Inverted A2O Process in Underground Wastewater Treatment: Case Study & Performance Data

Application of an Inverted A²/O Process in an Underground WWTP

Overview

With the acceleration of urbanization, the discharge of domestic wastewater has increased significantly, imposing higher requirements on the effluent quality of wastewater Treatment Plants. To alleviate land resource constraints and urban environmental pressures,underground wastewater treatment plants have gradually emerged as an important construction model.

The traditional Anaerobic-Anoxic-Oxic (A²/O) process is well-established in removing COD and NH₃-N but has certain limitations in the advanced removal of Total Nitrogen (TN) and Total Phosphorus (TP), making it difficult to consistently meet the Class 1A discharge standards. This study takes an underground wastewater treatment plant in Jinan City as an example, where the inverted A²/O process was adopted. By analyzing actual operational data, the effectiveness of this process in removing Chemical Oxygen Demand (COD), NH₃-N, TN, and TP was evaluated, providing technical insights for optimizing underground wastewater treatment processes.

Project Overview

Located in Jinan City, this project adopts an underground construction model, covering an area of 18,500 m². It is designed to treat urban domestic wastewater with a capacity of 50,000 m³/d. The treatment process includes "aerated grit chambers + inverted A²/O process + high-efficiency sedimentation tanks / sand filters." The inverted A²/O process effectively removes TN. The effluent quality strictly complies with the Class 1A standards of the "Discharge Standard of Pollutants for Municipal Wastewater Treatment Plants" (GB 18918—2002), with further reference to the Class IV water body standards of the "Environmental Quality Standards for Surface Water" (GB 3838—2002) to minimize the impact on receiving water bodies. After actual operation, the plant's effluent quality has been excellent, with all pollutant indicators meeting or exceeding design requirements, fully demonstrating the advanced nature and applicability of the selected process.

Application of the Inverted A²/O Process

1. Process Selection and Water Quality Analysis

Based on water quality testing results, the domestic wastewater in this region exhibits good biodegradability. The designed influent parameters are as follows: COD₆ᵣ 300 mg/L, BOD₅ 130 mg/L, SS 200 mg/L, TN 40 mg/L, NH₃-N 30 mg/L, TP 4 mg/L, and pH 6–9. The calculated BOD/COD₆ᵣ > 0.3, BOD/TN > 3, and BOD/TP > 20 indicate favorable biodegradability, suitable for biological treatment processes. Given the requirement for effluent TN to be ≤15 mg/L (removal rate of 62.5%), the traditional A²/O process may not consistently meet this standard. Thus, the inverted A²/O process, which prioritizes TN removal, was selected. By altering the sequence of anaerobic and anoxic zones, the process enhances denitrification and improves TN removal efficiency. To ensure TP meets the Class 1A standard of 0.3 mg/L, advanced treatment units, including high-efficiency sedimentation tanks and sand filters, were incorporated to further remove TP and SS, ensuring stable compliance with discharge standards.

2. Aerated Grit Chamber Design

The aerated grit chamber removes inorganic particles larger than 0.2 mm with a density greater than 2.65 g/cm³ to prevent wear and clogging of downstream equipment. With a design flow rate Q of 50,000 m³/d, two parallel aerated grit chambers were implemented. The total retention time was set at 4.29 minutes, with a horizontal flow velocity of 0.1 m/s per chamber to ensure effective grit removal. The aeration rate was designed at 0.2 m³ of air per cubic meter of wastewater, resulting in a total daily aeration volume of 10,000 m³/d and an average air flow rate of 416.7 m³/h. Three Roots blowers (two operational, one standby) were installed, each with a capacity of 208.3 m³/h. Each chamber is equipped with a 0.55 kW chain-type grit scraper for periodic grit removal. The grit discharge system includes two 5 kW grit pumps (one operational, one standby) and a grit-water separator (0.37 kW) to achieve a grit moisture content below 60%.

3. Biochemical Tank Design

The biochemical tank employs the inverted A²/O process, comprising anoxic, anaerobic, and oxic zones. Its advantage lies in prioritizing denitrification to enhance TN removal efficiency. The anoxic zone has a hydraulic retention time (HRT) of 3.7 hours, with an internal reflux ratio of 200% and a sludge return ratio of 100%. Based on the maximum denitrification rate, 12 submersible propellers (eight with 2.5 kW power, four with 3.4 kW power) were installed to ensure uniform flow. The anaerobic zone has an HRT of 1.5 hours. To promote phosphorus release by polyphosphate-accumulating organisms, four 3.4 kW submersible propellers were installed, with one additional standby unit. The oxic zone has an HRT of 7.92 hours. To ensure dissolved oxygen supply, three 129 kW air suspension blowers (two operational, one standby) were installed, along with 5,580 corundum disc fine Bubble Diffusers, with an air-to-water ratio of 5.7:1.0. Mixed liquor reflux is handled by five 11 kW horizontal propeller pumps (four operational, one standby), and sludge回流 is managed by four 3 kW submersible pumps (two operational, two standby). This design ensures sufficient organic matter degradation capacity in the oxic zone.

4. Post-Treatment Unit Design

The post-treatment units include secondary sedimentation tanks, high-efficiency sedimentation tanks, and sand filters. The secondary sedimentation tanks use a co-current flow design, divided into five rectangular units. The HRT is 3 hours, with surface loads of 1.09 m³/(m²·h) at average flow and 1.46 m³/(m²·h) at peak flow, meeting standard requirements. Each unit is equipped with a 0.55 kW non-metal chain-type sludge scraper for regular sludge removal. The high-efficiency sedimentation tanks integrate mixing, flocculation, and sedimentation functions. The mixing zone has an HRT of 1.09 minutes, the flocculation zone 10 minutes, and the sedimentation zone a surface load of 18 m³/(m²·h). Three 4 kW mixers and three 5.5 kW flocculation mixers enhance chemical mixing. The sedimentation zone features honeycomb inclined tubes (1.2 m length, 60° inclination) to improve efficiency. Sludge reflux is set at 3%, achieved by four 11 kW sludge return pumps. The sand filters consist of 10 units, each with a filtration area of 39.25 m², a filtration rate of 7.11 m/h, a filter media thickness of 1.5 m, and an effective particle size of 1.2 mm. Backwashing employs combined air and water, with a cycle of 24 hours, using three 45 kW water pumps and three 31.5 kW Roots blowers for thorough cleaning.

Effectiveness of the Inverted A²/O Process

1. COD Removal Efficiency

The inverted A²/O process achieved exceptionally high COD₆ᵣ removal efficiency. Actual operational data showed an influent COD₆ᵣ concentration of 80–450 mg/L (annual average 180 mg/L), while the effluent COD₆ᵣ concentration was 4–13 mg/L (annual average 7.1 mg/L), with a removal rate of 96%. In 2022, effluent COD₆ᵣ consistently remained below the Class IV standard limit of 15 mg/L (GB 3838—2002), with minimal fluctuations. This high efficiency is attributed to the synergistic effects of the optimized anaerobic, anoxic, and oxic zones in the inverted A²/O process.

2. NH₃-N Removal Efficiency

NH₃-N removal is a core indicator of wastewater treatment performance. The inverted A²/O process demonstrated outstanding NH₃-N removal, with an influent concentration of 4–38 mg/L (annual average 7 mg/L) and an effluent concentration of 0.01–0.06 mg/L (annual average 0.07 mg/L), achieving a removal rate of 99%. In 2022, effluent NH₃-N levels were well below the Class IV standard limit of 1.5 mg/L (GB 3838—2002). This excellent performance is due to the efficient collaboration between nitrification in the anoxic zone and denitrification in the oxic zone.

3. TN Removal Efficiency

The inverted A²/O process showed significant technical advantages in TN removal. The influent TN concentration was 16–110 mg/L (annual average 32 mg/L), while the effluent TN concentration was 4.2–14 mg/L (annual average 6.08 mg/L), with a removal rate of 81%. In 2022, effluent TN levels remained stable below the Class 1A standard of 15 mg/L. Compared to the traditional A²/O process, which typically achieves 50%–65% TN removal, the inverted process's improvement stems from its innovative design, which optimizes carbon source utilization during denitrification.

4. TP Removal Efficiency

The process also demonstrated excellent performance in TP removal. The influent TP concentration was 1–18 mg/L (annual average 3 mg/L), while the effluent TP concentration was 0.01–0.1 mg/L (annual average 0.06 mg/L), with a removal rate of 98%. In 2022, effluent TP fully complied with Class 1A standards. This high efficiency is due to the enhanced phosphorus release in the anaerobic zone and uptake in the oxic zone, further reinforced by the high-efficiency sedimentation tanks. Compared to traditional processes, the inverted A²/O process offers significant advantages in TP removal efficiency and effluent stability.

Conclusion

This study systematically validates the excellent performance of the inverted A²/O process in wastewater treatment. The results indicate that the process offers significant advantages in optimizing denitrification, enhancing anaerobic phosphorus release, and improving aerobic phosphorus uptake, achieving high removal efficiencies for COD, NH₃-N, TN, and TP. The effluent quality consistently meets Class 1A discharge standards, with some indicators surpassing Class IV surface water standards, fully demonstrating the value of the inverted A²/O process in enhancing the stability and compliance of wastewater treatment plants.