Anammox-driven advanced nitrogen removal from high-strength ammonia wastewater: Multi-pathway synergism and microbiome spatial remodeling.

The anammox process demonstrates significant potential as an energy-efficient nitrogen removal technology in wastewater treatment. However, its operational strategies and nutrient transformation mechanisms for treating complex high-ammonia wastewater require further investigation. This study developed a continuous-flow anoxic-aerobic integrated fixed-film activated sludge (A/O IFAS) system centered on anammox to treat mature landfill leachate (MLL) with an extremely low BOD5/N ratio (<0.1). By synergistically adjusting the hydraulic retention time to 3 days, sludge reflux ratio to 200% ± 5%, and aerobic zone dissolved oxygen to 1.0 mg/L, the system achieved a total nitrogen removal efficiency (NRE) of 98.2 ± 3.5% and an average effluent total nitrogen (TN) of 29.1 ± 4.3 mg/L without external carbon addition. Based on 15N isotope tracing and nitrogen transformation mechanism analysis, this study innovatively evaluated multi-pathway nitrogen removal and synergistic contributions, with anammox as the core pathway contributing 87.7% of TN removal and the aerobic zone as the hotspot achieving 89.4%. The IFAS configuration fostered micro-anoxic niches within the aerobic environment, driving the spatial redistribution of the key anammox bacteriaCandidatus Kueneniafrom floc sludge (0.5%) to biofilm carriers (5.8%). This work provides an efficient, low-energy autotrophic nitrogen removal strategy for high-ammonia, extremely low C/N wastewater and serves as a reliable technical case to advance the engineering application of anammox.