Three methods of domestic sewage treatment. The basic treatment methods are as follows: (1) primary treatment uses physical treatment methods, such as grille, screen, sand sedimentation tank, sedimentation tank, grease trap, etc. To remove solid suspended matter and oil slick in sewage, preliminarily adjust pH and reduce the corrosion degree of sewage. After primary treatment, the wastewater is generally not up to the discharge standard, and the BOD removal rate is only 25-40%. Therefore, the pretreatment stage is generally used to reduce the load of the subsequent processing process and improve the treatment effect. (2) Secondary treatment is the use of biological treatment and some chemical methods to remove biodegradable organic matter and some colloidal pollutants in sewage. The removal rate of BOD after secondary treatment can reach 80-90%, that is, BOD concentration does not exceed 30 mg/l. After secondary treatment, agricultural irrigation standards and sewage discharge standards can be reached, so secondary treatment is an important link of sewage treatment. The water after secondary treatment still contains a certain amount of suspended matter, insoluble organic matter, soluble inorganic matter, nitrogen and phosphorus containing viruses and bacteria and other algae nutrients. As a result, emission standards do not meet the requirements. For example, treated streams with low discharge rates and poor dilution capacity cause pollution and cannot be used directly as a source of supply for water, industry and groundwater. (3) Basic methods of domestic sewage treatment: tertiary treatment is phosphorus, nitrogen, organic matter, inorganic pollutants, pathogens and so on. Chemical methods (chemical oxidation, chemical precipitation, etc.) are used for the tertiary treatment of sewage. Physical chemistry (adsorption, ion exchange, membrane separation, etc.). Remove specific contaminants. Tertiary sewage treatment investment is large, can make full use of water resources.