The primary treatment of sewage is the physical treatment of sewage, which is the first stage in the sewage treatment level, also known as the pretreatment of sewage. Through simple sedimentation, filtration or proper aeration, it can remove suspended solids and some suspended pollutants (including oil film and oil droplets) in sewage, adjust pH value and reduce the degree of corruption of sewage. The treatment method generally consists of sieving method, gravity sedimentation method, flotation and pre-aeration method, etc., to remove most of the particles in the sewage with a particle size of more than 100 microns; sieving can remove larger substances; gravity sedimentation can remove inorganic substances. Particles and cohesive organic particles with a relative density greater than 1; flotation can remove particles (oils, etc.) with a relative density less than 1. The processing equipment and methods used mainly include grids, screens, sedimentation (sand settling), filtration, microfiltration, air flotation, centrifugal (cyclone) separation, etc. Wastewater generally still fails to meet the discharge standard after primary treatment.

Secondary treatment of sewage is the purification process of colloidal and dissolved organic matter in sewage after primary treatment, and biological method and flocculation method are commonly used. Biological method is the use of microorganisms to treat sewage, mainly to remove organic matter in sewage after primary treatment, which can be divided into aerobic biological treatment method and anaerobic biological treatment method, such as activated sludge, biological filter, biological turntable, sewage irrigation, oxidation The flocculation method is to destroy the stability of the colloid by adding a flocculant (for example: Poly(Dimethyl diallyl ammonium Chloride), polyacrylamide, etc.), so that the colloidal particles flocculate, and flocculation occurs and adsorption occurs, mainly to remove the primary treatment Inorganic suspended solids and colloidal particles or low-concentration organic matter in the sewage. Secondary treatment is primarily used to adequately remove the biological content of human waste, food waste, soapy water and wash water.