A modern wastewater treatment plant operates as a closed-loop water purification assembly line. Urban domestic sewage and industrial wastewater converge through pipe networks, undergo multi-stage purification, and finally turn into qualified clean water for discharge or reclaimed reuse.This article elaborates on the complete process, functions of each unit and core working principles in the actual flow sequence of sewage, easy to understand for all readers. I. Overall Structure of Wastewater Treatment Plant All treatment units operate in series along the water flow direction to form a complete purification system, consisting of 7 major functional zones: 1. Influent Pumping Station (with coarse bar screen) 2. Pretreatment Unit (fine screen, grit chamber) 3. Primary Sedimentation Tank 4. Biological Reaction Tank (with blower house) 5. Secondary Sedimentation Tank 6. Disinfection, Discharge & Reuse Unit 7. Sludge Treatment Unit Sewage goes through the process in sequence: physical interception → primary sedimentation → biological degradation → mud-water separation → disinfection & discharge/reuse. All sludge and residue generated in the whole process are collected uniformly, reduced in volume, and transported out for disposal, realizing dual operation of water treatment + sludge disposal. II. Detailed Explanation of Each Treatment Unit 1. Influent Pumping Station (with Coarse Bar Screen) It is the first gateway for sewage entering the plant, undertaking three core functions: sewage collection, water level lifting and coarse filtration. Municipal sewage mainly flows by gravity. The water level is too low upon arriving at the plant to directly enter subsequent structures. Water pumps are required to lift the water level, enabling sewage to flow by gravity through all subsequent treatment units. Core EquipmentCoarse bar screen, submersible lifting pump, manual/electric sluice gate, collecting tank, deodorization system Working Principle • Coarse Bar Screen: With a gap of 10~20 mm, it acts like a giant comb to intercept large floating matters and suspended solids such as tree branches, plastic bags, stones and cloth strips, preventing blockage of subsequent pipelines, pumps and precision equipment. • Lifting Pump: Stably lifts sewage in the collecting tank to the designed water level of the pretreatment unit, ensuring continuous and stable water flow. • Sluice Gate: Used for equipment maintenance, flow switching and influent flow regulation. • Deodorization System: Collects malodorous gas in the pumping station, which is treated by washing and adsorption to meet emission standards and reduce odor inside the plant. 2. Pretreatment Unit (Fine Screen & Grit Chamber) Sewage after coarse screening still contains a large amount of fine suspended solids, sand and cinder. Further pretreatment is necessary to protect subsequent biochemical and sedimentation equipment. Core EquipmentFine screen, vortex grit chamber / aerated grit chamber, screw conveyor, sand-water separator, deodorization system Working Principle • Fine Screen: With a gap of 3~5 mm, it further intercepts fine floating matters such as hair, fiber and paper scraps; screen residues are collected and transported out by screw conveyors. • Grit Chamber: Adopts gravity and vortex action to rapidly settle high-density sand and stones and separate them from organic matter. Vortex grit chambers are widely used for small floor area and high sand removal efficiency, effectively preventing pump abrasion and tank siltation caused by sand grains. • Sand-Water Separator: Separates sand-water mixture from the grit chamber; sand is transported out while separated water flows back to the sewage system. 3. Primary Sedimentation Tank Pretreated sewage enters the primary sedimentation tank for physical sedimentation, serving as a key load reduction link before biochemical treatment. Core FunctionsRemoves 20%~30% of COD and BOD₅, and more than 50% of suspended solids (SS) in sewage.It significantly reduces the organic load of subsequent biological reaction tanks, cuts aeration energy consumption and extends service life of equipment.Key NoteNot all processes are equipped with a primary sedimentation tank. Integrated processes such as oxidation ditch, SBR and MBR usually omit it, allowing sewage to directly enter the biochemical system.Working PrincipleSewage flows slowly inside the tank; settleable solids sink to the tank bottom by gravity to form primary sludge, which is collected and discharged by a sludge scraper. The relatively clear supernatant flows into the biological reaction tank. 4. Biological Reaction Tank (with Blower House) It is the core heart of wastewater treatment. Microorganisms in activated sludge "decompose and consume" organic matter, ammonia nitrogen, total nitrogen and total phosphorus in sewage to achieve advanced purification. Mainstream ProcessesAAO (Anaerobic-Anoxic-Oxic), oxidation ditch, SBR, CAST, MBR, among which AAO is the most widely applied.Core EquipmentBlower, aeration disc/aeration pipe, submersible mixer, sludge return pump, nitrifying liquid return pump, chemical dosing system Working Principle • Anaerobic Zone: Mixers keep sludge in suspension; phosphorus-accumulating bacteria release phosphorus to prepare for phosphorus absorption in the subsequent oxic zone. • Anoxic Zone: Mixers mix water flow; denitrification is realized by utilizing nitrate in returned nitrifying liquid to remove total nitrogen. • Oxic Zone: Blowers deliver compressed air, which forms micro bubbles through aerators to supply oxygen for microorganisms. Microorganisms propagate massively to decompose organic matter and oxidize ammonia nitrogen; meanwhile, phosphorus-accumulating bacteria absorb excessive phosphorus. • Return System: Sludge return pumps send sludge from the secondary sedimentation tank back to the biochemical tank to maintain microbial concentration; nitrifying liquid return improves nitrogen removal efficiency. • Chemical Dosing System: Dose carbon sources and phosphorus removers as required to ensure stable compliance of total nitrogen and total phosphorus. 5. Secondary Sedimentation Tank Effluent from the biological reaction tank contains a large amount of activated sludge. Complete mud-water separation by gravity sedimentation is required to guarantee clear effluent. Common TypesVertical flow, radial flow and horizontal flow secondary sedimentation tanks. Working Principle • Supernatant: Clear water flows into the disinfection unit for final standard-compliant discharge preparation. • Settled Sludge: Part of the sludge is returned to the biological reaction tank to maintain microbial quantity; the rest is surplus sludge discharged to the sludge treatment unit to avoid excessive sludge accumulation. 6. Disinfection, Discharge & Reuse Unit Although the effluent of the secondary sedimentation tank has reached basic standards, it still contains a small number of bacteria and viruses. Disinfection is mandatory for safe discharge or reuse. Common Disinfection Methods • UV Disinfection: No chemical agent, no residual substance, short contact time (several seconds to more than ten seconds), simple operation and maintenance; widely used in municipal wastewater plants. • Sodium Hypochlorite Disinfection: Convenient dosing and low cost; required contact time ≥ 30 minutes; residual chlorine needs strict control to prevent secondary pollution. • Ozone Disinfection: Extremely strong oxidative sterilization capacity, with simultaneous decolorization and deodorization; high equipment investment and operating cost, mostly applied in high-standard reuse scenarios. Final Disposal Routes • Standard Discharge: Discharged into receiving water bodies such as rivers and lakes. • Reclaimed Reuse: Applied to urban greening, road flushing, industrial cooling water, landscape water replenishment, etc., realizing recycling of water resources. 7. Sludge Treatment Unit Primary sludge from the primary sedimentation tank and surplus sludge from the secondary sedimentation tank feature high moisture content and easy putrefaction. They must undergo volume reduction, stabilization and harmless treatment to avoid secondary pollution. Complete Sludge Treatment FlowThickening → Conditioning → Dewatering → Outward DisposalCore EquipmentSludge thickening tank/thickener, sludge conditioning tank, plate-and-frame filter press / belt dewaterer / screw press dewaterer, sludge silo, deodorization system Working Principle • Thickening: Remove free water in sludge to reduce volume and lower the load of subsequent treatment. • Conditioning: Dose chemicals such as PAM (Polyacrylamide) to improve sludge dewatering performance. • Dewatering: Mechanical extrusion reduces sludge moisture content below 80% to form solid sludge cakes. • Outward Disposal: Sludge cakes are transported to qualified sites for harmless disposal such as landfill, incineration, building material production and land utilization. III. Summary of Wastewater Treatment Process The purification logic of a wastewater treatment plant is essentially:Physical interception & sedimentation → Biological degradation → Secondary sedimentation & mud-water separation → Advanced purification → Sludge disposal 1. Physical Impurity Removal: Coarse/fine screens intercept residues, and grit chambers remove sand to eliminate solid impurities first; 2. Preliminary Load Reduction: Primary sedimentation tank settles suspended sludge to reduce biochemical load; 3. Core Purification: Microorganisms decompose organic matter and realize nitrogen and phosphorus removal to meet water quality standards; 4. Mud-Water Separation: Secondary sedimentation tank separates clean water from activated sludge to ensure clear effluent and recycle activated sludge; 5. Sterilization & Compliance: Disinfect pathogenic microorganisms for clean water discharge or reuse; 6. Sludge Disposal: Surplus sludge is thickened, dewatered, volume-reduced and disposed harmlessly.
