Roles of nitrate recycling ratio in the A2/O - MBBR denitrifying phosphorus removal system for high-efficient wastewater treatment: Performance comparison, nutrient mechanism and potential evaluation.

The long-term effect of nitrate recycling ratios (R = 100%-500%) on the denitrifying phosphorus removal (DPR) characteristics was studied in a novel two-sludge system, which coupled Anaerobic Anoxic Oxic (A2/O) with Moving Bed Biofilm Reactor (MBBR) for simultaneous nitrogen (N) and phosphorus (P) removals. During the 220 days' operation, effluent COD (30.87-45.15 mg/L) can meet the discharge standard completely, but N and P removals were significantly affected by the R-value, including CODintra removal efficiency (CODintra-Re: 56.09-85.98%), TN removal (TN-Re: 52.06-80.50%), anaerobic PO43- release (PO43--An: 10.66-29.02 mg/L) and oxic PO43- absorption (PO43--O: 2.22-6.26 mg/L). Meanwhile, N and P displayed close correlation with the ΔPO43-/ΔNO3- ratio of 4.20-4.41 at R = 300%-400%, resulting in the high-efficient anoxic poly-ß-hydroxyalkanoates (PHA) utilization (ΔPHAA: 64.88 mgCOD/gVSS). Based on the stoichiometry methodology, at R of 300%-400%, the percentages of phosphorus accumulation organisms (PAOs) and glycogen accumulating organisms (GAOs) contributed to ΔPHAAn (ΔGlyAn) were 71.7%, 28.3% (61.3%, 38.7%) in the anaerobic stage, respectively, while N denitrification rate (NDRA: 3.91-3.93 mg N/(gVSS·h)) and P uptake rate (PURA: 3.76-3.90 mg P/(gVSS·h)) reached the peak, suggesting superior DPR performance with higher contribution of denitrifying PAOs (DPAOs) (70%) than denitrifying GAOs (DGAOs) (30%) in the anoxic stage. Microbial community analysis showed that Accumulibacter (27.66-30.01%) was more enriched than Competibacter (13.41-14.34%) and was responsible for the improved C, N, P removals and DPR characteristics. For optimizing operation, the combined effect of nitrate recycling ratio with other process parameters especially economic evaluation should be considered.