Balancing sulfur accumulation and nitrous oxide emissions during sulfide autotrophic denitrification in moving-bed...

Sulfide autotrophic denitrification (SADN) is a promising strategy for integrating biogas desulfurization with nitrogen removal in anaerobic digestion-based treatment systems. However, its application in moving bed biofilm reactors (MBBRs) remains limited by biosulfur (S⁰) accumulation and nitrous oxide (N₂O) emissions. This study investigated the operational tradeoffs governing SADN performance in an MBBR under different nitrate-to-sulfide (NO₃^-/S^2-) ratios, electron acceptors (NO₃^- or NO₂^-), and hydraulic retention times (HRTs). Batch assays, microbial community analysis, and techno-economic evaluation were conducted to elucidate biofilm-planktonic biomass interactions and operational feasibility. Complete sulfide removal (>99%) was achieved under all tested conditions. Increasing the NO₃^-/S^2- ratio reduced S⁰ accumulation from 16.5% to 5.2%, mitigating carrier clogging, but simultaneously promoted incomplete denitrification and N₂O formation. In contrast, operation at an NO₃^-/S^2- ratio of 1.6 achieved complete denitrification without detectable N₂O emissions while maintaining effective sulfur control, representing the optimal techno-economic condition. Using NO₂^- as electron acceptor enabled complete denitrification and suppressed N₂O emissions, but increased S⁰ accumulation due to limited electron-accepting capacity for complete sulfide oxidation. Microbial analysis identified Sulfurimonas as the dominant sulfur-oxidizing genus, while mixotrophic denitrifiers contributed to residual sulfur formation. Activity assays showed that biofilm biomass dominated electron transfer, whereas planktonic biomass enhanced reactor performance through synergistic interactions. Decreasing HRT from 24 to 12 h, with increased S^2- and NO₃^- loadings, maintained S^2- and NO₃^- removal while reducing reactor footprint. These findings demonstrate that sustainable SADN-MBBR operation requires balancing sulfur control, denitrification completeness, greenhouse-gas mitigation, and reactor stability for scalable low-carbon biogas upgrading applications.