Mass balances, nitrifying plants
By entering the sludge age, Expression (4.15), is found :
1
e x A = - - -
,
!lobs,A,net
(6.3)
Knowing the net growth rate of the nitrifying bacteria, !lobs,A,net it is possible to find
the necessary sludge age, Sx,A from Expression (6.3), or the necessary volume of the
nitrification tank by means of Expression (6.2).
The sludge age is the easiest parameter to use, as it is very easily calculated for a
specific plant seeing that it is assumed to be the same for all the sludge components.
Hence it can be calculated based on an arbitrary measuring unit, SS, VSS, COD, etc.
and consequently no knowledge of the concentration of nitrifying biomass, Xs,A in
the plant is required.
The sludge age to be calculated is the aerobic sludge age because it was assumed,
when writing the mass balance in Expression (6.1), that removals, growth and decay
only take place in the aeration tank (with volume Vz).
Fig 6.2 shows the necessary aerobic sludge age for the maintenance of the nitrification under normal operating conditions in activated sludge treatment plants.
Expression (6.3) can be used to find the ammonium concentration of the aeration
tank, SNH4,:?J when the sludge age is known:
1
!lobs,A,net = - 8 -
X,A
SNH4,2
So2,2
b
1
!lmax,A · S
·
A=-NH4,2 + Ks,NH4,A Ks,02,A + So2,2
ex,A
Full nitrification
10
5
No nitrification
' 1. Temperature ("C)
0~--------~----------~----------~
5
10
15
20
(6.4)
Fig 6.2
Relationship between temperature and the ne cessary aerobic sludge age for the
achievement of nitrification in activated sludge treatment plants. (Operating conditions: Oxygen concentration 2 g!m 3 ).
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