In this process, the materials are naturally digested or degraded by bacteria in sealed airtight containers with an absence of oxygen. A range of characteristics describe the different digester types.
Low or high solid digesters – These treat the material that is in liquid state with 15-20% solids. These low solid digesters need continuous mixing. High solid digesters usually hold the material with 20-40% solids and this process does not have any internal mixing.
Single or multi-stage systems – The digestion process has many different phases. The important phase involves hydrolysis or acidogenesis. In this stage, material is broken into organic acids and then methanogenesis, where the methane is produced, takes place. Single digesters take in all stages of digestion in a single container, but the multi-stage digesters takes up the process in different containers.
Mesophilic and thermophilic systems – The mesophilic system works at 35oC and need less external heating. This produces less gas and the retention times taken to get the material digested are around 15-30 days. The thermophilic system usually operates at 55o C and needs more heating energy. This produces more gas, is harder to control, and the retention times are only 12-15 days.
Continuous or batch flow systems – As the name clearly suggests, some systems work in a batch mode in which the materials are processed in one go and left for the full retention time. These tend to be smaller systems, whereas the continuous system is a larger system that gradually feeds material and draws gas from the digestate at the same time.