Aerobic digestion is a biosolids treatment process which utilizes psychrophilic microorganisms (organisms that thrive at temperatures near 20ºC)and is similar to the activated sludge process. However, in the aerobic digestion process the available food supply becomes depleted. This forces the microorganisms to enter into a state of endogenous decay where the microorganisms begin to consume their own protoplasm in order to obtain energy for cell maintenance reactions.

The biochemical reactions that occur due to endogenous decay result in a significant reduction in the microorganisms and any available food source and, consequently - destruction of volatile suspended solids. The end product of aerobic digestion is an odorless, humus-like, biologically stable material that reflects general fertilizer characteristics.