Digestion of the manure occurs in a digestor, which must be strong enough to withstand the buildup of pressure and must provide anaerobic conditions for the bacteria inside. Digestors are usually built near the manure source, and several are often used together to provide a continuous gas supply. Products put into the digestor are composed mainly of carbohydrates with some lipids and proteins.
The digestion has three main stages. The first, hydrolysis, involves breaking down the large macromolecules to sugars, amino acids, and fatty acids by bacteria under aerobic conditions. The second stage is acetogenesis, during which acetogenic bacteria convert sugars into short-chain acids, mainly acetic acid. The third stage is methanogenesis, which is carried out by anaerobic bacteria. Here, the acids are converted into methane.