Anaerobic digestion is the process for the reduction of sludge without the use of air or elemental oxygen. In anaerobic digestion, organic pollutants are converted by anaerobic micro-organisms to a gaseous product (including methane gas) that has the potential for reuse. It is a very low energy process, that produces a fraction of the waste sludge generated through aerobic biological processes. It is particularly recommended for treating wastewater which is high in soluble BOD and/or COD.