Full-scale mainstream partial nitritation/anammox process: A three-year demonstration for municipal wastewater treatment

• Stable full-scale mainstream anammox was operated across a wide temperature range (11.6–28.9°C). • An average effluent total nitrogen of 4.5 ± 2.3 mg/L was achieved, meeting stringent standards. • Efficient biological nitrogen and phosphorus removal under low-temperature conditions in winter (<15°C). • Ca. Brocadia demonstrated high resilience and dominated the anammox community. Mainstream partial nitritation/anammox (PN/A) is recognized as a promising method for sustainable nitrogen removal from wastewater. However, its practical implementation remains challenging to date. In this study, a full-scale mainstream partial-nitritation activated sludge system, coupled with an anammox fixed-film (PN-AF) process, has been developed to treat municipal wastewater. The process was initiated by inoculating fixed biofilm with a mature anammox consortium sourced from a side-stream PN/A. The integrated application of chemical inhibition and ecological niche selection was practically implemented in engineering for the first time, enabling the PN-AF process to achieve two years of continuous operation within a wide temperature range (11.6–28.9°C). The average total nitrogen (TN) removal efficiency was 91.8 ± 4.6%, with an average effluent TN concentration of 4.5 ± 2.3 mg/L and a nitrogen removal rate of 0.072 ± 0.01 kg N·m −3 ·d −1 . Molecular analysis revealed a remarkably high relative abundance of Candidatus Brocadia (17.87%), accompanied by an eight times higher activity of anammox than that of denitrification. Overall, this work presents a reliable mainstream partial nitritation/anammox process for sustainable wastewater treatment.