Modern sewage treatment technology can be divided into primary, secondary, and tertiary treatment according to the degree of treatment. Generally, the degree of sewage treatment is determined based on the water quality and the destination of the treated water.

Primary treatment: mainly removes suspended solid pollutants from wastewater, while most physical treatment methods can only meet the requirements of primary treatment. After primary treatment of sewage, BOD can generally be removed by about 30%, which does not meet the discharge standards. The first level processing belongs to the preprocessing of the second level processing.

Secondary treatment: mainly removes colloidal and dissolved organic pollutants (BOD, COD substances) from wastewater, with a removal rate of over 90%, ensuring that organic pollutants meet discharge standards and suspended solids are removed at a rate of 95%. The effluent effect is good.

Third level treatment: Further treatment of recalcitrant organic matter, nitrogen, phosphorus, and other soluble inorganic substances that can lead to eutrophication of water bodies. The main methods include biological denitrification and phosphorus removal, coagulation precipitation, sand filtration, activated carbon adsorption, ion exchange, and electrodialysis.

The entire process involves lifting the raw sewage through a coarse grid through a sewage lift pump, passing through a grid or a screening rate device, and then entering a sedimentation tank. The sewage separated by sand and water enters the primary sedimentation tank, which is treated in a first stage (i.e. physical treatment). The effluent from the primary sedimentation tank enters the biological treatment equipment, including the activated sludge method and biofilm method (where the reactors of the activated sludge method include aeration tanks, oxidation ditches, etc., and the biofilm method includes biofilters, rotary tables, biological contact oxidation methods, and biological fluidized beds). The effluent from the biological treatment equipment enters the secondary sedimentation tank, and the effluent from the secondary sedimentation tank is disinfected and discharged or enters the third stage treatment. After the first stage treatment is completed, it is treated in a second stage, which includes biological removal. Nitrogen phosphorus removal method, coagulation precipitation method, sand filtration method, activated carbon adsorption method, ion exchange method, and electrodialysis method. Part of the sludge from the secondary sedimentation tank flows back to the primary sedimentation tank or biological treatment equipment, while another part enters the sludge concentration tank and then enters the sludge digestion tank. After being dehydrated and dried by the equipment, the sludge is utilized.