How does Anaerobic Digestion work?

ANAEROBIC digestion is the breakdown of organic matter into carbon dioxide and methane (biogas), plus water and organic materials (digestate).

Biogas can be burned to produce heat and electricity. Methane can be used as vehicle fuel, or added into the gas grid.

Digestate is nutrient-rich and can be used as a fertiliser; a feedstock for ethanol production; and in low-grade building materials, such as fibreboard. The left over water, after treatment, can be recycled.

Anaerobic digestion is done by two groups of micro- organisms: bacteria and archaea. There are four main stages, which break the matter into smaller and smaller parts:

• Hydrolysis: Breaks down complex organic matter – carbohydrates, fats and proteins – into simple sugars, fatty acids and amino acids.
• Acidogenesis: Single sugar molecules, fatty acids and amino acids are broken down into alcohols and volatile fatty acids, producing carbon dioxide, ammonia and hydrogen sulphide.
• Acetogenesis: Volatile fatty acids and alcohols are converted into hydrogen, carbon dioxide, and acetic acid.
• Methanogenesis: Methanogenic archaea con- vert the remaining hydrogen and acetic acid into methane and more carbon dioxide.