Continuous flow partial nitrification/anammox anaerobic-oxic-anoxic (PN/A-AOA) system for advanced nitrogen removal of low carbon to nitrogen ratio wastewater at different temperature: Long-term performance and mechanism

A continuous flow PN/A-AOA system integrating endogenous denitrification (EnD), partial nitrification (PN), anammox (Amx) and exogenous denitrification (Dn) was run for 360 days treating low carbon to nitrogen ratio (C/N) wastewater at different temperature (15.1 °C–27.2 °C). Results showed that the PN/A-AOA system with intermittent NH₂OH (2 mg/L) dosage and bypass operation strategy could achieve stable partial nitrification in oxic tank (nitrite accumulation ratio, NAR > 90%) and advanced nitrogen removal (total inorganic nitrogen removal efficiency, TINRE > 96% and effluent TIN < 3 mg/L). EnD and Amx were the dominant pathways, with contribution proportion of 59.4%–94.2% and 11.9%–28.7% to nitrogen removal, respectively, and the flexible bypass (anaerobic tank to anoxic tank) strategy enhanced Amx contribution proportion in anoxic tank. Ex situ batch tests, enzymes activity and metagenomic analysis indicated the growth of nitrite oxidizing bacteria (NOB) was successfully inhibited, while denitrifying glycogen accumulating organisms (DGAOs) and anaerobic ammonium oxidizing bacteria (AnAOB) were highly enriched despite the drop of temperature. Moreover, the synergistic effects among DGAOs, AnAOB, ammonium oxidizing bacteria (AOB) and denitrifying bacteria (DNB) was the key of PN/A-AOA system to achieve stable and advanced nitrogen removal performance during the long-term operation.