activated sludge design

8/3/2019 Activated Sludge Design

1/37

Design of an activated sludge

plant with recycle


2/37

Definitions

Q: flowC: concentrationX: particulatesS: solubleM: massSludge recycle rate: R = Q4/Q1


3/37


4/37

Fractionation of COD


5/37

Fractionation of nitrogen


6/37

Treatment efficiency

E = (C1-C3)/C1

or better:

E = (M1-M3)/M1


7/37

Volumetric loading

BV = Q1*C1/V2

or better

BV = M1/V2


8/37

Sludge concentration

Measured as

SS (same as TSS),

VSS,TS,VS


9/37

Sludge Mass

MX = V*X


10/37

Sludge loading

BX = Q1*C1/(V2*X2)

or better

BX = M1/(V2*X2)


11/37

Sludge production

FSP = Q3*X3 + Q5*X5

or better

FSP = M3 + M5


12/37

Excess (surplus) sludge production

FOSP = Q5*X5

or better

FOSP = M5


13/37

Observed yield

FSP = Yobs (C1-C3)*Q1

or better

FSP = Yobs (M1-M3)


14/37

Sludge age/ sludge retention time

X = MX/FSP


15/37

Typical sludge retention times


16/37


17/37

Aerobic sludge age

X, aerobic = MX, aerobic/FSP

MX, aerobic = Vaerobic*X


18/37

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.


19/37

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)


20/37

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)


21/37

SVI determines the MLSS concentration in the

tanks when the tank size is given


22/37

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!


23/37

Design of activated sludgetreatment plants for

nitrogen removal andphosphorous removal

A manual


24/37

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?


25/37

Design of nitrification and denitrification

Estimate the necessary aerobic sludge age ( X,aerobic)5

31

2

4


26/37

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.


27/37

The same graph from anotherbook


28/37

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.


29/37

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


30/37

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


31/37

Determine the necessary aerobic sludge mass.

MX,aerobic = X,aerobic FSP


32/37

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


33/37

Estimate the denitrification rate (rX,S(NO3)) based on thebiodegradability of the wastewater.

Determine anoxic sludge mass. MX,anoxic = MN,denit/

rX,S(NO3)


34/37

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


35/37

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)


36/37

Check that ratio between CBOD and CN,denit at theinlet to the denitrification reactor is > 4kgBOD/kgN


37/37

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.

activated sludge design

Documents