of COD
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Fractionation of nitrogen
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Treatment efficiency
E = (C1-C3)/C1
or better:
E = (M1-M3)/M1
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Volumetric loading
BV = Q1*C1/V2
or better
BV = M1/V2
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Sludge concentration
Measured as
SS (same as TSS),
VSS,TS,VS
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Sludge Mass
MX = V*X
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Sludge loading
BX = Q1*C1/(V2*X2)
or better
BX = M1/(V2*X2)
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Sludge production
FSP = Q3*X3 + Q5*X5
or better
FSP = M3 + M5
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Excess (surplus) sludge production
FOSP = Q5*X5
or better
FOSP = M5
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Observed yield
FSP = Yobs (C1-C3)*Q1
or better
FSP = Yobs (M1-M3)
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Sludge age/ sludge retention time
X = MX/FSP
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Typical sludge retention times
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Aerobic sludge age
X, aerobic = MX, aerobic/FSP
MX, aerobic = Vaerobic*X
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C/N ratio and C/P ratio
For both denitrification and biological phosphorousremoval enough organic matter is needed. The ratios
are used as an estimation of if enough organic matteris present.
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Sludge Volume Index, SVI
Take 1000 ml MLSS from an
activated sludge tank and let itsettle in a cylindrical glass for
30 minutes.
SVI = (ml sluge after settling) /(g SS/l before settling)
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Diluted Sludge Volume Index,DSVI
Take 1000 ml MLSS from an activated
sludge tank, dillute it e.g. 1:1, 1:2, 1:3, 1:4and let it settle in a cylindrical glass for 30
minutes.
DSVI = (ml sludge after settling) / (g SS/lbefore settling but after dilution)
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SVI determines the MLSS concentration in the
tanks when the tank size is given
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Treatment plants can bedesigned by:
Volumetric loading: BV = M1/V2 Sludge loading: BX = M1/(V2*X2)
Sludge age (nitrification) and denitrificationrate (organic matter quality)
Computer aided design
Should only be used for plant optimization notfor basic design. Here rule of thumbs aresafer!
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Design of activated sludgetreatment plants for
nitrogen removal andphosphorous removal
A manual
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Key data for design
Chose the lowest temperature at whichnitrogen removal must take place.
Find the flow and the loadings of, COD, BOD,
N, and P that the plant must be design to treat. Find the expected discharges of SS, COD,
BOD, N, and P. Note that this is not the sameas the outlet standards.
Estimate the degradability of the wastewater. Isit easily or not so easily degradable?
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Design of nitrification and denitrification
Estimate the necessary aerobic sludge age ( X,aerobic)5
31
2
4
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Estimate the observed yield (Yobs) without chemicalsludge production in either the unit of kgSS/kgBODor kgCOD/kgCOD. In general it is better to use the
later unit as the yield chosen hence is independenton chemical additions to the plant.
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The same graph from anotherbook
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Estimate the chemical sludge production CP,chem.
The chemical sludge production can be calculatedbased on the assumption that MePO4 and Me(OH)3 are
the end products of the process. I.e. First all PO4 isprecipitated by Me and then the rest of the metal goesinto the side reaction producing metal hydroxide. This isnot entirely true, but good enough for estimating the
chemical sludge mass produced. The molar rationeeded to obtain good P-removal depends on whetherchemical P-removal is the only process applied or ifthere is also biological P-removal.
In the first case, the needed molar ratio is 1.2-1.6 (moleMe) / (mole Pinlet). In the second case, the needed molarratio is 0.2-0.4 (mole Me) / (mole Pinlet). These valuesare based on experiences in full-scale treatment plants.
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Estimate the outlet concentrations from the
treatment plant (C3). Typical values are BOD: 2-3 gm-3; SS: 5-15 g m-3; N: 4-6 g m-3; P: 0.3-1.0 g m-3.
Determine the sludge production (F ) and the excess
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Determine the sludge production (FSP) and the excesssludge production (FESP): FSP = Yobs (MBOD1 MBOD3) when the unit of Yobs is kgSS/kgBOD
If the unit of Yobs
is kgCOD/kgCOD then FSP
= Yobs
(MCOD1
- MCOD3
).But now the unit of FSP becomes kgCOD/d and one must convert fromkgCOD/d to kgSS/d. This is done by multiplying with a conversion factorbetween COD and VSS in the sludge, typically 1,4-1,5 kgCOD/kgVSSand then multiplying with the ratio of VSS to SS in the sludge. The lateris around 0,8 kgVSS/kgSS when no chemicals are added to the plant.
The exact value can be found from the analysis of the actual excesssludge of the plant in question.
NOTE that the ratio of kgVSS/kgSS decreases when chemicals areadded for P-precipitation! For systems without biological P-removal theratio is roughly 0.6-0.7 gVSS/gSS.
If there is chemical sludge production:
FSP = Yobs (MCOD1 - MCOD3) + MP,chem FESP = M5 = FSP X3
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Determine the necessary aerobic sludge mass.
MX,aerobic = X,aerobic FSP
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Find the nitrogen that is to be denitrified(MN,denit). Estimate the nitrogen concentration inthe excess sludge. This is done either based on
the actual analysis of the sludge or, if thesevalues are not available, on a theoretic estimateof the N-content of sludge.
Estimate the denitrification rate (r ) based on the
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Estimate the denitrification rate (rX,S(NO3)) based on thebiodegradability of the wastewater.
Determine anoxic sludge mass. MX,anoxic = MN,denit/
rX,S(NO3)
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Estimate the suspended solids (SS)concentration in the process tanks (X2). Typicalvalues are 4-6 kg SS/m3.
F l i h l d i h bi d
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For plants with recycle determine the aerobic andanoxic reactor volume. For alternating plantsdetermine the total reactor volume and the fractions
of time where the plant must operate in eachphase.
tanoxic = MX,anoxic/ (MX,anoxic + MX,aerobic)
taerobic = MX,aerobic/ (MX,anoxic + MX,aerobic)
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Check that ratio between CBOD and CN,denit at theinlet to the denitrification reactor is > 4kgBOD/kgN
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Design of biologicalphosphorous removal
Estimate the volume of the anaerobic tank (1 3hours retention time)
Check that the ratio between soluble COD and
soluble P is > 10 gCODsol/gPsol. If nitrate ispresent at the inlet to the anaerobic tank thenthis ratio must be increased.
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