4.2.1. Biogas

Biogas is produced in an oxygen-free environment by anaerobic digestion of organic matter using microorganisms. Biogas processing requires several stages of a number of microorganisms: Microorganisms turn complex organic compounds into less complex organic compounds that are then converted into organic acids in initial hydrolysis reactions. Then methane forming microorganisms use these acids to form methane, the principal component of biogas [2,11]. Biogas is a mixture of gases containing typically 50–70% of methane. In addition, anaerobic digestion can produce hydrogen either as a component of the biogas, or as the major product. The latter requires that particular species, such as *Rhodobacter* or *Enterobacter* sp., to dominate the microbial population [41,44]. Current models indicate significantly greater recovery of energy from the digestion of biomass as methane. Hydrogen fermentation may become more attractive with the advancement of fuel-cell technology. Biogas can be used with little alteration in many applications (Stoves, Boilers). The gas needs substantial modifications for applications in combustion engines (generators, motor car engines) in order to eliminate non-methane components [2,29].
