When organic compounds are maintained at 5-70°C and neutral pH under anaerobic conditions, spontaneously biomethanation will happen. Biogas is often produced from underground of landfill. Kitchen garbage and sewage sludge have been used as substrates of biomethanation. Organic wastewater from food factory containing sugars and starch has been also used as substrate for biomethanation.

Biomethanation is composed of hydrolysis, acetogenesis and methanogenesis. Fig. 5.1.1

indicates an outline of biomethanation. Polysaccharides are decomposed to monosugars, proteins are to amino acids, and fats are to fatty acids and glycerol. Fermentative bacteria are for example Bacteroides spp. and Clostridium spp. Sugars and amino acids are decomposed to acetate and propionate by acidogens. Finally, methanogens convert acetate or hydrogen and carbon dioxide to methane. Acidogenesis is a complex process in which microflora of anaerobes collaboratively decomposes organic compounds to low molecular organic acids. Acetate, lactate, succinate, ethanol, butanol, acetone and etc. can be produced from glucose by acidogens. In wastewater treatment, 70% of methane is produced from acetate, and 30% is produced from hydrogen and carbon dioxide. Formula of acetoclasic reaction is as follows;

CH3COOH—CH4+CO2

Formula of hydrogenotrophic reaction is as follows;

CO2+4H2——CH4+2H2O

Methanogens are anaerobes which can grow using acetate or hydrogen and can produce methane. Representative methanogens are Methanobacter spp. and Methanosaeta spp.

Methanogens are killed by exposure to oxygen; therefore, methane formation requires obligate anaerobic conditions. Phylogenetic analyses indicate that methanogens are placed in Archaea group, distinguished from eukaryote and prokaryote. Methanogens can only use hydrogen, formate, acetate, 2-propanol, 2-buthanol, methylamine, methanol, methylmercaptan to produce methane.