1.5 Process of anaerobic digestion

Anaerobic digestion is a natural process that converts, in the absence of oxygen, organic matter, such as faecal or sewage sludge, food waste, and agricultural waste, into biogas and a nutrient-rich by-product called digestate.

To convert organic matter into biogas, there are four stages involved.

Hydrolysis: During this stage, complex organic compounds such as proteins, carbohydrates, and fats are broken down into simpler molecules by enzymes produced by bacteria. The enzymes convert complex organic compounds into simpler compounds such as amino acids, sugars, and fatty acids.

Acidogenesis: During acidogenesis, bacteria create organic acids, CO₂ , and hydrogen gas from simple molecules made in hydrolysis. Examples of organic acids made are acetic, propionic, and butyric.

Acetogenesis: In this stage, acetogenic bacteria convert the organic acids produced during the acidogenesis phase into acetic acid, hydrogen, and carbon dioxide.

Methanogenesis: This is the final stage of anaerobic digestion, where methane-producing bacteria called methanogens convert the acetic acid, hydrogen, and carbon dioxide produced in the previous steps into biogas, primarily methane and carbon dioxide.

Please note that:

• Degradation stages occur at the same time if not physically separated.
• The interdependent processes require intermediate products to proceed to the next step.
• Due to interdependency, mutual inhibition can occur, i.e., intermediate products may accumulate.
• Hydrolysis takes hours to days and is the fastest, while methane formation takes 5 to 15 days and is the slowest.
• An anaerobic, oxygen-free environment is specifically required for the methanogenesis stage.