High-Efficiency mixotrophic biological nitrogen removal process at low Temperature: Sulfur Disproportionation-Mediated greenhouse gas mitigation

Conventional sulfur-driven autotrophic denitrification (SDAD) is limited by the low bioavailability of elemental sulfur (S⁰). While sulfur disproportionation (SD) enhances S⁰ utilization, its performance under mixotrophic conditions and seasonal temperature fluctuations remains unquantified. This study developed a novel SD-mediated mixotrophic biological nitrogen removal (SD-MixBNR) system under low-temperature conditions to enhance biological nitrogen removal efficiency. Long-term experiments using actual influent from secondary sedimentation tanks demonstrated that the SD-MixBNR system achieved superior nitrate removal rates (1.32–3.33 mg N/L/h), significantly exceeding those reported for SDAD under comparable conditions. Furthermore, the system reduced 64.14–84.20 % N₂O emissions compared to conventional SDAD processes. Interestingly, batching test revealed substantial contributions (17–39 %) of ammonium production through dissimilatory nitrate reduction to ammonium (DNRA), suggesting potential for ammonia recovery. Microbial community analysis elucidated nitrogen-sulfur conversion mechanisms among functional microorganisms. SD bacteria (Desulfocapsa, 1.89 %) utilize S⁰ to generate sulfides and polysulfides, which act as supplementary electron donors for autotrophic sulfur-oxidizing nitrate-reducing bacteria (Thiobacillus, 9.30 %), thereby accelerating denitrification rates. Concurrently, these sulfur intermediates stimulate DNRA bacteria (Geobacter, 2.70 %) to reduce nitrate into ammonium, while effectively mitigating N₂O emissions. This work validates the operational stability and environmental adaptability of SD-MixBNR systems under real municipal wastewater and low-temperature conditions, proposing an innovative approach for ammonium recovery in carbon–neutral wastewater treatment.