Biogas refers to a gas mixture produced by the biological decomposition of organic matter in the absence of oxygen (anaerob), which comprises of about 50-70 % methane and other components, such as 30-40% carbon dioxide and trace amount of hydrogen sulfide, nitrogen, carbon monoxide and water.

Methane-producing microbes prefer moist environment void of air. During their vital processes they decompose organic matter, whereby biogas is produced. Examples of natural biogas formation:

  • marsh gas,
  • biogas emitted on rice fields,
  • biogas produced in communal waste landfill sites,
  • biogas produced in the digestive system of ruminant livestock, termites and whales.

Biogas formation is a multiple-step process: In the first phase proteins, fats and carbohydrates decompose into simpler compounds, like amino acids, fatty acids and sugars. In the second phase, acetogenic bacteria transform these substances into organic acids with only a few carbon atoms (acetic acid, propion acid, butyric acid), hydrogen. Finally, a group of methane-producing microorganisms breaks down the organic acids into methane, carbon dioxide and water. At the same time, another group of methanogens transforms part of the arising carbon dioxide into methane by using the hydrogen produced by the acetogens. In a biogas plant, these steps are not separated, they take place simulataneously, which explains the sensitive nature of biogas technology.