Stabilisation

Aerobic stabilisation can be performed simultaneously in an activated sludge plant whereby primary and secondary sludges are continuously aerated for long periods of time. In aerobic digestion the microorganisms extend into a respiration phase where materials previously stored by the cell are oxidized, resulting in a reduction of the biologically degradable organic matter. Thus, aerobic stabilisation of the entire excess sludge (including primary sludge) is energy consuming. Additionally, it calls for extra reactor volume. 
Sludge digestion is carried out in the absence of free oxygen by anaerobic organisms. The facultative and anaerobic organisms break down the complex molecular structure of these solids setting free the "bound" water and obtaining oxygen and food for their growth. Anaerobic stabilisation processes work at normal temperatures (< 40°C) or within the range of thermophile bacteria, where 50-65°C are reached alone by the heat development of the biochemical processes. The chemical stabilisation of the sludge by means of wet oxidation or addition of quicklime and thermal stabilisation under high temperature and pressure, are applied less often.

The anaerobic sludge digestion takes place in the highly visible digesters.

Digesters (source: klaerwerk-online.de)

More information about sludge treatment and reduction are on the following pages:

Thickening
Dewatering
Sludge drying
Copious growth of filamentous organisms - problems and solutions

Filter presses for sludge treatment

Centrifugation and centrifuges