Enhanced nitrogen removal from low C/N municipal wastewater at ambient temperatures through integration of partial denitrification/anammox and partial nitrification/anammox in step-feed continuous-flow pure-biofilm anoxic/oxic/anoxic system.

The limited enrichment and retention of anaerobic ammonium-oxidizing bacteria (AnAOB), coupled with inadequate nitrite supply, remain major obstacles to implementing anammox-based processes in mainstream wastewater treatment. Hence, in this research, a step-feed continuous-flow pure-biofilm anoxic/oxic/anoxic (AOA) process was designed, integrating partial denitrification/anammox (PD/A) and partial nitrification/anammox (PN/A) to enhance the nitrogen removal efficiency from municipal wastewater with a low carbon-to-nitrogen ratio (C/N) at ambient temperatures. During the stable operating phase, under a temperature as low as 15.2 °C and a C/N of 3.1, the pure-biofilm AOA process achieved average ammonium removal and total inorganic nitrogen (TIN) removal efficiencies of 92.2% and 88.2%, respectively. Notably, the pre-anoxic, oxic, and post-anoxic zones contributed 51.2% (46.9% by PD/A), 13.8% (8.0% by PN/A), and 35.0% (20% by PD/A) of the overall TIN removal, respectively. In situ profiling of pollutant removal pathways and batch activity tests revealed that the pure-biofilm AOA process established a stable and complementary dual nitrite supply network. Microbial community analysis demonstrated that AnAOB were enriched across all functional zones, with relative abundances ranging from 0.1% to 10.3%. Correlation and co-occurrence network analyses (positive: 63.38% > negative: 36.62%) further indicated complex synergistic associations among key functional microorganisms, including anaerobic ammonium-oxidizing bacteria, ammonia-oxidizing bacteria and denitrifying bacteria. Overall, this study developed and demonstrated the feasibility of the pure-biofilm AOA process incorporating dual nitrite-supply pathways, offering a promising strategy for the removal of nitrogen from mainstream wastewater with low C/N at ambient temperatures.