The "Three-Stage Evolution" Guide to Turning Sewage into Clear Water: A Complete Analysis of Process Details and Combination Schemes

Proper treatment of urban sewage is a critical line of defense for safeguarding water environment quality and public health security. For turbid sewage to meet discharge standards or even be recycled for reuse, it relies on the scientific closed-loop process of primary pretreatment → secondary biochemical treatment → tertiary advanced treatment. This article breaks down the core processes, practical control points, and mainstream combination modes of each treatment stage, guiding you through the complete logic of sewage purification.

1. Primary Treatment: The "Preliminary Screening" of Sewage Purification (Core of Physical Treatment)

The core goal of primary treatment is to remove large-diameter suspended solids, grit, and floating matter from sewage, reduce the load on subsequent processes, and protect equipment from abrasion — essentially performing "rough processing" on sewage. After this stage, the removal rate of SS (Suspended Solids) can reach 40%–50%, COD (Chemical Oxygen Demand) removal rate is about 15%–20%, and BOD (Biochemical Oxygen Demand) removal rate is only around 30%, requiring further advanced treatment.

Typical Processes

1. Grit Chambers: Utilizing the principle of gravitational sedimentation to remove inorganic particles such as sand and gravel. Common types include vortex grit chambers and aerated grit chambers, preventing hard particles from wearing out pumps, aerators, and other equipment.
2. Primary Sedimentation Tanks: The core sedimentation unit, divided into horizontal flow and radial flow types. They allow heavy suspended particles in sewage to settle and form primary sludge, further purifying water quality.

Control Points

• Control the flow velocity through the grilles at 0.6–1.0 m/s, remove screenings promptly to prevent clogging, and transport the dewatered screenings off-site for disposal.
• Maintain the hydraulic retention time (HRT) of grit chambers at 20–30 seconds; for aerated grit chambers, keep the air-to-water ratio at 0.2–0.3 m³/m³, discharge sand regularly, and carry out sand-water separation.
• Ensure the surface loading rate of primary sedimentation tanks is 1.0–1.5 m³/(m²·h) with an HRT of 1.5–2.0 hours. Sludge must be discharged timely to avoid anaerobic fermentation, and surface scum should be skimmed off simultaneously.
• For wastewater treatment plants handling purely domestic sewage, the primary sedimentation tank can be omitted based on factors such as influent water quality characteristics, treatment scale, and carbon source requirements, to retain more organic matter for subsequent biological treatment.

1. Secondary Treatment: The "Core Battlefield" for Pollutant Degradation (Led by Biochemical Treatment)

Secondary treatment is the key link for removing organic matter, nitrogen, and phosphorus. It degrades colloidal and dissolved pollutants through microbial metabolism, significantly improving treatment efficiency and serving as the "core process" of sewage purification. Mainstream technologies are all based on the activated sludge process, achieving a COD removal rate of 85%–95% after treatment, which meets basic discharge standards.

Typical Processes

1. AAO Process: Currently the most widely applied technology (accounting for over 60% of new projects in 2023). The flow is anaerobic zone → anoxic zone → aerobic zone → secondary sedimentation tank. Simultaneous nitrogen and phosphorus removal is achieved through mixed liquor internal recirculation (100%–200%) and sludge recirculation (50%–100%).
2. Oxidation Ditch Process: Features a circular aeration tank design with a mixed liquor circulation velocity of 0.3–0.5 m/s, creating an alternating "aerobic-anoxic" environment. It produces 15%–20% less sludge than the AAO process, making it suitable for existing wastewater treatment plants.
3. SBR Process: Adopts cyclic operation of influent → aeration → sedimentation → effluent → idle. Multiple tanks operate alternately with high automation requirements, and the tank volume utilization rate is approximately 70%–80%. It is less commonly used in new construction projects.

Control Points

• For the AAO process, precisely control the retention time of each zone: ensure nitrification in the aerobic zone (ammonia nitrogen removal rate ≥ 90%), enhance denitrification in the anoxic zone (total nitrogen removal rate 55%–80%), and promote phosphorus release by phosphorus-accumulating organisms (PAOs) in the anaerobic zone (total phosphorus removal rate 60%–80%).
• For the oxidation ditch process, maintain the sludge age at > 20 days and HRT at 10–40 hours. Optimize denitrification effects (total nitrogen removal rate 55%–85%) by adjusting aerator distribution.
• For the SBR process, control a single cycle at 5–8 hours. Maintain sufficient dissolved oxygen (DO) during aeration, avoid sludge disturbance during sedimentation, and rely on automated systems to ensure continuous operation.
• Control the operation and maintenance cost at 0.8–1.4 CNY per ton of water. Adjust parameters such as aeration rate and recirculation ratio according to influent water quality fluctuations to prevent adverse effects on microbial activity.

1. Tertiary Treatment: The "Final Polishing" for Water Quality Upgrading (Advanced Treatment Finishing Stage)

Tertiary treatment, also known as advanced treatment, targets residual TP (Total Phosphorus), TN (Total Nitrogen), SS, and refractory organic matter in secondary effluent. It further purifies water through physical and chemical methods, ensuring effluent meets higher standards such as Grade 1-A Discharge Standard or Surface Water Class IV Standard.

Typical Processes

1. Coagulation-Sedimentation: Add coagulants such as PAC (10–100 mg/L) and PAM (1–3 mg/L) to form flocs that remove residual suspended solids and phosphorus. Common equipment includes vertical flow sedimentation tanks and inclined tube sedimentation tanks.
2. Filtration Process: Quartz sand filters, activated carbon filters, or ultrafiltration membranes are commonly used to trap fine suspended solids and colloids, reducing SS to below 10 mg/L, improving water transparency, and creating favorable conditions for subsequent disinfection.
3. Disinfection Process: Disinfectants such as ozone, chlorine, and chlorine dioxide are used to kill bacteria and viruses, ensuring the hygienic safety of effluent. Chlorine disinfection has low costs but may produce harmful by-products; ozone disinfection has excellent efficacy with no secondary pollution but requires high investment and operation costs; chlorine dioxide disinfection offers high efficiency and few by-products. For projects with strict nitrogen and phosphorus removal requirements, additional nitrogen and phosphorus removal units can be added to enhance advanced treatment.

Control Points

• In the coagulation-sedimentation stage, precisely control the dosage of chemicals and maintain an HRT of 2.0–3.0 hours to avoid chemical waste or residue.
• Regularly backwash the filtration unit to prevent filter media clogging and maintain stable filtration performance.
• In the disinfection stage, control the dosage of disinfectants and contact time to balance disinfection efficacy and the risk of secondary pollution.
• Monitor water quality after advanced treatment to ensure it meets reuse or discharge standards and prevent water eutrophication.

1. Common Process Combination Schemes (Tailored to Specific Needs)

Sewage treatment process combinations must be customized based on factors such as influent water quality, discharge standards, and land availability. Currently, over 85% of urban wastewater treatment plants in China adopt the classic process of pretreatment + biochemical treatment + advanced treatment. Three mainstream combinations are listed below:

1. Standard Version for Urban Domestic Sewage
• Combination: Coarse Screens → Fine Screens → Vortex Grit Chambers → AAO Process → Secondary Sedimentation Tanks → Coagulation-Sedimentation → Filtration → Disinfection
• Application Scenarios: Newly built wastewater treatment plants requiring compliance with Grade 1-A Discharge Standard, with simultaneous nitrogen and phosphorus removal (total nitrogen removal rate 55%–80%, total phosphorus removal rate 60%–80%).
• Core Advantages: Stable treatment efficiency, controllable operation costs (0.8–1.2 CNY per ton of water), moderate land occupation, suitable for most urban needs.
2. Upgrade and Renovation Version for Existing Plants
• Combination: Existing Screens + Grit Chambers → Oxidation Ditch Process (with upgraded aeration system) → Newly Added Coagulation-Sedimentation → Disinfection
• Application Scenarios: Upgrading of existing wastewater treatment plants with limited land space and sludge reduction requirements, achieving a total nitrogen removal rate of 55%–85%.
• Core Advantages: Utilizes existing facilities for renovation, low investment costs, simple operation and maintenance, suitable for upgrading needs of existing projects.
3. Enhanced Version for Reclaimed Water Reuse
• Combination: Screens → Grit Chambers → AAO Process → Secondary Sedimentation Tanks → Coagulation-Sedimentation → Ultrafiltration → Reverse Osmosis → Disinfection
• Application Scenarios: Projects targeting reclaimed water reuse for industrial purposes or landscape irrigation, requiring effluent to meet or exceed Surface Water Class IV Standard.
• Core Advantages: Deep removal of refractory pollutants, high water purity, realization of water resource recycling, and reduction of freshwater consumption.

Sewage purification is a systematic project. From the "physical screening" of primary treatment, to the "biochemical degradation" of secondary treatment, and the "advanced polishing" of tertiary treatment, every step requires precise control of process parameters. Reasonable selection of process combinations can not only ensure water quality compliance but also achieve cost reduction and efficiency improvement, providing solid support for the sustainable development of water environments.

If you have filter cartridge requirements, please contact us. We provide 7×24 uninterrupted technical support to further optimize your selection scheme. For inquiries, please call: +86 15936809132. We can customize filter cartridges tailored to your specific working conditions!