Pilot-scale implementation of mainstream anammox for municipal wastewater treatment against cold temperature

Applying anammox to municipal wastewater treatment promises enormous energy and resource savings; however, seasonally cold conditions pose a considerable challenge, impeding its future applications towards non-tropical regions. In this study, we establish a pilot-scale wastewater treatment plant (50 m³/d) in northern China and implement the partial denitrification coupling anammox process on actual municipal wastewater. Despite seasonal cooling, the nitrogen removal efficiency remains high, ranging from 75.0 ± 4.6% at 27.8-20.0 °C to 70.4 ± 4.5% at 10-7.5 °C. This process exhibits remarkable low-temperature tolerance, achieving an in-situ anammox rate of 32.7 ± 4.7 g-N/(m³·d) at 10-7.5 °C and contributing up to 39.7 ± 6.7% to nitrogen removal. Further ¹⁵N stable isotope tracing and kinetic tests reveal that the partial denitrification is capable of supplying increasingly abundant NO₂^- to anammox with decreasing temperature, enabling robust mainstream anammox against seasonal cooling. From 27.8 °C to 7.5 °C, anammox bacteria not only survive but thrive under mainstream conditions, with absolute and relative abundances increasing by 429.1% and 343.5%, respectively. This pilot-scale study sheds fresh light on extending mainstream anammox towards non-tropical regions, taking a necessary step forward toward the sustainability goals of the wastewater treatment sector.