Enhanced nutrients removal and microbial mechanisms in a pilot-scale anaerobic-oxic-anoxic (A/O/A) system: Synergistic roles of denitrifying polyphosphate accumulating organisms and endogenous denitrifiers

The realization of low-cost and high-standard nutrients removal are not acquired effectively in conventional pre-denitrification biological nitrogen removal processes (e.g., anaerobic/anoxic/oxic (A/A/O) process) for the treatment of domestic sewage. In this study, a pilot-scale bioreactor system based on the synergism of denitrifying phosphorus removal (DPR) and endogenous denitrification (ED) was proposed for advanced nutrients removal, which was operated under alternating anaerobic/oxic/anoxic (A/O/A) mode with no extracellular carbon source in the anoxic stage. The results showed that A/O/A and A/A/O systems were successfully established and operated in four stages with different mixed liquor suspended solid (MLSS, I: 3.32 ± 0.25 g/L, II: 5.89 ± 0.94 g/L, III: 8.66 ± 0.51 g/L, IV: 9.11 ± 5.72 g/L). Compared with the A/A/O system, the A/O/A system showed significantly higher nitrogen and phosphorus removal effectiveness within hydraulic retention time of 10 h. At MLSS of 8.66 ± 0.51 g/L, the A/O/A system could achieve simultaneously efficient removal of COD (91.47% ± 2.06%, 2.97 ± 0.26 mg COD/(g MLSS·h)), NH4+-N (97.27% ± 1.23%, 0.37 ± 0.06 mg NH4+-N/(g MLSS·h)), TN (89.94% ± 1.47%, 0.45 ± 0.03 mg TN/(g MLSS·h)), and PO43--P (96.67% ± 0.88%, 0.04 ± 0.01 mg PO43--P/(g MLSS·h)), and the effluent concentrations of COD, NH4+-N, TN, and PO43--P were 24.0 ± 3.1, 0.9 ± 0.4, 4.4 ± 0.6, and 0.13 ± 0.03 mg/L, respectively. In addition, microbial community results indicated that denitrifying polyphosphate accumulating organisms and endogenous denitrifiers synergistically enhanced the nutrients removal capacity in the A/O/A system. Overall, this work provides new insight for further scaling and optimization of the A/O/A system.