Modeling of Biogas Reactors

176 6 Modeling of Biogas Reactors
Table 6.3 Equilibrium constants for wastewater of bakers’ yeast production, temperature 37.4 °C.
ã1 0.73
Activity Coefficients
Monovalent ion
ã2 0.28 Bivalent ion
Dissociation Constant [mol L –1 ]
KD,Ac KD,Pro K I D,Ac K II D,CO
2 K I D,H
2
S K II D,H
2
S KD,NH3 1.17·10 –5
1.3·10 –5
tions of CO 3 2– and S 2– can be neglected in the relevant range of pH between 6.6 and
7.4. Therefore, calculation can be handled like a 1-step dissociation. Eqs. 3, 9–14 give
the pH value in implicit form. The pH can be evaluated by a stepwise iterative procedure.
6.3.1
Acetic Acid, Propionic Acid
4.94·10 –7
5.79·10 –11
1.44·10 –7
2.7·10 –14
6.83·10 –6
Many experiments have been reported on the growth rate of acetic acid converting
organisms with respect to the concentration of acetic acid in the fermentation broth.
A reliable collection of those data is presented in Figure 6.12. Because it is very difficult
to measure the growth rate of methane producing organisms directly (in a
mixed culture it may be impossible) and because it is assumed that this growth rate
is directly proportional to the methane production rate, usually the latter value is taken
as a measure of bacterial activity. In Figure 6.12 the methane production rate, related
to the maximum methane production rate of a biogas reactor, is denoted on the
ordinate of the graph.
Fig. 6.12 Methane production as a function of the concentration of total acetic acid.