Basic Biological Processes
Expression (3.12) converted into COD-units is:
b · MWbiom · 1 .42
a · MW0 rg · 1.07
Yobs
If a = 1 and other known values are assigned, b is found
b · 113 g biom/mol · 1.42 COD/g biom = 0. 55 COD/ COD
1 · 60 g HAc/mol · 1.07 COD/g HAc
9
g
b =0.22
Hence the equation of reaction is:
CH3COOH + 0.22 NH;t + 0.22 OW+ 0.9 02 ->
0.22 CsH7N02 + 0.9 C02 + 1.78 H20
(3.12)
The expression demonstrates that nitrogen is needed for the growth so that it cannot
take place solely on the basis of acetic acid.
The same applies to other nutrients (P, S, K etc.).
From the expression it is seen that the yield constant on a molar basis is Y obs = 0.22
mole of biomass/mole of HAc.
In biological treatment plants, the yield constant varies, both with the type of
wastewater and the load of the plant. This is illustrated in Fig 3.5 with an example
from an activated sludge plant.
Yield constant
kgSS/kgBOD kgCOD(B)Ikg
added
COD(S) added
1,0
0 ,8
0,5
0,4
0,3
0,2
0,1
0,1
1,0
~--------:~ Raw
~------~ wastewater
0,5
2,0
1,0
Sludge load
kgBOD/(kgSS·d)
kgCOD(S)/(kgSS·d)
Fig 3.5
Obseroed yield constant as a function of the sludge load in activated sludge plants.
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