Analysis of the Impact of Staged Oxygen Supply in the Aerobic Zone of the AAO Process on Pollutant Removal Efficiency
Overview
The AAO process is a widely used wastewater treatment technology, primarily comprising anaerobic, anoxic, and aerobic stages, which work synergistically to effectively remove pollutants from wastewater. The aerobic stage is a critical component of the AAO process, and the method of oxygen supply directly impacts the overall operational efficiency of the entire system. To further enhance the effectiveness of the AAO process in practical applications, researchers have proposed a staged oxygen supply scheme. By establishing multiple zones with different dissolved oxygen (DO) concentrations within the system, this scheme aims to optimize the metabolic activity of aerobic microorganisms and improve pollutant removal efficiency. Therefore, analyzing the impact of staged oxygen supply in the aerobic zone of the AAO process on pollutant removal has significant practical value.
Overview of Staged Oxygen Supply in the Aerobic Zone of the AAO Process
The aerobic zone is the primary site for the oxidation and decomposition of organic matter. Through staged oxygen supply, DO concentrations in different zones can be flexibly adjusted based on the degradation rate of organic matter and the oxygen demand of microorganisms, ensuring uniform and sufficient degradation of organic matter across zones. This approach helps improve organic matter removal rates and stabilize effluent quality. In the aerobic zone, ammonia nitrogen is oxidized to nitrate by nitrifying bacteria. Staged oxygen supply ensures nitrifying bacteria operate efficiently under suitable DO concentrations, avoiding adverse effects on the nitrification process caused by excessively high or low DO levels. Simultaneously, by controlling the recirculation ratio and mixed liquor concentration, the nitrification process can be further optimized, enhancing ammonia nitrogen removal efficiency. The AAO process performs simultaneous nitrogen and phosphorus removal. Under staged oxygen supply conditions in the aerobic zone, phosphorus-accumulating organisms (PAOs) can fully absorb phosphorus under appropriate DO concentrations and achieve phosphorus removal by discharging phosphorus-rich sludge in subsequent stages. Meanwhile, by adjusting operational parameters in the anoxic and aerobic zones, the denitrification process can be optimized, improving total nitrogen removal efficiency.
Experimental Methodology for Analyzing the Impact of Staged Oxygen Supply on Pollutant Removal Efficiency
During the experiment, methods such as aeration valve control systems, automatic control systems, and the number of blower devices were used to regulate aeration intensity, thereby reflecting DO concentration. The process flow of the experimental setup is shown in Figure 1.
As shown in Figure 1, the aerobic zone of the AAO system is divided into three regions: the head, middle, and tail sections. The hydraulic retention time (HRT) of the system was set to 2 hours. The reactor dimensions were 160 cm × 125 cm × 100 cm (length × width × height), with a mixed liquor height set at 60 cm. Flow direction between reaction tanks was controlled using guide walls and baffles.
Effluent samples were collected from the primary sedimentation tank of a municipal wastewater treatment plant. The wastewater quality was relatively stable, with all relevant indicators within standard ranges: TP concentration ranged from 3.0 to 5.5 mg/L, TN concentration from 26 to 49 mg/L, and COD from 255 to 485 mg/L.
Each aerobic section was equipped with a vortex air pump and an independently configured perforated pipe system to form the aeration system for aeration operations. During system operation, each vortex air pump operated independently and stably, maintaining DO concentrations within the ranges of 4–5 mg/L, 3–4 mg/L, and 2–3 mg/L, respectively. The DO concentrations and effluent quality from different sections were measured and analyzed to determine the specific impact on pollutant removal efficiency.
3 Analysis of the Impact of Head Section DO Concentration on Pollutant Removal Efficiency
3.1 COD Removal Efficiency Analysis
Analysis of COD removal in the head section of the AAO aerobic zone under three different DO concentration conditions showed effluent COD values of 41.2, 40.2, and 40.8 mg/L, with removal efficiencies of 91.3%, 90.5%, and 90.8%, respectively. Specific details are shown in Figure 2.
Data analysis indicates that while COD removal efficiency in the head section varied to some extent under different DO concentrations, the overall variation was minimal and did not show a clear correlation. When DO concentration increased from the 2–3 mg/L level to the 3–4 mg/L level, effluent COD and removal efficiency decreased by 1.0 mg/L and 0.8%, respectively. However, when DO concentration increased to the 4–5 mg/L level, effluent COD and removal efficiency increased by 0.6 mg/L and 0.3%, respectively. Different DO concentrations did not significantly impact COD removal efficiency.
3.2 TN Removal Efficiency Analysis
Analysis of TN removal in the head section showed effluent TN concentrations of 12.8, 12.3, and 13.1 mg/L under the three DO conditions, with removal rates of 68.0%, 66.8%, and 67.7%, respectively.
Data analysis indicates that TN removal efficiency in the head section varied to some extent under different DO concentrations, but the overall variation was minimal and did not show a clear correlation. Thus, it can be concluded that different DO concentrations did not significantly impact TN removal efficiency.
3.3 TP Removal Efficiency Analysis
Analysis of TP removal in the head section showed effluent TP concentrations of 0.60, 0.51, and 0.48 mg/L under the three DO conditions, with removal rates of 88.1%, 90.7%, and 91.7%, respectively.
Data analysis indicates that TP removal efficiency in the head section varied with DO concentration. Increasing DO concentration reduced effluent TP concentration and further improved removal efficiency. Thus, it can be concluded that the DO concentration level of 4–5 mg/L achieved the relatively highest removal efficiency.
Comprehensive analysis suggests that setting the DO concentration in the head section to the 4–5 mg/L level results in higher phosphorus uptake efficiency.
4 Analysis of the Impact of Middle Section DO Concentration on Pollutant Removal Efficiency
4.1 COD Removal Efficiency Analysis
Analysis of COD removal in the middle section showed effluent COD values of 39.9, 38.9, and 40.4 mg/L under the three DO conditions, with removal efficiencies of 91.0%, 90.9%, and 91.2%, respectively. Specific details are shown in Figure 3.
Data analysis indicates that while COD removal efficiency in the middle section varied to some extent under different DO concentrations, the overall variation was minimal and did not show a clear correlation. When DO concentration increased from the 2–3 mg/L level to the 3–4 mg/L level, effluent COD and removal efficiency decreased by 1.0 mg/L and 0.1%, respectively. However, when DO concentration increased to the 4–5 mg/L level, effluent COD and removal efficiency increased by 0.5 mg/L and 0.3%, respectively. Different DO concentrations did not significantly impact COD removal efficiency.
4.2 TN Removal Efficiency Analysis
Analysis of TN removal in the middle section showed effluent TN concentrations of 13.8, 13.0, and 12.9 mg/L under the three DO conditions, with removal rates of 62.5%, 66.3%, and 66.4%, respectively. Comparatively, DO concentration levels of 3–4 mg/L and 4–5 mg/L resulted in better TN removal efficiency.
4.3 TP Removal Efficiency Analysis
Analysis of TP removal in the middle section showed effluent TP concentrations of 0.57, 0.52, and 0.46 mg/L under the three DO conditions, with removal rates of 88.5%, 90.8%, and 91.5%, respectively. Comparatively, DO concentration levels of 3–4 mg/L and 4–5 mg/L resulted in better TP removal efficiency.
Comprehensive analysis suggests that setting the DO concentration in the middle section to the 3–4 mg/L level achieves higher pollutant removal efficiency.
Analysis of the Impact of Tail Section DO Concentration on Pollutant Removal Efficiency
5.1 COD Removal Efficiency Analysis
Analysis of COD removal in the tail section showed a removal efficiency of 91.8% under all three DO concentration conditions. Different DO concentrations did not significantly impact COD removal efficiency.
5.2 TN Removal Efficiency Analysis
Analysis of TN removal in the tail section showed effluent TN concentrations of 11.5, 12.7, and 13.4 mg/L under the three DO conditions, with removal rates of 72.7%, 67.9%, and 66.5%, respectively. Comparatively, the DO concentration level of 2–3 mg/L resulted in better TN removal efficiency.
5.3 TP Removal Efficiency Analysis
Analysis of TP removal in the tail section showed that when DO concentration was below 2.0 mg/L, removal efficiency did not exceed 96%. In this experiment, the removal rate under all three DO conditions was 90%, and effluent concentrations met the primary standard.
In summary, setting the DO concentration in the tail section to the 2–3 mg/L level achieves higher pollutant removal efficiency.
Conclusion
To investigate the specific impact of staged oxygen supply in the aerobic zone of the AAO process on pollutant removal efficiency, the aerobic zone was divided into head, middle, and tail sections during the study. Analysis of COD, TN, and TP removal efficiencies across these sections, combined with the research results, indicates that setting DO concentration levels in the three aerobic zones to 4–5 mg/L, 3–4 mg/L, and 2–3 mg/L, respectively, achieves better overall pollutant removal efficiency. This approach can provide support and reference for ecological environmental protection, energy conservation, and emission reduction efforts.