Infobox 2: Which processes are carried out in a biogas plant?
The conversion of biomass into biogas and fermentation residues in biogas reactors is down to a variety of microorganisms whose metabolic activity depends on external conditions such as temperature, biomass water content and substrate pH. Practice has shown that the microorganisms of biogas plants prefer stable parameters. This is why the most important parameters are monitored 24/7.
Biomass digestion is divided into four phases:
1) Hydrolysis: During hydrolysis, polymeric macromolecules (carbohydrates, fats, proteins) are broken down into their short-chain monomers (e.g. monosaccharides, amino acids and fatty acids) by enzymes derived from certain microorganisms.
2) Acidogenesis: Acidifying microorganisms break down the hydrolysis products into short-chain organic acids and alcohols (e.g. butyric and propionic acid), hydrogen sulphide and ammonia.
3) Acetogenesis: Certain acetate bacteria convert the acidogenesis products into acetic acid and hydrogen; the pH decreases.
4) Methanogenesis: This final phase, which is the most important one, involves producing methane (CH4) and carbon dioxide (CO2) from acetic acid using methanogenic archaea. At the same time, elementary hydrogen (4H2) and CO2 are turned into methane and water.