The recirculation of the activated sludge is a main unit of the process of biological oxidation. In fact, it allows maintaining a sufficiently high concentration of biomass in the aeration tank, making possible a regular and intense growth of bacteria, regardless of the concentration of the inlet organic load. The active sludge is continuously re-circulated by pumps from the secondary sedimentation tank into the biological oxidation tank (or nitrification).  During the start-up of the plant, when a quick bacteria growth is required, the sludge is completely pumped back. When the plant reaches a good steady state, most of the sludge is pumped back while the remaining effluent, the excess sludge, should be undergone other treatments and thickened (several methods are available) before being discharged in a landfill.

This technology is an integral part of the treatment process of a Biological plant.