Anaerobic digestion to biogas

Definition biogas

Biogas is produced during a biological process in the absence of oxygen. From organic matter, a gas mixture consisting of 50-75% methane and 25-50% carbon dioxide is formed. There are also small amounts of hydrogen, hydrogen sulfide, ammonia and other trace gases in the biogas. For further information, please visit the page, raw biogas.

Anaerobic digestion process

The biogas production process consists of four biochemical reactions performed by different groups of microorganisms, taking place simultaneously in a one-step process. 

• Hydrolysis: The first reaction is liquefaction. Long-chain organic compounds (fats, proteins,…) are split and decomposed into simple organic compounds (amino acids, sugars,…) by enzymes released by bacteria.
• Acidogenesis: The products of the hydrolysis are converted into organic acids by acid-forming bacteria. Acetate, hydrogen and carbon dioxide, which are used as starting substances for the formation of methane, are also formed.
• Acetogenesis: Organic acids and alcohols are converted into acetic acid, hydrogen and carbon dioxide by acetogenic bacteria. It is important that the hydrogen produced is consumed by the methanogenic microorganisms, as too much hydrogen inhibits the formation of acetic acid.
• Methanogenesis: The products of the previous reactions are converted to methane, carbon dioxide and water by methanogenic microorganisms.

These individual biochemical reactions must be coordinated well.

Process conditions

The methanogenic microorganisms that form methane grow most slowly and react most sensitively to disturbances. Therefore the process conditions have to be adapted to the requirements of these microorganisms.

The following process conditions are required: absence of oxygen, constant temperature at a predetermined level, a pH-value between 5,2 and 8, presence of necessary macro- and micronutrients and the absence of inhibitors.

From 10 kg of plant biomass about 3 kWh of electricity or 4 kWh of heat can be produced with a gas engine and a generator.