Decanter is batch activated sludge treatment of wastewater will be equipped with a mechanical device. Intermittent type activity sludge method of sewage processing mechanism and general activity sludge method same, its difference is original sewage not sequence flows through all processing unit but release to single reaction pool within, by time order achieved different purpose of operation, its basic operation program by water (fill), and reaction (react), and precipitation (settlc), and water (draw) and standby (idlc), five a basic process composition. From the sewage into the start to the end of standby time for one cycle