Abstract:The study explores the optimization of nitrogen removal efficiency in the eAnMBR-PN/A integrated process. Simulation results reveal that oxygen load significantly impacts the system's performance. When the oxygen load is maintained between 0.12 and 0.14 g/(m3·d), the system achieves a peak total nitrogen (TN) removal efficiency of 64.45%, coinciding with the highest relative abundance of anaerobic ammonium-oxidizing bacteria (AnAOB) in the biofilm. However, when the oxygen load exceeds 0.16 g/(m3·d), the elevated dissolved oxygen levels inhibit AnAOB activity and promote the proliferation of nitrite-oxidizing bacteria (NOB), leading to nitrate accumulation and a decline in TN removal efficiency. The carbon-to-nitrogen (C/N) ratio is also shown to play a crucial role in regulating the system′s performance. At low influent  concentrations (<35 g/m3), increasing the C/N ratio enhances TN removal by stimulating the denitrification activity of heterotrophic bacteria (HB). Conversely, at high  concentrations (≥35 g/m3), maintaining a low C/N ratio (<2.0) is essential to prevent HB from competitively suppressing AnAOB functionality. The research concludes that the eAnMBR-PN/A process can effectively remove nitrogen from municipal wastewater with low C/N ratios through oxygen transfer and substrate allocation optimization. This provides valuable theoretical insights and a technical pathway for the low-carbon transformation of wastewater treatment plants. The study emphasizes the importance of balancing the oxygen load to avoid both the inhibition of ammonia-oxidizing bacteria (AOB) activity at low oxygen levels and the suppression of AnAOB activity at high oxygen levels. Additionally, when the influent  concentration is low, increasing the influent COD concentration can enhance TN removal, although this may lead to higher operational costs. On the other hand, when the influent -N concentration is high, maintaining a lower influent COD concentration helps AnAOB participate in TN removal, reducing operational costs. The research also provides insights into the microbial community dynamics within the biofilm under different oxygen loads and C/N ratios. At an oxygen load of 0.12 g/(m3·d), a stable symbiotic system of AnAOB, AOB, and HB is formed in the biofilm. However, as the oxygen load increases to 0.16 g/(m3·d) and above, the AnAOB abundance gradually decreases, while the AOB abundance first increases and then decreases. This is attributed to the inhibition by high dissolved oxygen. In summary, the eAnMBR-PN/A integrated process offers an effective solution for nitrogen removal in municipal wastewater with low C/N ratios. By optimizing oxygen load and C/N ratio, the system can achieve efficient nitrogen removal while reducing operational costs. This study advances the understanding of nitrogen removal processes and highlights the potential of the eAnMBR-PN/A process in promoting the enrichment of dominant bacterial species for efficient nitrogen removal in municipal wastewater treatment. Close-