Analysis of Pollutant Removal Efficiency in Wastewater Using a Modified AAO System
Overview
The Anaerobic-Anoxic-Oxic (AAO or A²/O) process is a widely adopted biological wastewater treatment technology designed for simultaneous removal of organic carbon, nitrogen, and phosphorus. It consists of three interconnected zones:
- Anaerobic Zone: Lacking oxygen and nitrate, facultative bacteria break down organic compounds, releasing phosphorus.
- Anoxic Zone: Denitrifying bacteria use organic carbon as an electron donor to reduce nitrates/nitrites (returned from the oxic zone) into nitrogen gas, achieving nitrogen removal.
- Oxic Zone: Aerobic microorganisms oxidize remaining organic matter and facilitate nitrification (ammonia to nitrate), while phosphorus-accumulating organisms uptake phosphates.
In the field of wastewater treatment, while the conventional AAO system can remove pollutants from sewage, the increasingly complex composition of wastewater has led to a certain decline in the treatment efficiency of the AAO process. To ensure the application level and effectiveness of the AAO process, it is necessary to conduct specific research on modified systems, which holds significant practical importance for relevant enterprises and departments in improving their operational quality.
Modified AAO System
1. Basic Principles of the AAO System
Taking the AAO wastewater treatment system in a specific wastewater treatment plant as an example, the existing system is a conventional AAO process, primarily consisting of four components: an anaerobic tank, an aerobic tank, an anoxic tank, and a secondary sedimentation tank, as detailed in Figure 1.
In the conventional AAO system, microorganisms grow and metabolize under various environmental conditions. Through interactions among different microbial communities, effective removal of pollutants is achieved via chemical reactions such as ammonification, nitrification, and denitrification, which significantly eliminate organic pollutants.
The conventional AAO system offers advantages such as low technical costs, simple operation, and short hydraulic retention time (HRT). However, it also suffers from drawbacks including poor phosphorus removal efficiency and stringent requirements for sludge age and carbon source design, making it difficult to meet expected standards in some wastewater pollutant removal projects.
2. Design Analysis of the Modified AAO System
Based on previous research, improvements were made to the conventional AAO system, primarily focusing on the anaerobic tank. The modified anaerobic tank system consists of three parts: a sludge-water mixing zone, a sludge-water separation zone, and a media zone, as shown in Figure 2.
In the modified AAO system (Figure 3), the sludge-water mixing zone and the media zone are designed with identical dimensions (15 cm length × 20 cm width × 60 cm height), each with an effective volume of 9 L. The hydraulic retention time (HRT) for both the sludge zone and the media zone is 2 hours.
3. Analysis of COD Removal Efficiency in the Modified AAO System
The chemical oxygen demand (COD) removal efficiency of the modified AAO system was analyzed. When the influent COD was 447 mg/L, the effluent COD from the anaerobic stage was approximately 147 mg/L, and the final effluent COD was 42 mg/L, meeting the Class A discharge standard. The COD removal efficiency in the early anaerobic stage was unstable, with significant fluctuations and relatively low mixed liquor suspended solids (MLSS) levels. However, after 7 days, the removal rate stabilized at 94%. The anaerobic stage primarily removes pollutants through microbial degradation and dilution by reflux, demonstrating effective pollutant removal.
Using Minitab software, the removal performance of the modified and conventional AAO systems was compared via independent sample t-test analysis, with results shown in Figure 4.
At a 95% confidence interval, the t-value was 0.26, and the p-value was 0.605. The data analysis indicated no significant difference in mean removal rates between the two systems. The modified AAO system showed relatively high variability in COD removal efficiency, primarily due to differences in early-stage data, including the acclimation phase. Overall, the modified AAO system demonstrated effective COD removal.
4. Analysis of Ammonia Nitrogen Removal Efficiency in the Modified AAO System
The ammonia nitrogen (NH₃-N) removal efficiency was analyzed. When the influent NH₃-N concentration was 36 mg/L, the effluent NH₃-N from the anaerobic stage was approximately 19 mg/L. In the early stage, the effluent concentration was relatively high, and removal efficiency fluctuated significantly. However, after 12 days of acclimation, the removal rate increased to around 81%, with nitrifying bacteria in the logarithmic growth phase. Subsequently, the removal rate stabilized, reaching an average of 93%, with an effluent NH₃-N concentration of 4 mg/L, meeting the Class A discharge standard.
At a 95% confidence interval, the t-value was 3.41, and the p-value was 0.998. The data analysis indicated no significant difference in mean removal rates between the two systems. The modified AAO system showed relatively high variability in NH₃-N removal efficiency, primarily due to differences in early-stage data, including the acclimation phase. Overall, the modified AAO system demonstrated effective NH₃-N removal.
5. Analysis of Total Phosphorus and Total Nitrogen Removal Efficiency in the Modified AAO System
5.1 Total Phosphorus Removal Efficiency
The total phosphorus (TP) removal efficiency was analyzed. When the influent TP concentration was 3.6 mg/L, an 11-day acclimation period was required. The effluent TP concentration from the entire system reached 2.8 mg/L, while the anaerobic stage effluent TP concentration was 4.2 mg/L, indicating significant phosphorus release. After acclimation, the TP removal performance improved markedly, with the anaerobic stage effluent TP concentration decreasing to 2.7 mg/L and the removal efficiency reaching 17%. In the later stage, the TP removal rate stabilized above 60%, and the effluent TP concentration approached 0.5 mg/L, meeting the Class B discharge standard.
Comparison of the two systems showed that the modified AAO system required an initial acclimation period with relatively low removal efficiency. However, after acclimation, its TP removal performance significantly improved, demonstrating enhanced efficiency compared to the conventional AAO system.
5.2 Total Nitrogen Removal Efficiency
The total nitrogen (TN) removal efficiency was analyzed. When the influent TN concentration was 34 mg/L, the anaerobic stage effluent TN concentration was approximately 18 mg/L. In the early stage, the effluent concentration was relatively high, and removal efficiency fluctuated significantly. After 10 days of acclimation, the TN removal rate increased to 68%, with an effluent concentration of 9 mg/L, meeting the Class A discharge standard.
At a 95% confidence interval, the t-value was 0.72, and the p-value was 0.753. The data analysis indicated no significant difference in mean removal rates between the two systems. The modified AAO system showed relatively high variability in TN removal efficiency, primarily due to differences in early-stage data, including the acclimation phase. Overall, the modified AAO system demonstrated effective TN removal.
Conclusion
In summary, the modified AAO system demonstrates robust performance in removing key wastewater pollutants-COD, ammonia nitrogen, total nitrogen, and total phosphorus-meeting Class A or B discharge standards after a brief acclimation period.
While statistical analysis (t-test, p-value) showed no significant difference in mean removal efficiency compared to the conventional system, the modified design exhibited higher stability and improved treatment outcomes over time, despite greater data variability during initial operation. The enhancements, particularly in the anaerobic zone with optimized mixing, separation, and media zones, contribute to increased process resilience and efficiency.
These findings underscore the potential of modified AAO systems to address complex wastewater compositions effectively, supporting their practical application for upgrading existing treatment infrastructure.