Performance improvement and microbial mechanisms of an integrated A/O biofilm reactor through optimized aeration orifice design and inflow velocity.

To mitigate eutrophication in water bodies caused by high organic matter and nitrogen-phosphorus loads from rural domestic sewage, this study developed an integrated A/O biofilm reactor. Numerical simulations were conducted to optimize the flow field in both the aeration and influent zones, identifying the optimal aeration configuration as five holes, each with a diameter of 20 mm. The inlet flow velocity was maintained below 0.1 m/s, effectively mitigating short-circuiting, backflow vortices, and biofilm detachment. Experimental validation demonstrated that the optimized reactor significantly improved pollutant removal efficiency: the removal efficiencies of COD, NH4+-N and TN reached 92% (effluent: 28.1 mg/L), 96% (effluent: 2.11 mg/L) and 58.5% (effluent: 20.9 mg/L), respectively. Meanwhile, the average dissolved oxygen concentration increased from 1.65 mg/L under the original single-hole aeration condition to 2.25 mg/L under the optimized configuration. Microbiological analysis revealed that the enhanced nitrogen removal efficiency was associated with a substantial increase in the relative abundance of Nitrospira and Terrimonas, which facilitated simultaneous nitrification-denitrification and anaerobic ammonia oxidation within the aerobic packing. This study provides valuable theoretical insights for improving nitrogen removal performance in integrated A/O biofilm reactors.