Primary treatment, also called physical treatment, it’s whole process is that the raw sewage passing through the coarse grid is lifted by the sewage lifting pump, passes through the grid or the sand filter, and then enters the grit chamber, and the sewage separated by the sand water enters the primary sedimentation tank. The effluent from the primary sedimentation tank enters the biological treatment equipment, and has activated sludge method and biofilm method. (The reactor of the activated sludge method has an aeration tank, an oxidation ditch, etc., and the biofilm method includes a biological filter, a biological turntable. , biological contact oxidation method and biological fluidized bed), then the effluent of the biological treatment equipment enters the secondary sedimentation tank, and the effluent of the secondary settling tank is disinfected or discharged into the tertiary treatment, which is the end of secondary treatment. The third stage treatment includes biological nitrogen, phosphorus removal, coagulation sedimentation, sand filtration, activated carbon adsorption, ion exchange and electrodialysis. Some of the sludge from the secondary sedimentation tank is returned to the primary sedimentation tank or biological treatment equipment, others enters the sludge concentration tank, and then enters the sludge digestion tank. After dewatering and drying equipment, the sludge can be finally utilized.
The first-stage treatment mainly removes the solid pollutants in the suspended state in the sewage, and most of the physical treatment methods can only fulfill the requirements of the primary treatment. 
