Single-stage bioreactor integrating anammox and sulfur-driven autotrophic denitrification for nitrogen removal from...

An integrated anaerobic ammonium oxidation-sulfur autotrophic denitrification (Anammox-SAD) bioreactor was developed for nitrogen removal from real landfill leachate without external organic carbon. The system was operated continuously for 105 days under stepwise-decreasing hydraulic retention time (HRT), with a stable influent NO₂^--N/NH₄⁺-N ratio of 1.2-1.5 provided by partial nitrification. The anammox zone maintained robust performance, yielding average effluent NH₄⁺-N, NO₂^--N and NO₃^--N of 18 ± 5, 6 ± 9, and 110 ± 16 mg/L, respectively. At an HRT as low as 0.5 d, the maximum nitrogen removal rate reached 2.04 kg N/(m³ d). Despite influent fluctuations (total nitrogen of 600-840 mg/L), the integrated reactor achieved a stable total nitrogen removal of 85-89 %. The downstream SAD zone effectively polished anammox-derived nitrate, reducing NO₃^--N from 80-150 to 35-60 mg/L. Sulfur oxidation was confirmed by elevated sulfate concentrations, while anammox-associated alkalinity generation provided intrinsic pH buffering, sustaining favorable conditions for SAD. Metagenomic analyses revealed coordinated nitrogen-sulfur metabolic networks, showing that shortening HRT enhanced microbial diversity and functional redundancy but revealed kinetic limitations in substrate reduction. Overall, the integrated anammox-SAD system provides a robust and low-carbon strategy for treating high-strength landfill leachate.