Effect of Three-Stage Reflux Ratio on the Performance of AAOA-MBR Process for Municipal Wastewater Treatment

• • Optimal performance achieved at R1:R2:R3 = 300%:200%:100% (total R=6), yielding effluent COD, TN, TP, and NH3-N meeting discharge standards; reducing total R to 3 increased effluent COD and TN but decreased TP, highlighting the trade-off between nitrogen and phosphorus removal. • • Lowering total reflux ratio from R=6 to R=3 shortened SRT, reducing denitrifier abundances (Thauera from 0.68% to 0.42%, Ottowia from 0.51% to 0.24%) and weakening nitrogen removal, while Candidatus_Accumulibacter increased from 0.78% to 1.12%, improving phosphorus removal. • • Membrane fouling control requires R1 ≥ 300%; reducing R1 to 200% caused sludge accumulation in the membrane tank, accelerating transmembrane pressure rise and reducing permeate efficiency. • • Recommended strategy: maintain high R1 (300%) and moderate R2 (0–100%) to balance nitrogen removal, phosphorus removal, and membrane fouling control, achieving energy savings and stable operation.