A biodigester or digester is an airtight chamber in which anaerobic digestion of manure, biosolids, food waste, other organic wastewater streams or a combination of these feedstocks occurs. This process produces commodities such as biogas (a blend of methane and carbon dioxide), animal bedding, and fertilizer.

A biogas digester, also known as a methane digester, is a system of equipment which can turn organic waste into usable fuel. In addition to providing a source of renewable fuel, biogas digesters also provide low-cost fuel to people in poverty, and they help to dispose of waste materials which would otherwise be discarded. A number of nations have invested in research on biogas digesters, ranging from devices which can be used by a single household to industrial-scale equipment which could be used to generate large amounts of power.

The biogas digester relies on decomposition of biomass, waste material which is biological in origin, ranging from kitchen scraps to cow dung. As anyone who has walked past a poorly maintained outhouse or compost pile is aware, they emit a number of distinctive gases, most notably methane, in the process of digestion. These gases are usually viewed as a symptom of inefficiency and they are vented away for disposal, but they can actually be very useful.

The following are the various forms of biodigester systems and their target groups