The AOA process optimizes and adjusts the traditional sewage treatment process, which mainly includes anaerobic zone, aerobic zone, and anoxic zone. This process arrangement enables effective conversion and utilization of carbon sources in the treatment of wastewater.
◇ Anaerobic zone: In the anaerobic zone, organic matter in wastewater is converted into intermediate products such as volatile fatty acids (VFA) by microorganisms under anaerobic conditions, and internal carbon sources such as polyhydroxyalkanoates (PHA) are synthesized and stored in microbial bodies.
Aerobic zone: Sewage then enters the aerobic zone where nitrification occurs, converting ammonia nitrogen into nitrate nitrogen. Meanwhile, some organic compounds are also oxidized and decomposed under aerobic conditions. However, in the AOA process, most of the dissolved oxygen in the aerobic zone is used for nitrification, so only a small amount of organic matter is oxidized here, and most of the organic matter (especially COD) remains in the system as a carbon source for the subsequent anoxic zone.
◇ Hypoxia zone: In the anoxic zone, internal carbon sources (such as PHA) stored in the anaerobic zone are used for denitrification, reducing nitrate nitrogen to nitrogen gas to achieve denitrification. Due to the utilization of internal carbon sources stored in anaerobic zones in hypoxic areas, the demand for external carbon sources is reduced.
Reasons why there is basically no need to add carbon sources
◇ Endogenous Denitrification: In the AOA process, especially in the design after the anoxic stage, due to the anoxic stage being located after the aerobic stage, the carbon source produced by endogenous respiration of microorganisms in the aerobic stage (i.e. the decomposition of microbial cell material) is used for denitrification. This endogenous denitrification mechanism reduces the demand for external carbon sources.
Efficient utilization of organic matter: In the anaerobic stage, the organic matter in the influent is converted by microorganisms into easily biodegradable organic matter such as volatile fatty acids (VFAs), which are stored in the microbial body as an internal carbon source. These internal carbon sources are released in the subsequent anoxic stage for denitrification, thereby achieving efficient utilization of organic matter.
◇ Sludge reflux: AOA process usually includes sludge reflux, which returns the sludge from the aerobic section or secondary sedimentation tank to the anaerobic section or anoxic section. This sludge reflux not only helps maintain the biomass in the system, but also brings the internal carbon source in the microbial body back to the anoxic zone, further reducing the demand for external carbon sources.
◇ Non reflux of nitration solution: Compared with traditional A/O or A ²/O processes, AOA process eliminates the reflux step of nitration solution. This reduces energy consumption and avoids the potential additional carbon source consumption caused by the reflux of nitration solution.
Process optimization: By optimizing process parameters such as hydraulic retention time (HRT), sludge age (SRT), dissolved oxygen (DO) concentration, etc., the utilization efficiency of AOA process for carbon sources can be further improved, thereby reducing the demand for external carbon sources.
Advantages of AOA process
◇ Reduce the demand for external carbon sources: Due to the full utilization of carbon sources in raw water by the AOA process, the demand for external carbon sources is reduced, resulting in lower operating costs.
◇ Improve denitrification efficiency: With sufficient carbon sources, AOA process can achieve nearly 100% nitrogen removal efficiency, improving sewage treatment efficiency.
◇ Reduce sludge production: Due to the fact that microorganisms in the AOA process mainly utilize internal carbon sources for denitrification, the sludge production is relatively small, reducing the cost of sludge treatment.
In summary, the AOA process optimizes the process flow and parameter settings to fully utilize the carbon source in the raw water for denitrification, thereby reducing the demand for external carbon sources. This process design not only reduces operating costs, but also improves sewage treatment efficiency and denitrification efficiency.
