What is the purpose of tertiary treatment in general municipal wastewater treatment plants?

From the Internet: Modern wastewater treatment processes can be classified according to the level of treatment into primary, secondary, and tertiary treatment. The quality of the water output at each stage varies; generally, the level of treatment is determined based on the quality of the water and the intended use of the treated water. Below is a brief introduction to the three treatment levels for wastewater. Primary treatment, also known as pretreatment, is primarily used to remove solid pollutants in wastewater that are in a suspended state. Most physical treatment methods can only meet the requirements of primary treatment. For wastewater that has undergone primary treatment, the BOD removal rate can reach around 30%, which is not sufficient to meet the discharge standards. Wastewater treatment – secondary treatment: Secondary treatment in wastewater treatment is primarily used to remove organic pollutants in colloidal and dissolved forms present in wastewater (such as BOD and COD), with a removal rate of over 90%, thereby ensuring that these organic pollutants meet the discharge standards ； The suspended solids removal rate reaches 95%, resulting in excellent effluent quality. Tertiary treatment: Tertiary treatment can further deal with the refractory organic substances that were not removed during primary and secondary treatment, as well as soluble inorganic substances such as nitrogen and phosphorus, which can cause eutrophication of water bodies. The main methods employed include biological nitrogen and phosphorus removal, coagulation sedimentation, sand filtration, activated carbon adsorption, ion exchange, and electrodialysis. Tertiary treatment process: The wastewater passes through a coarse grid, is then lifted by a sewage lift pump, and goes through a grid or screen before entering a grit chamber. The wastewater after sand-water separation enters a primary sedimentation tank; this constitutes the primary treatment phase (i.e., physical treatment). The effluent from the primary sedimentation tank is sent to biological treatment units, which include the activated sludge method and the biofilm method. (The equipment used in the activated sludge method includes aeration tanks and oxidation ditches, while the biofilm method involves biological filters, biological rotors, biological contact oxidation processes, and biological fluidized beds.) The effluent from these biological treatment units enters a secondary sedimentation tank. The effluent from the secondary sedimentation tank is either discharged after disinfection or sent for tertiary treatment. Once the primary treatment is complete, this marks the end of the secondary treatment phase. Tertiary treatment includes biological nitrogen and phosphorus removal methods, coagulation and sedimentation, sand filtration, activated carbon adsorption, ion exchange, and electrodialysis. Part of the sludge from the secondary sedimentation tank is returned to the primary sedimentation tank or biological treatment equipment, while another part goes to the sludge thickening tank and then to the sludge digestion tank. After dehydration and drying, it is finally utilized.

Tertiary treatment: Advanced treatment of wastewater, which includes the removal of nutrients and the disinfection of wastewater using chlorination, ultraviolet radiation, or ozone technology.