Comparative Analysis of Nitrogen and Phosphorus Removal Efficiency Between Bardenpho and AAO Processes
1 Project Overview and Process Flow
1.1 Project Overview
The Xi'an No.5 Reclaimed Water Plant (formerly "No.5 Wastewater Treatment Plant," hereinafter referred to as "WuWu") has a total design capacity of 400,000 m³/d, covering an area of 387.57 mu (approximately 258,380 m²). It serves a total area of about 5,330 hectares and a population of approximately 900,000. The plant can treat domestic sewage and industrial wastewater using either the conventional AAO process or the five-stage Bardenpho process. The main wastewater treatment structures include coarse screens, lift pump stations, fine screens, aerated grit chambers, primary sedimentation tanks, biological reaction tanks, secondary sedimentation tanks, high-efficiency sedimentation tanks, V-type filters, and contact disinfection tanks, with the final effluent discharged into the Ba River. The effluent quality complies with the Grade A standard specified in Table 1 of the "Shaanxi Province Yellow River Basin Comprehensive Wastewater Discharge Standard" (DB61/224-2018). (Note: The TN limit follows the requirement of 12 mg/L stipulated in the "Xi'an Municipal Urban Wastewater Treatment Plant Reclamation Upgrade, Covering, and Deodorization Project Three-Year Action Plan (2018-2020)" (Municipal Office Document [2018] No. 100)). The design influent and effluent water quality are shown in Table 1.
1.2 Process Flow
Flowcharts comparing the Bardenpho process with the traditional AAO process are shown in Figures 1 and 2.
2 Design Parameters
2.1 Design Influent and Effluent Water Quality
2.2 Operating Parameters
The biological tanks participating in the comparison share identical dimensions. Each biological tank is divided into 3 channels, with a single channel dimension of L × W × H = 86 m × 15 m × 9 m. The average MLSS concentration in the biological tanks ranges between 6,500~7,000 mg/L. The hydraulic retention times (HRT) for the conventional AAO process are: Anaerobic Zone 1.983 h, Anoxic Zone 5.534 h, Aerobic Zone 9.029 h, totaling 16.546 h. The HRTs for the Bardenpho process are: Anaerobic Zone 1.983 h, First Anoxic Zone 4.643 h, First Aerobic Zone 7.163 h, Second Anoxic Zone 1.973 h, Second Aerobic Zone 0.822 h, totaling 16.584 h.
3 Project Background, Research Objective, and Methodology
3.1 Project Background and Research Objective
The main biological treatment processes at WuWu are the conventional AAO process and the Bardenpho process. The conventional AAO process is a common biological treatment method in wastewater treatment plants. With the continuous improvement of China's wastewater discharge standards, the Bardenpho process, derived from the conventional AAO process and known for its higher nitrogen removal efficiency, has been widely adopted by domestic wastewater treatment plants. To facilitate better process selection, WuWu conducted a comprehensive comparison of the conventional AAO and Bardenpho processes from the perspective of nitrogen and phosphorus removal. This provides a basis for the upgrade of other municipal domestic wastewater treatment plants and the design of new projects.
3.2 Research Methodology
Each biological tank at WuWu has a daily treatment capacity of 50,000 m³/d. For this comparative experiment, the A1 and B1 series biological tanks were selected. The A1 series employs the Bardenpho process, with its biological system sequentially divided into: Anaerobic Zone, First Anoxic Zone, First Aerobic Zone, Second Anoxic Zone, and Second Aerobic Zone. The B1 series employs the conventional AAO process, with its biological system sequentially divided into: Anaerobic Zone, Anoxic Zone, and Aerobic Zone. During the experiment, both series operated under identical conditions, with sampling points distributed along the process flow as required.
Pollutant measurement methods: TP was measured using the Ammonium Molybdate Spectrophotometric Method; TN using the Alkaline Potassium Persulfate Digestion UV Spectrophotometric Method; NH₃-N using the Nessler's Reagent Spectrophotometric Method; COD using the Potassium Dichromate Spectrophotometric Method.
4 Operational Challenges and Current Status
The conventional AAO process is also a variant of the AO activated sludge process. Its TN removal entirely depends on recirculation. Higher effluent standards and greater required removal rates necessitate larger recirculation flows, accompanied by increased energy and chemical consumption. For Grade A standards, the conventional AAO process is still acceptable. However, for stricter TN standards, conventional processes are clearly no longer suitable.
The Bardenpho process is a typical five-stage process. By adding a post-denitrification zone after the conventional AAO process, it breaks the limitation of TN removal being dependent on the recirculation ratio, thereby enhancing nitrogen removal. As wastewater treatment plants face increasingly stringent TN discharge standards, the Bardenpho process demonstrates significant advantages.
5 Research Results and Discussion
5.1 NH₃-N Removal
NH₃-N levels at the influent of the anaerobic zones and the effluent of the biological tanks for A1 and B1 were monitored repeatedly over 15 days. Results are shown in Figure 3. The average NH₃-N removal for the Bardenpho process was 12.7 mg/L, while for the conventional AAO process it was 11.68 mg/L. The results indicate that under the same seasonal conditions, time period, uniform influent distribution, and with carbon source addition in the pre-anoxic zone, the Bardenpho process achieved better NH₃-N removal than the conventional AAO process.
5.2 TN Removal
TN levels at the influent of the anaerobic zones and the effluent of the biological tanks for A1 and B1 were monitored repeatedly over 10 days. Results are shown in Figure 4. The average TN removal for the Bardenpho process was 6.23 mg/L, while for the conventional AAO process it was 2.65 mg/L. The results indicate that under the same conditions, the Bardenpho process achieved better overall TN removal than the conventional AAO process.
5.3 TP Removal
TP levels at the influent of the anaerobic zones and the effluent of the biological tanks for A1 and B1 were monitored repeatedly over 22 days. Results are shown in Figure 5. The average TP removal for the Bardenpho process was 0.561 mg/L, while for the conventional AAO process it was 0.449 mg/L. The results indicate that under the same conditions, the Bardenpho process achieved better overall TP removal than the conventional AAO process.
5.4 COD Removal
COD levels at the influent of the anaerobic zones and the effluent of the biological tanks for A1 and B1 were monitored repeatedly over 9 days. Results are shown in Figure 6. The average COD consumption for the Bardenpho process was 13 mg/L, while for the conventional AAO process it was 19 mg/L. The results indicate that under the same conditions, the conventional AAO process had a higher COD demand than the Bardenpho process.
6 Conclusion and Outlook
6.1 Conclusion
Under the same seasonal operating conditions, the Bardenpho process demonstrated an overall trend of superior removal efficiency for TN, TP, and NH₃-N in wastewater compared to the conventional AAO process.
Currently, the annual phosphorus removal agent usage for treating wastewater with the conventional AAO process at WuWu is approximately 2,961 tons; for the Bardenpho process, it is approximately 2,000 tons. This translates to an annual cost saving of about 450,000 RMB, demonstrating significant economic benefits.
The operation of the Bardenpho process will greatly meet the requirements of China's continuously tightening wastewater discharge standards and reduce pollution in the downstream Ba River water system. This will lead to significant improvements in water quality, both perceptually and in terms of reduced pollution levels, gradually restoring environmental functions. It holds particular importance for protecting the ecological environment of downstream water bodies. Fundamentally, wastewater treatment controls the pollution of urban wastewater to groundwater sources. Therefore, it plays a protective role for urban water supply sources and downstream water sources, gradually restoring polluted ecological environments. This will significantly improve the living environment for urban residents and the production environment for Industry and Commerce, enhance the city's external image, and contribute to the healthy and sustainable development of the economy and society.