A tank in which sludge is placed to allow decomposition by microorganisms.  Digestion occurs under anaerobic conditions where NO free or dissolved oxygen is present.  These microorganisms obtain their oxygen by breaking down chemical compounds that contain oxygen, such as sulfate (SO42-).  Carbon dioxide (CO2) and methane (CH4) are the gaseous breakdown products produced from anaerobic digestion.  A portion of the methane produced is reused to heat digesters and buildings while the excess is burned off in the methane waste burner.  Upcoming projects are slated to make full use of this energy resource.