What is biogas?

Biogas is only produced by anaerobic digestion. The process is divided into four steps: hydrolysis, acidogenesis, acetogenesis and methanogenesis.

Hydrolysis – the first step. In this step, the complex compounds in the starting material (e.g. hydrocarbons, proteins, fats) are split to form simpler, organic compounds (e.g. amino acids, sugars, fatty acids). The hydrolytic bacteria involved release enzymes that degrade the material biochemically.

Acidogenesis (acidification) – the intermediates produced are then further degraded by fermentative
(acidogenic) bacteria to form simple fatty acids (acetic, propionic and butanoic acids) as well as carbon dioxide and hydrogen. Small amounts of lactic acid and alcohols are also produced.

Acetogenesis (acetic acid formation) – these simple fatty acids are now converted into the biogas precursors (acetic acid, hydrogen, carbon dioxide) by acetogenic bacteria. The hydrogen partial pressure is of great significance at this stage.

Methanogenesis – the last step in biogas formation. At the end of this step acetic acid as well as hydrogen and carbon dioxide have been converted to methane by anaerobic methanogenic archaea. Approximately 70% of the methane formed results from acetic acid cleavage.

Biogas comprises the following gases:
Methane (50 to 70%), CO2 (30 to 50%), nitrogen (4%), oxygen ( 2%), hydrogen (1%), hydrogen sulphide (2%).


Biogas Chart 2

What is biogas produced from?
From organic waste and residues from:
Slaughterhouses • agriculture • restaurants • supermarkets • animal production
From renewable resources:
Maize, sorghum

Most important products:
• Biogas (can be upgraded to natural gas quality)
• Electricity
• Thermal energy
• Manure (digestate)

Biogas 1Biogas 2Biogas 3