Anaerobic digestion is a series of biological processes in which microorganicisms break down biodegradable material in the absence of oxygen. The process takes place within an enclosure, known as a digester. One of the end products is biogas, which can be processed into renewable natural gas and transportation fuels. Separated digested solids can be composted, utilized for dairy bedding, directly applied to cropland, or converted into other products.
Source: American Biogas Council