In Activated sludge process, air or oxygen is forced into sewage liquor to develop a biological floc which reduces the organic content of the sewage. The sludge is returned to the aeration system to re-seed the new sewage entering the tank. Activated sludge is also the name given to the active biological material produced by activated sludge plants. Excess sludge is called “surplus activated sludge” or “waste activated sludge” and is removed from the treatment process to keep the ratio of biomass to food supplied in the wastewater in balance.
