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methane production

The degradation of organic compounds to produce biogas can be divided in four interdependent main metabolic pathways: Hydrolysis, Acidogenesis, Acetogenesis and Methanogenesis.


Hydrolysis

The hydrolysis is the first step in the anaerobic digestion process. Water-insoluble biopolymers are broken down into dissoluble fragments or into their monomeric parts. This reduction into particle size is performed by facultative anaerobe microorganisms and their exoenzymes. Carbohydrates such as starch and cellulose are digested into their oligomer or monomer sugars. Fat is turned into glycerine and high fatty acids. At first proteins are split into poly- and oligopeptides and are further broken down into amino acids by peptidases. The rate of hydrolysis of hardly bio-degradable substrates such as cellulose-rich substrates determines the velocity of the whole anaerobic digestion process.


Acidogenesis

The acidification proceeds parallel to the hydrolysis stage. During acidification the small molecular compounds produced in hydrolysis are converted into short-chain organic acids (volatile organic acids like acetic acid, propionic acid, butyric acid and valerianic acid), alcohols, H2 and CO2. Facultative anaerobe microorganisms are able to gain energy during acidification.


Acetogenesis

In the third step of anaerobic digestion, acetogenic microorganisms transform organic acids and alcohols into acetate / acetic acid, H2 and CO2, respectively.


Methanogenesis

Strictly anaerobic microorganisms have two different possibilities to produce methane. In the first possible metabolic pathway acetic acid (CH3COOH) is transformed into methane (CH4) and carbon dioxide (CO2). This pathway provides 70 % of the produced methane. In the second pathway (30 %) methane is produced by the reduction of carbon dioxide with hydrogen. Microorganisms gain energy by performing one of these metabolic pathways.