Primary treatment mainly removes suspended solid pollutants from sewage, and most physical treatment methods can only meet the requirements of primary treatment. After primary treatment, BOD can generally be removed by about 30%, which can not meet the discharge standard. Primary treatment belongs to the pretreatment of secondary treatment.
The secondary treatment mainly removes colloid and dissolved organic pollutants (BOD, COD) in sewage, and the removal rate can reach over 90%, so that the organic pollutants can meet the discharge standards.
Tertiary treatment, further treatment of refractory organic matter, soluble inorganic matter such as nitrogen and phosphorus that can lead to eutrophication of water body. The main methods include biological nitrogen and phosphorus removal, coagulation and sedimentation, sand filtration, activated carbon adsorption, ion exchange and electrodialysis.
The whole process is that the raw sewage passing through the coarse grid is lifted by the sewage lifting pump, passes through the grid or sand filter, and then enters the grit chamber, and the sewage separated from sand and water enters the primary grit chamber, which is the primary treatment (i.e. physical treatment). The effluent from the primary sedimentation tank enters biological treatment equipment, including activated sludge method and biofilm method. (The reactors of activated sludge process include aeration tank and oxidation ditch, etc. Biofilm method includes biological filter, biological turntable, biological contact oxidation method and biological fluidized bed). The effluent from biological treatment equipment enters the secondary sedimentation tank, and the effluent from the secondary sedimentation tank is discharged after disinfection or enters the tertiary treatment. The primary treatment ends here as secondary treatment, and the tertiary treatment includes biological nitrogen and phosphorus removal, coagulation and precipitation, sand filtration, activated carbon adsorption, ion exchange and electrodialysis. Part of the sludge in the secondary sedimentation tank flows back to the primary sedimentation tank or biological treatment equipment, and part of it enters the sludge concentration tank and then enters the sludge digestion tank. After dehydration and drying equipment, the sludge is finally utilized.