A novel partial nitrification-synchronous anammox and endogenous partial denitrification (PN-SAEPD) process for advanced nitrogen removal from municipal wastewater at ambient temperatures

Mainstream anammox still faces the challenges of non-ideal NO2-/NH4+ ratios and excess nitrate resulted from the instability of partial nitrification (PN) in municipal wastewater. To address these problems, in this study, we developed a novel two-sludge process that combined PN with synchronous anammox and endogenous partial denitrification (SAEPD); the process was tested with pre-treated domestic sewage at ambient temperatures for 205 d. High nitrogen removal efficiency of 91.2% was achieved with an influent C/N ratio of 1.7 at 15.4 °C; the success was attributed to the fact that EPD replenished the deficient nitrite by reducing nitrate and the excess nitrite was further reduced to nitrogen gas. With a non-ideal NO2-/NH4+ ratio of 0.89, the contribution of the SAEPD-sequencing batch reactor (SBR) during the anoxic stage reached 98.2% and the proportional contributions of the anammox and denitrification pathways were 77.2% and 22.8%, respectively. Although the low nitrite accumulation (66.1%) caused 10.8 mg N/L of nitrate to be transported into the SAEPD-SBR and the anammox reaction also converted 20% of nitrite to nitrate, only 1.1 mg N/L of nitrate remained in the effluent. High-throughput sequencing analysis revealed that although NH2OH was added, some genera of nitrite-oxidizing bacteria (0.73%) remained in the PN-SBR and potentially resulted in the oxidation of nitrite to nitrate. In the SAEPD-SBR, anammox and endogenous denitrifying bacteria co-existed and synergistically achieved the removal of ammonium, nitrite, and nitrate. Overall, the PN-SAEPD process has great potential for achieving cost-effective and energy-efficient municipal wastewater treatment.