Faecal sludge collected from bucket or vault latrines has a very high biochemical oxygen demand (BOD) and is generally putrid and odorous. Primary and secondary sludges from an sludge treatment plant also have a high BOD and may be difficult to dewater. Even sludge from a septic tank, which has undergone bacterial decomposition over at least a year, still has a high BOD. Stabilisation is the term used to denote the process of BOD reduction. The stabilisation process can be carried out under aerobic or anaerobic conditions.

(UNEP 2000)

Aerobic stabilisation

Anaerobic digestion