This is processes by which microorganisms break down biodegradable material. The digestion process begins with bacterial hydrolysis of the input materials. Insoluble organic polymers, such as carbohydrates, are broken down to soluble derivatives that become available for other bacteria.
Acidogenic bacteria then convert the sugars and amino acids into carbon dioxide, hydrogen, ammonia, and organic acids. These bacteria convert these organic into acetic acid, Finally, methanogens convert these products to methane and carbon dioxide