Microbiology
The three steps of biogas production
Biogas microbes consist of a large group of complex and differently acting microbe species,
notable the methane-producing bacteria. The whole biogas-process can be devided into
three steps: hydrolysis, acidification, and methane formation (Figure 2). Three types of
bacteria are involved (Figure 3).
Figure 2: The three-stage anaerobic fermentation of biomass
from: Production and Utilization of Biogas in Rural Areas of Industrialized and
Developing Countries, Schriftenreihe der gtz, No. 97, p. 54; after: Märkl, H.: Mikrobielle
Methangewinnung; in: Fortschritte der Verfahrenstechnik, Vol. 18, p. 509, Düsseldorf,
FRG
Hydrolysis
In the first step (hydrolisis), the organic matter is enzymolyzed externally by extracellular
enzymes (cellulase, amylase, protease and lipase) of microorganisms. Bacteria decompose
the long chains of the complex carbohydrates, proteins and lipids into shorter parts. For
example, polysaccharides are converted into monosaccharides. Proteins are split into
peptides and amino acids.
Acidification
Acid-producing bacteria, involved in the second step, convert the intermediates of fermenting
bacteria into acetic acid (CH3COOH), hydrogen (H2) and carbon dioxide (CO2). These
bacteria are facultatively anaerobic and can grow under acid conditions. To produce acetic
acid, they need oxygen and carbon. For this, they use the oxygen solved in the solution or
bounded-oxygen. Hereby, the acid-producing bacteria create an anaerobic condition which is
essential for the methane producing microorganisms. Moreover, they reduce the compounds
with a low molecular weight into alcohols, organic acids, amino acids, carbon dioxide,
hydrogen sulphide and traces of methane. From a chemical standpoint, this process is
partially endergonic (i.e. only possible with energy input), since bacteria alone are not
capable of sustaining that type of reaction.
Acid-producing bacteria, involved in the second step, convert the intermediates of fermenting
bacteria into acetic acid (CH3COOH), hydrogen (H2) and carbon dioxide (CO2). These
bacteria are facultatively anaerobic and can grow under acid conditions. To produce acetic
acid, they need oxygen and carbon. For this, they use the oxygen solved in the solution or
bound oxygen. Hereby, the acid-producing bacteria create an anaerobic condition which is
essential for the methane producing microorganisms. Moreover, they reduce the compounds
with a low molecular weight into alcohols, organic acids, amino acids, carbon dioxide,
hydrogen sulphide and traces of methane. From a chemical standpoint, this process is
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