Modeling of Biogas Reactors

178 6 Modeling of Biogas Reactors
kinetic data of authors who worked with more defined microbial systems are studied.
These data are presented in Figure 6.14. It can be seen that experiments performed
with Methanothrix sp. result in a smaller K HAc value (K HAc = 0.02 mmol L –1 )
than those performed with Methanosarcina barkeri, which correspond to a somewhat
higher value (K HAc = 0.12 mmol L –1 ). This result may be due to the fact that the relative
surface of the filamentous Methanothrix sp. is larger than that of the rodshaped
Methanosarcina sp.
Altogether, the K HAc value based on undissociated acetic acid is roughly in the
range between 0.02 mmol L –1 and 0.12 mmol L –1 . In a mixed culture an average value
of 0.07 mmol L –1 may be adequate. If this mixed culture contains mainly Methanothrix
sp. to convert acetic acid, the value will be smaller. If Methanosarcina sp.
dominate, the value of K HAc should be higher. The author of this paper used K HAc =
0.07 mmol L –1 .
Witty and Märkl (1986) reported that propionic acid inhibits the conversion of acetic
acid to methane and CO 2. Experiments with different amounts of propionic acid
resulted in an inhibition coefficient of K HPro = 0.97 mmol L –1 . Therefore, a concentration
of 0.97 mmol L –1 of undissociated propionic acid causes an inhibition of 50%
of the rate without propionic acid (Eq. 18).
2 c c
H ã 1
2
K c
D,Pro+c H ã 1
c HPro = c Pro (16)
2 c c
H ã 1
cH2S = cS,tot (17)
2
K c
D,H2S+c H ã1 Fig. 16.14 Methane production as a function of the concentration of the undissociated acetic acid.
Experimental data of cultures of Methanothrix-like sp. and Methanosarcina barkeri.