Comparison of phosphorus removal efficiency between reversed and conventional A²/O processes

Through the monitoring and analysis on the whole reversed A²/O process in a sewage treatment plant in Xi'an, the adverse factors for poor phosphorus removal were ascertained. The study shows that the denitrification is incomplete in the anoxic tank so that lots of nitrate flows into the anaerobic tank and inhibits phosphorus release, which leads to the poor phosphorus removal. The phosphorus release was inhibited gradually with the increase of nitrate when the nitrate concentration was between 1 mg/L and 4 mg/L. When nitrate concentration was above 4 mg/L, phosphorus release was inhibited significantly. In order to avoid the adverse effect of nitrate on phosphorus accumulating bacteria for phosphorus release, nitrate concentration in the anaerobic tank should be below 1 mg/L. The adverse effect of nitrate on phosphorus accumulating bacteria indicates that carbon source is required for phosphorus release. Therefore, the effect of nitrate on phosphorus release was studied by using sodium acetate and raw water as carbon source sampled from the wastewater treatment plant. The study shows that the easily biodegradable organic matter was more beneficial for phosphorus release in anaerobic tank. Because of the anoxic tank set as the first step in reversed A²/O process, a large amount of easily biodegradable organic matter is consumed first for denitrification in the anoxic tank, which lead to the poor effect of phosphorus release in anaerobic tank due to lack of easily biodegradable organic matter. In response to this study, a reconstruction scheme that the reversed A²/O process shall be adjusted to a conventional A²/O process was put forward. After the reconstruction project, the concentration of nitrate in anaerobic tank decreases from a previous one of 3.57 mg/L to the current one of 0.89 mg/L with the amount of phosphorus release increased by 1.8 times. The total phosphorus in the effluent is 0.66 mg/L which is 0.21 mg/L lower than that of the reversed A²/O process.