At present, there are many sewage treatment plants in China. For modern sewage treatment plants, sewage treatment technology is mainly classified according to the degree of treatment effect. It is what we usually refer to as primary, secondary and tertiary sewage treatment.
For the primary sewage treatment: it mainly removes the suspended solid pollutants in the sewage, and most of the physical treatment methods can only complete the requirements of the primary treatment. After primary treatment of sewage, BOD can generally be removed by about 30%, which does not meet the discharge standard. The primary treatment belongs to the preprocessing of the secondary treatment.
For secondary sewage treatment: It mainly removes organic pollutants (BOD, COD substances) in colloidal and dissolved state in sewage, and the removal rate can reach more than 90%, so that organic pollutants can meet the discharge standard.
For the tertiary treatment: further treatment of refractory organic matter, soluble inorganic matter such as nitrogen and phosphorus that can lead to eutrophication of water bodies, etc. The main methods are biological denitrification and phosphorus removal, coagulation sedimentation method, sand rate method, activated carbon adsorption method, ion exchange method and electroosmosis analysis method, etc.
In three grades for wastewater treatment technology. The whole process is that the raw sewage that passes through the coarse grid is lifted by the sewage lifting pump, passes through the grid or sieve, and then enters the grit chamber, and the sewage separated by the sand and water enters the primary sedimentation tank. The above is the primary treatment (ie physical treatment), the effluent of the primary sedimentation tank enters the biological treatment equipment, including activated sludge method and biofilm method, (the reactor of the activated sludge method includes aeration tank, oxidation ditch, etc., and the biofilm method includes biological filter, biological Turntable, biological contact oxidation and biological fluidized bed), the effluent from the 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. Primary treatment includes biological denitrification and phosphorus removal, coagulation sedimentation, sand filtration, activated carbon adsorption, ion exchange and electrodialysis.
Part of the sludge in the sedimentation tank is returned to the primary sedimentation tank or biological treatment equipment, and part of it enters the sludge thickening tank, and then enters the sludge digestion tank. After dewatering and drying equipment, the sludge is reused.