Proper Treatment of Urban Wastewater: A Comprehensive Guide to the "Three-Stage Purification" Process

Proper Treatment of Urban Wastewater: A Comprehensive Guide to the "Three-Stage Purification" Process

Proper treatment of urban wastewater serves as a critical line of defense for safeguarding water environment quality and public health security. To achieve compliant discharge or even reclamation and reuse of turbid wastewater, a scientific closed-loop process of "Primary Pretreatment → Secondary Biological Treatment → Tertiary Advanced Treatment" is indispensable. This article breaks down the core technologies, practical control points, and mainstream combination modes of each treatment stage, helping readers understand the complete logic of wastewater purification.

1. Primary Treatment: "Preliminary Screening" of Wastewater (Core of Physical Treatment)

The core objective of primary treatment is to remove large suspended solids (SS), gravel, and floating debris from wastewater, reducing the load on subsequent processes and protecting equipment from abrasion—analogous to "rough processing" for wastewater. After this stage, the SS removal rate reaches 40%-50%, Chemical Oxygen Demand (COD) removal rate is approximately 15%-20%, and Biochemical Oxygen Demand (BOD) removal rate is only about 30%, requiring further advanced treatment.

Typical Technologies

• Gratings: The "first sieve" of wastewater treatment, classified into coarse gratings (5-20mm), fine gratings (1-5mm), and ultra-fine gratings (<1mm, paired with MBR processes). Primarily intercept large floating particles such as plastic bags, branches, and fibers.
• Grit Chambers: Utilize gravity sedimentation to remove inorganic particles like sand and stones. Common types include vortex grit chambers and aerated grit chambers, preventing hard particles from wearing pumps, aerators, and other equipment.
• Primary Sedimentation Tanks: Core sedimentation units, available in horizontal flow and radial flow types. Allow high-specific-gravity suspended solids in wastewater to settle into primary sludge, further purifying water quality.

Key Control Points

• Control the flow velocity through gratings at 0.6-1.0m/s, promptly remove screenings to prevent clogging, and transport dewatered screenings for disposal.
• Maintain a hydraulic retention time (HRT) of 20-30s in grit chambers; for aerated grit chambers, keep the air-to-water ratio at 0.2-0.3m³/m³, periodically discharge grit, and implement sand-water separation.
• Maintain a surface loading rate of 1.0-1.5m³/(m²·h) and HRT of 1.5-2.0h in primary sedimentation tanks; promptly discharge sludge to avoid anaerobic fermentation and skim surface scum.
• For domestic wastewater treatment plants, the primary sedimentation tank may be omitted based on influent water quality, treatment scale, and carbon source requirements to retain more organic matter for subsequent biological treatment.

1. Secondary Treatment: "Core Battlefield" for Pollutant Degradation (Dominance of Biological Treatment)

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

Typical Technologies

• AAO Process: The most widely used technology (accounting for over 60% of new projects in 2023), featuring a flow of "Anaerobic Zone → Anoxic Zone → Aerobic Zone → Secondary Sedimentation Tank". Synchronous nitrogen and phosphorus removal is achieved through internal mixed liquor recirculation (100%-200%) and sludge recirculation (50%-100%).
• Oxidation Ditch Process: Designed with a circular aeration tank, maintaining a mixed liquor circulation velocity of 0.3-0.5m/s to form an alternating "aerobic-anoxic" environment. Sludge production is 15%-20% lower than the AAO process, suitable for existing wastewater treatment plants.
• SBR Process: Adopts periodic operations of "Influent → Aeration → Sedimentation → Effluent → Idle", with multiple tanks operating alternately. Requires high automation, with a tank volume utilization rate of 70%-80%, and is rarely used in new projects.

Key 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 (TN) removal rate 55%-80%), and promote phosphorus release by polyphosphate-accumulating organisms (PAOs) in the anaerobic zone (total phosphorus (TP) removal rate 60%-80%).
• For the oxidation ditch process, maintain a sludge age (SRT) >20d and HRT of 10-40h; optimize denitrification efficiency (TN removal rate 55%-85%) by adjusting aerator distribution.
• For the SBR process, control a single cycle at 5-8h; maintain sufficient dissolved oxygen (DO) during aeration, avoid sludge disturbance during sedimentation, and rely on automated systems to ensure continuous operation.
• Control operating costs at 0.8-1.4 yuan per ton of water; adjust parameters such as aeration rate and recirculation ratio according to fluctuations in influent water quality to prevent impacts on microbial activity.

1. Tertiary Treatment: "Final Polishing" for Water Quality Upgrade (Conclusion of Advanced Treatment)

Tertiary treatment, also known as advanced treatment, targets residual TP, TN, SS, and refractory organic matter in secondary effluent. It further purifies water through physical and chemical methods, ensuring effluent meets higher standards such as Class IA Discharge Standard or Surface Water Class IV Standard, enabling reclamation for landscape use, industrial reuse, etc.

Typical Technologies

• Coagulation-Sedimentation: Dosing agents such as Polyaluminum Chloride (PAC, 50-100mg/L) and Polyacrylamide (PAM, 1-3mg/L) to form flocs that remove residual suspended solids and phosphorus. Common types include vertical flow and inclined-tube sedimentation tanks.
• Filtration Processes: Commonly use quartz sand filters, activated carbon filters, or ultrafiltration (UF) membranes to intercept fine suspended solids and colloids, reducing SS to below 10mg/L, improving water transparency, and creating conditions for subsequent disinfection.
• Disinfection Processes: Adopt disinfectants such as ozone, chlorine, and chlorine dioxide to kill bacteria and viruses, ensuring effluent hygiene and safety. Chlorine disinfection has low costs but may produce harmful byproducts; ozone disinfection offers excellent efficacy without secondary pollution but requires high investment and operating costs; chlorine dioxide disinfection boasts high efficiency and fewer byproducts. For projects with strict nitrogen and phosphorus removal requirements, additional nitrogen and phosphorus removal units can be added to enhance advanced treatment.

Key Control Points

• Precisely control chemical dosage during coagulation-sedimentation, maintain an HRT of 2.0-3.0h, and avoid chemical waste or residues.
• Regularly backwash filtration units to prevent filter media clogging and maintain stable filtration efficiency.
• Control disinfectant dosage and contact time during disinfection to balance bactericidal efficacy and secondary pollution risks.
• Monitor water quality after advanced treatment to ensure compliance with reuse or discharge standards and prevent eutrophication.

1. Common Process Combination Schemes (Customized to Local Conditions)

The combination of wastewater treatment processes should be customized based on influent water quality, discharge standards, land availability, and other factors. Currently, over 85% of urban wastewater treatment plants in China adopt the classic process of "Pretreatment + Biological Treatment + Advanced Treatment". Below are three mainstream combinations:

1. Standard Version for Urban Domestic Wastewater

• Combination Form: Coarse Grating → Fine Grating → Vortex Grit Chamber → AAO Process → Secondary Sedimentation Tank → Coagulation-Sedimentation → Filtration → Disinfection
• Applicable Scenarios: Newly built wastewater treatment plants requiring compliance with Class IA Discharge Standard and simultaneous nitrogen and phosphorus removal (TN removal rate 55%-80%, TP removal rate 60%-80%).
• Core Advantages: Stable treatment efficiency, controllable operating costs (0.8-1.2 yuan per ton of water), moderate land occupation, suitable for most urban needs.

2. Upgrade Version for Existing Plant Renovation

• Combination Form: Existing Gratings + Grit Chambers → Oxidation Ditch Process (aeration system renovation) → New Coagulation-Sedimentation → Disinfection
• Applicable Scenarios: Upgrading of existing wastewater treatment plants with limited land and requirements for reduced sludge production; TN removal rate up to 55%-85%.
• Core Advantages: Utilizes existing facilities for renovation, low investment costs, simple operation and maintenance, suitable for upgrading existing projects.

3. Enhanced Version for Reclaimed Water Reuse

• Combination Form: Gratings → Grit Chambers → AAO Process → Secondary Sedimentation Tank → Coagulation-Sedimentation → Ultrafiltration → Reverse Osmosis (RO) → Disinfection
• Applicable Scenarios: Reclamation goals such as industrial reuse and landscape water; effluent meeting Surface Water Class IV Standard or higher.
• Core Advantages: Deep removal of refractory pollutants, high water purity, realization of water resource recycling, and reduction of fresh water consumption.

Wastewater purification is a systematic project. From the "physical screening" of primary treatment to the "biological degradation" of secondary treatment and the "advanced polishing" of tertiary treatment, each link requires precise control of process parameters. Reasonable selection of process combinations not only ensures compliance with water quality standards but also achieves cost reduction and efficiency improvement, providing solid support for the sustainable development of the water environment.

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