Anaerobic Wastewater Treatment
Component----t
Xs
Cs
5cH4
Reaction rate rv,x
Process J.
Methane production
-1
1-Ymax
Cs
1
-
J.lmaxCs + Ks · Xs
Ymax
Ymax
kgCOD / m 3
kgCOD/ m 3
kgCOD/m 3
Biomass
Org. matter
Methane
Table 9.2
Simplified process matrix, anaerobic process. Y ma. is the total maximum anaerobic yield constant.
With symbols from Fig 9.1, a COD mass balance for the wastewater can be written
as follows (by using the simplified process matrix in Table 9.2):
Q1 · C1-rv,x· Vz =Q3 · C3
(9.1)
1- Ymax
Cs
where rv,x =
· llmax
· Xe
Yrnax
Cs + Ks
The produced amount of methane corresponds to the COD removed from the
water and is therefore rv,x .Yz.
Suspended process
In a suspended anaerobic process, the sludge age will often be the design basis as
the plant is laid out in such a way that the slow-growing methane bacteria can
survive.
The observed net growth rate of the methane bacteria, f.l.ob51 can be found in Table 9.1
to be:
SHAc
llobs,M,net · Xs,M = llmax,M · S
K
· Xs,M - bH,M · Xs,M
HAc+ S,M
Hence a methane mass balance over the plant in Fig 9.2 is:
Ql · Xs,M,l + f.l.obs,M,net · Xs,M,2' Vz = Q3 · Xs,M,3+ Qs · Xs,M,S
(9.2)
Fig9.2
Suspended process/sludge blanket process, anaerobic treatment.
The amount of methane bacteria in the influent, 01 · Xs,M,1 can normally be
assumed to be zero. By introducing sludge production Fsr = Q3 · Xs,M,3 + Qs · Xs,M,S
(see Expression (4. 12)), the sludge mass Mx = Xs,M,2 · Vz (see Expression (4. 10)) and
the sludge age Sx,M (see Expression (4.14)), we find:
Sx,M = 1 I f.l.obs,M,net
(9.3)
287