The production of biogas is a complex biotechnological
process that can be divided into four reaction steps based on
each other (Fig. 1). First, the polymeric biomass components
are digested by hydrolysis, to release organic constituents.
Therefore, the educts (fats, proteins, carbohydrates) are
broken by enzymes in soluble oligomers and monomers
(peptides, amino acids, fatty acids, glycerol, oligo- and
monosaccharides). In acidogenesis this substances are converted
to hydrogen, CO2, organic acids, acetic acid, and alcohols.
At best, this reaction step leads to acetic acid, since it can
be converted directly by methanogenic archaea (methanogenesis).
However, this depends on several factors, e.g.
species-specific metabolic pathways and hydrogen partial
pressure have a significant influence on the reaction products.
If hydrogen partial pressure is high, usually higher organic
acids such as propionic acid were generated. In acetogenesis