Biogas is similar to landfill gas in that it is produced by the decomposition of organic material under anaerobic conditions. The organic material can be wood chips, municipal sewage, rendering plants, animal manure from cattle or hog farms, or selected municipal waste such as yard or food waste.
The organic material is mixed with water and fed to tank, or "anaerobic digester", where it "cooks" for a period ranging from a few days to a few weeks, producing gas containing up to 65% methane and about 35% carbon dioxide, with small amounts of hydrogen sulfide. Depending on the particular digester system used, each ton of solids will produce from 6000 to 12000 cubic feet of methane.
More information on Anaerobic Digesters can be found at a U.S. government web site.