Stable nitrite accumulation and phosphorous removal from nitrate and municipal wastewaters in a combined process of endogenous partial denitrification and denitrifying phosphorus removal (EPDPR)

In this study, a novel process was developed by combining endogenous partial denitrification (EPD, NO3- -N -> NO2- -N) with denitrifying phosphorus removal (DPR) in an anaerobic/anoxic/aerobic sequencing batch reactor (SBR) to achieve a stable nitrite accumulation and phosphorus removal from high-strength nitrate (NO3- -N: 186.5 mg/L) and municipal wastewater (NH4+ -N: 52.6 mg/L, chemical oxygen demand (COD): 242.7 mg/L). After 138-day, high nitrate-to-nitrite transformation ratio (NTR) of 75.3% and PO43- -P removal efficiency of 92.3% were achieved by controlling the anaerobic drainage ratio and anoxic/aerobic durations. During the anaerobic stage (2.5 h), carbon source in raw municipal wastewater was efficiently utilized by phosphorus-andglycogen accumulating organisms (PAOs and GAOs) to enhance intracellular carbon storage (69.9% in anaerobic COD consumption), providing sufficient carbons for subsequent anoxic EPD and DPR. During the anoxic stage (2.5 h), high activity of DGAO(NO3-NO2) (denitrifying GAOs conducting NO3- -N -> NO2- -N) over DPAO(NO3) and DGAO(NO3) (DPAOs and DGAOs conducting NO3- -N -> N-2) in nitrate denitrification (77.7% > 8.2% and 5.7%) facilitated the nitrite accumulation and phosphorus uptake. In the last short aerobic stage (10 min), further phosphorus was removed by aerobic PAOs without initiating ammonia/nitrite oxidation. Thus, effluent NO2- -N/NH4+ -N ratio of 1.02: 1 (close to the theoretical value of 1.32: 1 for anammox process) with limited NO3- -N, PO43- -P and COD (1.8, 0.4 and 23.5 mg/L, respectively) were obtained. This provides the potential for advanced nutrient removal from nitrate and municipal wastewaters by combining EPDPR with anammox. Microbial community analysis revealed that Cluster I of Defluviicoccus-GAO group (similar to 7%) dominated the high nitrite accumulation performance, while Accumulibacter-PAO group (15 +/- 3%) was responsible for the P removal performance.