Nitrogen removal and nitrous oxide emission in the partial nitritation/anammox process at different reflux ratios

The partial nitritation/anammox (PN/A) process has been widely used in wastewater treatment owing to its notable advantages, including a low aeration rate and the non-requirement of an additional carbon source. In practical implementation, nitrite accumulation affects the nitrogen-removal efficiency and the amount of N₂O released during the PN/A process. By implementing wastewater reflux, the nitrite concentration can be decreased, thereby achieving a balance between the nitrogen-removal efficiency and N₂O release. This study conducted the CANON process with varying reflux ratios of 0 to 300 % and ~300 mg/L ammonium in the influent. The highest removal efficiency of ammonium and total nitrogen (98.2 ± 0.8 and 77.8 ± 2.3 %, respectively) could be achieved at a reflux ratio of 200 %. Further, a reflux ratio of 200 % led to the lowest N₂O emission factor (2.21 %), with a 31.74 % reduction in N₂O emission compared to the process without refluxing. Additionally, the reactor at a reflux ratio of 200 % presented the highest relative abundance of anaerobic ammonium-oxidizing bacteria (30.98 %) and the lowest proportion of ammonium-oxidizing bacteria (9.57 %). This study aimed to elucidate the impact of the reflux ratio on the nitrogen-removal efficiency of the CANON process and to theoretically explain the influence of different reflux ratios on N₂O release. These findings provide a theoretical framework for enhancing the nitrogen-removal efficiency and mitigating carbon emissions in practical applications of the CANON process.