BIOCHEMICAL CONVERSION: ANAEROBIC DIGESTION - Biomass as a Sustainable Energy Source for the Future - page 452

required to minimize the number of pathogenic bacteria that may originate from the
feedstock (like sewage sludge and pig manure) and thrive at around 37 C.

14.2.3.3 Potential Process Inhibition Inhibition of the anaerobic digestion proc-
ess may occur due to accumulation of inhibitors during the process. Typically, the
acetate-degrading methanogens (see Figure 14.2) are most strongly inhibited in the
process, due to the low Gibbs free energy yield of the process they catalyze. Unfavor-
able process conditions such as the accumulation of inhibitors require expenditure of
metabolic energy by the methanogens. Herewith, less metabolic energy is available
for growth and maintaining cellular homeostasis resulting in inhibition.
The toxicity of many compounds in anaerobic digestion is strictly related to the oper-
ational pH. This is due to the fact that undissociated weak acids and bases can penetrate
the cell membrane of microorganisms and dissociate after entering the cytoplasm,
resulting in interference with the membrane potential, which is essential for cellular
homeostasis. A graph demonstrating the undissociated fractions of common inhibitors
in the anaerobic digestion process as a function of the pH is shown in Figure 14.6.
Acetic acid is the main precursor for methane production, and elevated acetic acid
(and propionic and butyric acid, in general named VFA) concentrations can be obtained
with readily degradable substrates when the rates of the first steps in the degradation
scheme shown in Figure 14.2 exceed the methanogenic capacity of the biomass. In that
case, VFA will accumulate and the pH will decrease. Both the increase in VFA concen-
tration and the lower pH will increase the concentration of undissociated acetic acid,
amplifying the toxicity effect of VFA in the system. The toxicity of hydrogen sulfide
is related to the ratio of sulfate to organic carbon in the substrate. Elevated ratios of sul-
fate to organic carbon result in a high partial pressure of hydrogen sulfide in the biogas,
and at low pH values, significant inhibition can be the result. Ammonia inhibition
occurs during anaerobic digestion of PR-rich substrates (like manure) at high pH values.

1.0
0.9
0.8
0.7

Hydrogen
sulide

0.6
0.5
0.4
0.3
0.2
0.1

Ammonia

Acetic acid

0.0

2 4 6 8 10 12

pH (-)

FIGURE 14.6 pH dependency of the concentration of undissociated weak acids and bases as
encountered in the anaerobic digestion process.