I. Wastewater Treatment Processes and Process Selection
Wastewater treatment processes refer to various economical, rational, scientific, and effective methods for treating urban domestic sewage and industrial wastewater. Wastewater treatment is widely used in construction, agriculture, transportation, energy, petrochemicals, environmental protection, urban landscaping, medical care, catering, and other fields. The optimal urban wastewater treatment process should be determined after a comprehensive techno-economic comparison, based on the treatment scale, water quality characteristics, the environmental function of the receiving water body, and local conditions and requirements. Generally, actual wastewater often contains multiple pollutants, and using a single treatment method often fails to achieve the desired results. To obtain ideal treatment effects at a lower cost, it is often necessary to rationally combine several treatment technologies according to a certain hierarchy and sequence, based on factors such as wastewater quality, quantity, treatment level, the possibility of recovering useful substances, and available funds and site conditions, to form a complete purification and treatment system.
The main definitions of wastewater treatment processes include two aspects:
Definition 1:
The process of purifying wastewater by separating pollutants or converting them into harmless substances using various methods. Applied disciplines: Ecology (first-level discipline); Pollution Ecology (second-level discipline).
Definition 2:
Wastewater treatment measures employing physical, chemical, or biological methods. Applied disciplines: Water Resources Science and Technology (first-level discipline); Environmental Water Resources (second-level discipline); Water Pollution Control (Water Resources) (third-level discipline).
The specific criteria for selecting wastewater treatment processes include:
1) The urban wastewater treatment process should be selected based on a comprehensive technical and economic comparison, taking into account the treatment scale, water quality characteristics, environmental function of the receiving water body, and local conditions and requirements.
2) The main technical and economic indicators for process selection include: investment per unit of water treated, investment per unit of pollutant reduction, power consumption and cost per unit of water treated, power consumption and cost per unit of pollutant reduction, land area, operational reliability, ease of management and maintenance, and overall environmental benefits.
3) The influent wastewater quality should be determined realistically, and process design parameters should be optimized. A detailed investigation or measurement of the current wastewater quality characteristics and pollutant composition is necessary, along with reasonable analysis and prediction. When the wastewater composition is complex or unique, dynamic tests of the wastewater treatment process should be conducted, and pilot-scale studies should be carried out if necessary.
4) Adopt efficient and economical new processes actively and prudently. For new processes being used for the first time in China, pilot and production tests must be conducted to provide reliable design parameters before application.
II. Wastewater Treatment Process Level
Modern wastewater treatment technology can be classified into primary, secondary, and tertiary treatment based on the level of treatment.
Primary treatment mainly removes suspended solids from wastewater; most physical treatment methods can only meet the requirements of primary treatment. After primary treatment, BOD is typically reduced by about 30%, which is insufficient to meet discharge standards. Primary treatment is a pretreatment step for secondary treatment.
Secondary treatment mainly removes organic pollutants (BOD and COD) in colloidal and dissolved states from wastewater, with a removal rate of over 90%, enabling organic pollutants to meet discharge standards.
Tertiary treatment further removes recalcitrant organic matter, nitrogen, phosphorus, and other soluble inorganic substances that can lead to eutrophication. The main methods include biological nitrogen and phosphorus removal, coagulation and sedimentation, sand filtration, activated carbon adsorption, ion exchange, and electrodialysis.
The entire process begins with raw wastewater passing through a coarse screen, then being pumped up by a wastewater lift pump. After passing through a screen or sand filter, it enters a grit chamber. The wastewater, after sand-water separation, enters a primary sedimentation tank. This constitutes primary treatment (i.e., physical treatment). The effluent from the primary sedimentation tank enters biological treatment equipment, including activated sludge and biofilm processes (activated sludge reactors include aeration tanks and oxidation ditches; biofilm processes include biofilters, rotating biological contactors, biological contact oxidation, and biological fluidized beds). The effluent from the biological treatment equipment enters a secondary sedimentation tank. The effluent from the secondary sedimentation tank is then disinfected before being discharged or enters tertiary treatment. This completes the primary treatment phase, which is the second stage. Tertiary treatment includes biological nitrogen and phosphorus removal, coagulation 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 the rest enters a sludge thickening tank, then a sludge digestion tank. After dewatering and drying, the sludge is finally utilized.
