Robust nitrogen removal through simultaneous denitrification and anammox driven by alkaline sludge fermentation liquid at varying nitrification levels.

Operational instability of partial nitrification (PN) remains a major barrier to mainstream energy-efficient wastewater treatment. Here, a stable simultaneous denitrification-anammox system driven by alkaline sludge fermentation liquor (ASFL) under varying nitrification levels (PN, complete nitrification, and their coexistence) has been presented. Through a 240-day continuous operation treating municipal wastewater, this system achieved high nitrogen removal efficiency, ranging from 96.9 ± 2.0% under PN to 95.7 ± 2.4% under coexistence of PN and complete nitrification. Isotope tracing and microbial analyses indicated that ASFL-supported heterotrophic denitrification was the dominant nitrogen removal pathway, whereas anammox activity remained detectable but contributed only to a limited extent under the investigated conditions. Metagenomic analysis revealed functional shifts in nitrogen metabolism while the core carbon metabolic potential remained largely conserved. Combined with an optimized iron-assisted strategy for effluent polishing, this ASFL-driven simultaneous denitrification-anammox framework provides a sustainable, circular pathway for energy-efficient nitrogen removal, addressing the inherent vulnerability of mainstream PN applications.