The modern sewage treatment technology can be divided to primary, secondary and tertiary treatment based on the treatment degree. The sewage treatment degree is normally confirmed according to the water quality and water direction after treatment.
Primary Treatment
The primary treatment is mainly to remove the solid contaminants with the suspended state in the sewage. Most of physical treatment method can only meet the requirements of primary treatment. For the sewage after primary treatment, BOD can normally be removed about 30% and cannot meet the discharging standard. The primary standard belongs to the pretreatment of secondary treatment.
Secondary Treatment
The secondary treatment is mainly to remove the organic containments (BOD, COD) with the colloidal and dissolved state in the sewage. The removal rate can reach more than 90% so as to make the organic containment reach the discharging standard. The removal rate of suspended matters can reach 95% with good effluent results.
Tertiary Treatment
The tertiary treatment can further process the dissolved inorganic matters leading to the water eutrophication such as the degradation-resistant organic matters, nitrogen and phosphorus. The main methods include the biological nitrogen and phosphorus removal method, coagulant sedimentation method, sand filtration method, active carbon adsorption, ion exchange method and electrodialysis method.
The whole process is: the raw sewage thorough coarse grid enters into the grit basin via the grid or screen filter after lifted by the sewage lift pump and the sewage after grit-water separation enters into the primary sedimentation tank. The foregoing is the primary treatment (physical treatment). The effluent from the primary sedimentation tank enters into the biological treatment equipment. The activated sludge process and biofilm process are used (of which, there is aeration tank and oxidation ditch in the reactor with activated sludge process. The biofilm process includes the biofilter, biological rotating disc, biological contact oxidation process and biological fluidized bed). The effluent of biological treatment equipment enters into the secondary sedimentation tank. The effluent of secondary sedimentation tank is discharged or enters into the tertiary treatment after sterilization. The primary treatment ends and the secondary treatment starts. The tertiary treatment includes biological nitrogen and phosphorus removal, coagulant sedimentation, sand filtration method, active carbon adsorption method, ion exchange method and electrodialysis method. Parts of sludge in the secondary sedimentation tank flow back to the primary sedimentation tank or biological treatment equipment. Parts of sludge enter into the sludge thickening tank and then into the sludge digestion tank. After dehydration and drying equipment, the sludge is finally used.
