The Imhoff tank is a combined anaerobic biological pretreatment tank. The sedimentation (in the top part) and the anaerobic digestion of the organic material on the bottom take place in the same dual-compartment unit.
The sedimented solids precipitate through communication cracks in the accumulation and digestion compartment, where the organic substances undergo anaerobic fermentation which results in their stabilization and permits their treatment and the easy removal of sludge. The carbon dioxide and methane that develop in the lower compartment do not interfere at all with the sedimentation process that occurs in the upper compartment.
The performance of this system is similar to that of conventional septic tanks, about 50-70% of the total load and 20-30% of the original BOD5.