Continous Flow Stirred Tank Digestors - These digestered are continuously fed as well as stirred to achieve a very high mixing; the spent substrate is also continuously removed. They are suited to treat medium to high strength (2-10% solids) wastes.

Mixing is achieved either mechanically or by recirculation of the biogas. Gas may be collected in a separate tank or the digester itself may have a floating roof. These are the most common type of digesters used for sewage and animal excreta treatment; some of the digesters used for sewage treatment are of 12,000,000 1