Activated sludge is a process in sewage treatment in which air or oxygen is forced into sewage liquor to develop a biological floc which reduces the organic content of the sewage. In all activated sludge plants, once the sewage has received sufficient treatment, excess mixed liquor is discharged into settling tanks and the supernatant is run off to undergo further treatment before discharge. Part of the settled material, the sludge, is returned to the head of the aeration system to re-seed the new sewage entering the tank. The remaining sludge is further treated prior to disposal.