or as a saturation type of expression:
r
_ k .
XsfXB,S
. X
V,XS- hX Kx + (Xs/XB,S) B,S
(3.4)
The hydrolysis constants are lower under anaerobic conditions than under aerobic
conditions.
Example 3.14
The hydrolysis rate, rv,xs. in an anaerobic process is set at 0.38 kg COD(X)/(m 3 · d).
Furthermore we know:
Xs = 0.5 kg COD/m 3
Xs,s = 1 kg COD/m 3
khx = 0.4 kg COD(X)/(kg COD(B) · d)
Find the hydrolysis saturation constant, Kx.
From Expression (3.4), Kx can be found:
0.38 kg COD(X)/(m 3 ·d)= 0.4 d- 1
0 · 511 ' 0
· 1.0 kg COD/m 3
Kx + 0.5/1.0
0.2
0.38 = Kx + 0.5
Kx = 0.026 kg COD(X)/kg COD(B)
Acid step, anaerobic processes
The acid production can be described by Monod kinetics:
llrnax,S
S
X
rvs= - - .
· BS
'
Y rnax,S S + Ks,s
'
(3.47)
where S
is the substrate in the form of small dissolved molecules of organic matter,
XB,S
is the biomass of acid-forming bacteria,
Ks,s is the saturation constant.
Methane step
The methane production can also be described by Monod kinetics:
llrnax,M
S
X
rvs= - - - .
· BM
'
Y rnax,M S + Ks,M
'
where S
is the concentration of acetic acid,
XB,M is the biomass of methane producers.
(3.48)
Acetic acid can act as an inhibitor which can be described by using the following
kinetics /29 I:
105