Temperature-based strategy for enhanced nitrogen removal in mainstream via selectively strengthening anammox or denitrification

To address the instability challenges of Partial Nitrification and Anammox (PNA) at low temperatures, this study introduces a temperature-based nitrogen removal process and demonstrates its feasibility in a pilot-scale system. The temperature-based strategy allows for the selective enhancement of anammox at higher temperatures (>20 °C) or denitrification at moderate and lower temperatures (≤20 °C). Nitrogen removal efficiencies of 93.8%, 72.1%, and 59.1% were achieved under >20 °C, 15–20 °C, and <15 °C, with corresponding effluent qualities of 3.0 mg/L, 9.6 mg/L, and 13.7 mg/L. As temperatures decreased, anammox contributions to nitrogen removal weakened from 88.4% to 8.2%, while denitrification contributions increased from 10.1% to 90.1%. Anammox bacteria exhibit a competitive advantage over denitrifying bacteria at higher temperatures, evidenced by the abundance of Candidatus Kuenenia at 7.13%. Denitratesoma was enriched to 3.47% at moderate and low temperatures, effectively supporting nitrogen removal robustness. This study provides insights into the seasonal optimization of mainstream anammox processes.