Effect of Three-Stage Reflux Ratio on the Performance of AAOA-MBR Process for Municipal Wastewater Treatment
The AAOA-MBR (anaerobic-anoxic-oxic-anoxic membrane bioreactor) process is widely used in municipal wastewater treatment, but its multi-stage internal recirculation complicates sludge retention time (SRT) and carbon source distribution. This study systematically regulated three reflux ratios (R1: membrane tank to oxic tank; R2: oxic tank to anoxic I tank; R3: anoxic II tank to anaerobic tank) in a pilot-scale system (0.24 m3·d−1) to reveal their effects on nutrient removal and membrane fouling. When R1:R2:R3 = 300%:200%:100%, effluent COD, TN, TP, and NH3-N met discharge standards. Reducing R1 and R2, thereby decreasing total reflux ratio from R=6 to R=3, shortened SRT, which suppressed nitrifier accumulation and increased effluent COD and TN, but decreased TP. High-throughput sequencing of anoxic I and oxic tanks showed that denitrifying bacteria (Thauera and Ottowia) relative abundances decreased from 0.68% to 0.42% and 0.51% to 0.24%, respectively, while the phosphorus-accumulating organism Candidatus_Accumulibacter increased from 0.78% to 1.12%, enhancing phosphorus removal. Additionally, lowering R1 to 200% caused sludge accumulation in the membrane tank, exacerbating membrane fouling. Thus, internal recirculation ratios must be adjusted based on influent characteristics to balance nutrient removal and membrane performance.