What is the general process for wastewater treatment?

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Modern wastewater treatment technologies can be classified into primary, secondary, and tertiary treatment based on the degree of treatment. Primary treatment primarily removes 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, approximately 30% of the BOD can usually be removed, but this level does not meet the discharge standards. Primary treatment is a pre-treatment for secondary treatment. Secondary treatment primarily removes organic pollutants in the wastewater that are in colloidal or dissolved form (such as BOD and COD), with a removal rate of over 90%, thereby ensuring that these organic pollutants meet the discharge standards. Tertiary treatment involves further processing of refractory organic substances, as well as soluble inorganic substances such as nitrogen and phosphorus that can cause eutrophication in water bodies. The main methods include biological nitrogen and phosphorus removal, coagulation sedimentation, sand filtration, activated carbon adsorption, ion exchange, and electrodialysis. The entire process involves raw wastewater passing through a coarse grid; it is then lifted by a sewage lift pump and goes through a grid or sand filter, before entering a grit chamber. The wastewater after sand-water separation proceeds to a primary sedimentation tank. This constitutes the first stage of treatment (i.e., physical treatment). The effluent from the primary sedimentation tank enters the biological treatment equipment, which includes the activated sludge process and the biofilm process. (The reactors used in the activated sludge process include aeration tanks and oxidation ditches, while the biofilm process involves biological filters, biological rotating disks, biological contact oxidation, and biological fluidized beds.) The effluent from the biological treatment equipment then goes to the secondary sedimentation tank; the effluent from there is either discharged after disinfection or sent for tertiary treatment. Once the primary treatment is completed, this stage represents the secondary treatment. 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 passing through dewatering and drying equipment, the sludge is finally utilized.