Efficient nitrogen elimination and microbial community characteristics in a sulfur-based autotrophic bioreactor at high nitrate loading

This study systematically evaluated the nitrogen removal performance and microbial community characteristics of the sulfur-based autotrophic denitrification (SAD) process under various influent loads (15–140 mg N/L). After 22 days acclimation at 15 mg N/L, the system achieved >90% stable NO3−-N removal. Notably, the system demonstrated exceptional stability at a high load of 140 mg N/L, with an average total nitrogen removal rate of 98% (peaking at 99%). This performance indicates a successful adaptation to elevated loading conditions. The high removal efficiency is attributed to the coordinated metabolic processes within the system. As the nitrate load increased, sulfur oxidation accelerated, releasing protons and causing a decline in pH. The ratio of sulfate production to nitrate removal (6.8–10 mg/mg) remained consistent with the theoretical value of 7.54 mg/mg, confirming the integrity of the sulfur metabolic pathway across various loading conditions. Furthermore, the nitrate load acted as a selective pressure, promoting the dominance of Pseudomonadota and Thiobacillus to establish a robust functional microbial ecosystem. In conclusion, this study confirms the viability of the SAD process for treating wastewater with medium to high nitrate concentrations. It also reveals the metabolic and community mechanisms that support performance stability under various load conditions.