Anaerobic treatment of wastewater is a common sewage treatment technique that uses naturally growing bacteria to break down waste. This is efficient in two ways. Anaerobic treatment uses less energy than other processes, and also results in the production of gases such as methane that can be used to make energy, and can be lower in cost. The treatment type depends on the system used, called a digester, which commonly include covered lagoon, complete mix, and plug-flow varieties. Many other types are used for sewage treatment depending on the percentage of solids in the waste and the desired speed of the process.
The core of anaerobic treatment, the type of anaerobic digesters used also depends on how solid the waste is. Liquid waste, such as liquid manure on agricultural facilities, is processed most easily by covered lagoon digesters. The lagoons are usually large and covered to trap gases that form as the waste breaks down. For slurries that are partially liquid, a complete mix digester is typically used, in which waste is mixed inside an above-ground tank or one that is under the surface. Solids remain suspended as the anaerobic organisms break everything down.
Manure and other solid wastes are kept in an underground tank and broken down in a plug-flow digester. The lowest maintenance digester, it consists of a heated tank that stores remnant gas produced by the organisms. Sewage treatment and municipal landfills may call for one type of digester. Others may be used in agriculture, food processing, and wastewater treatment. There are also various other digesters used in anaerobic treatment, such as an upflow anaerobic sludge blanket, fixed-film anaerobic, or temperature-phased anaerobic digesters, which is typically used for low solids wastes.
The type of anaerobic treatment used can depend on how much it costs to design a system and use it, and how much methane can be sold or utilized to produce energy. The size of the tanks play a role, as well as the types of substances it will be used to break down. Anaerobic treatment involves pre-treatment, the processing of biogas, and the method of getting rid of or recycling the remaining waste products. The digestion process is a complex one in which waste molecules are broken down into simple compounds called monomers, which are converted into fatty acids. These break down into hydrogen and acetic acid in the presence of anaerobic bacteria, and are then further degraded into methane and carbon dioxide gas.