Optimization of AAO process for reduced NO emissions and enhanced nitrogen removal in municipal wastewater treatment: Exploring carbon supplementation and DO control strategies.

The anaerobic/anoxic/oxic (AAO) process remains a common nutrient removal process in municipal wastewater treatment, yet research focusing on concurrent optimization of process performance and NO emissions reduction is scarce. This study aimed to investigate the mitigation of NO emissions and enhance nitrogen removal efficiency in an AAO system treating low C/N domestic wastewater by establishing a fully enclosed gas-collecting continuous flow reactor and implementing carbon supplementation and dissolved oxygen (DO) control strategies. The results indicated that carbon supplementation in the anoxic zone effectively reduced nitrate concentrations and mitigated the accumulation of dissolved NO below 0.1 mgN/L. The moderate DO control (1-2 mg/L) could ensure the nitrification efficiency while reducing the gaseous NO emission rate to 63.48 mgN/d, and decreasing the dissolved NO concentration in the effluent to below 0.01 mgN/L. Both too high and too low DO levels were detrimental to NO emission mitigation. The optimized AAO process achieved a significant reduction in the NO emission factor to 0.85 % and an increase in nitrogen removal efficiency to 81.81 %. Additionally, the enrichment of anammox bacteria, Candidatus brocadia (0.15 %), positively contributed to the improvement in nitrogen removal efficiency. In conclusion, this study provides valuable insights into optimizing AAO process to mitigate NO emissions, enhance nitrogen removal, and lower carbon footprints associated with wastewater treatment.