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4. Finally, methanogenic organisms consume the acetate, hydrogen, <strong>and</strong> some
of the carbon dioxide to produce methane. Three biochemical pathways are
used by methanogens to produce methane gas. (EPA., 2008)
Figure 8.: Schematic diagram showing the main theoretical stages of the
anaerobic digestion process. (University of Strathclyde., 2009).
Methanol is shown as the substrate for the methylotrophic pathway, although
other methylated substrates can be converted. Sugars <strong>and</strong> sugar-containing
polymers such as starch <strong>and</strong> cellulose yield one mole of acetate per mole of
sugar degraded. Since acetotrophic methanogenesis is the primary pathway
used, theoretical yield calculations are often made using this pathway alone.
(EPA., 2008)
Acetogenesis produces a quantity of hydrogen. According to (EPA., 2008) for
every four moles of hydrogen consumed by hydrogenotrophic methanogens a
mole of carbon dioxide is converted to methane. Substrates other than sugar,
such as fats <strong>and</strong> proteins, can yield larger amounts of hydrogen leading to
higher typical methane content for these substrates. Furthermore, hydrogen
<strong>and</strong> acetate can be biochemical substrates for a number of other products as
well. Therefore, the overall biogas yield <strong>and</strong> methane content will differ
because of the diversity of substrates, biological consortia <strong>and</strong> digester
conditions. Typically, the methane content of biogas ranges from 40-70 percent
(by volume). (EPA., 2008)