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This is the last but most important step of generating biogas. Methane can be

metabolized out of acetic acid (CH3COOH), H2 and CO2, methanol of methylene
(CH3NH3). Responsible microorganisms are methanogenic bacteria. They are strictly
anaerobic and assigned to archaea. Methanogenic bacteria are "substrate experts",
which can transform only a few substances (Figure 9).

Molecular hydrogen is serving as a universal substrate, carbon dioxide is acting as
a carbon source and terminal electron acceptor.

Figure 9: Energy gaining reactions of methanogenic bacteria.

All species of methanogenic bacteria are able to transform CO2 and H2 respectively
hydrocarbonate (HCO3-), whereas just a few bacteria are able to transform acetic acid directly,
and just one species is able to convert methanol into methane.

Even though the formation of methane out of hydrogen is energetically favorable, just 27-30%
is generated by this path and 70% out of acetate.

Currently three classes of acetate consuming methane bacteria are known, but they have a
2-4 times lower conversion- respectively expansion rate than hydrogen converting bacteria:

  • Methanosarcina barkeri (quad package of cells),
  • Methanosaeta (bacillary, filament) and
  • Methanosarcina mazei (assembled or single).

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