TREATMENT OF SEWAGE WITH ACTIVATED SLUDGE

PURPOSE:To reduce the content of nitrogen, phosphorus, etc. in treated water, by growing aquatic plants on the free surface of the sewage in an aeration tank, ditch or settling basin in which the sewage added with activated sludge stagnates. CONSTITUTION:An aeration tank 34 is covered airtightly thereon with a light transmittable covering material 50 of PVC or the like and a gas is supplied thereon and is kept under the atmospheric pressure or above. Aquatic plants such as Eichhornia crassipes (Marius) (water hyacinth) Solms 54, Azolla imbricata Nakai 56 or the like are grown on the water surface of sewage 52. The flocs 58 of the activated sludge in the tank 34 suspend in the sewage and aerobic microbes propagate thereon. The aged stalks, leaves and roots as well as the sludge devoid of activity deposit as precipitate 60 on the bottom of the tank 34; therefore, the treatment system is divided to two systems, the covering 50 is removed, and the supply of the sewage and the supply of the gas are stopped properly alternately so that the aquatic plants are harvested with a harvester.