Environmental Engineering Reference
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3 CH
4
Electron acceptor reaction
12 e
C
6
H
12
O
6
3 CO
2
Electron donor reaction
FIGURE 14.1
Electron transfer during glucose fermentation to a mixture of methane and
carbon dioxide.
Particulate substrate
Hydrolysis/solubilization
Monomers
Acidogenesis
Fatty acids, alcohols
Acetogenesis
Hydrogen
Acetate
Methanogenesis
CH
4
+ CO
2
FIGURE 14.2
Simplified scheme of the sequence of conversion reactions occurring during
the anaerobic digestion process.
is both electron donor and acceptor of the chemical redox reaction. As an example,
the anaerobic degradation of glucose to a mixture of methane (CH
4
) and carbon
dioxide (CO
2
) in terms of an electron donor and acceptor reaction is shown in
Figure 14.1. From the six carbon atoms in glucose, three are oxidized to carbon diox-
ide, and the electrons are donated to the other three carbon atoms that are reduced to
methane.
As opposed to most biochemical degradation schemes, anaerobic digestion
requires the activity of a range of specialized microorganisms to catalyze the overall
fermentation process. Complex organic matter is degraded in a sequence of degrada-
tion reactions to methane-containing biogas according to the scheme shown in
Figure 14.2. Polymeric (particulate) organic material is first hydrolyzed and solubi-
lized. This reaction typically is catalyzed by extracellular enzymes produced by acido-
genic bacteria. Polymeric substrates may be proteins, polymeric carbohydrates, or
lipids. Hydrolysis products of these substrates are amino acids, sugar monomers,
and long-chain fatty acids and glycerol, respectively. The monomers are subsequently
fermented in the acidogenesis process, and volatile fatty acids (VFA) (acetic, propio-
nic, and butyric acid), carbon dioxide, hydrogen, and/or alcohols or other short-chain
organic acids are produced. Amino acid degradation is accompanied by the release of
ammonium in solution. The intermediately formed VFA and other small organic
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