Anaerobic digestion (AD) takes place over four stages, each a form of bacterial action:

1. Hydrolysis
2. Acidogenesis
3. Acetogenesis
4. Methanogenesis

Following the pre-processing of the feedstock, it is transferred to the digestion tank where hydrolysis initially begins to work on large inorganic molecules, such as proteins, lipids and carbohydrates. The bacteria breaks these down into simple molecules of amino acids, fatty acids and sugars.

The relatively simple molecules – now organic in nature – are then worked on by acidogenesis, where bacteria breaks them down into basic compounds such as organic acids.

Acetogenesis then sees bacteria work on the basic compounds and organic acids to produce acetic acid along with hydrogen monohydride (H2), ammonium (NH4), carbon dioxide (CO2).

The final stage is methanogenesis, and it is at this point that bacteria produce the final biogas – methane and carbon dioxide – by acting on the acetic acid.

Temperatures of about 35°C to 40°C are needed for standard mosphilic digestion to take place. The faster thermophilic digestion method requires temperatures of about 50°C to 55°C.

The remnants of the AD process are known as digestate, which can be used as a fertilizer.

Anaerobic digestion (AD) process

Page Last Updated: 30 May 2020