Sulfide-driven denitrification process for low C/N ratio wastewater treatment

This study presented a novel strategy employing sulfide (Na₂S) as the external electron donor and enzyme activity inhibitor to enhance nitrogen removal and enable nitrous oxide (N₂O) recovery in low carbon-to-nitrogen (C/N) ratio wastewater treatment. With the addition of Na₂S, the system achieved a maximum total nitrogen (TN) removal of 100.0 % while accumulating nitrogen oxide (N₂O) and elemental sulfur (S⁰) at an average yield of 60.5 % and 69.1 %, respectively. Systematic investigations under varying Na₂S concentrations (0.0-225.0 mg S/L) and C/N ratios (1.0:0.0-6.0:1.0) demonstrated that C/N ratio and Na₂S concentration had significant effects on the nitrogen removal performance and microbial community; high concentration of Na₂S (225.0 mg-S/L) would inhibit nitrogen removal efficiency, and with the decrease of C/N ratio and increase of Na₂S concentration, the microbial community structure gradually shifted from heterotrophic bacteria to autotrophic bacteria. Transcriptomic analysis revealed that Na₂S addition suppressed nitrous oxide reductase (N₂OR) activity (0.52 ± 0.04 μmol N/(min mg protein) → 0.23 ± 0.03 μmol N/(min mg protein)) and downregulated the nosZ gene cluster (-4.53), redirecting electron flux toward sulfur oxidation pathways dominated by sulfide:quinone oxidoreductase (SQR), favoring N₂O accumulation. By integrating autotrophic denitrification with targeted N₂O and S⁰ recovery, this work established a sustainable framework for low-C/N wastewater treatment, eliminating organic carbon dependency and transforming nitrogen pollution into valorized resources, contributing to the optimization of sulfur-mediated biological nitrogen removal (BNR) processes, and providing promising insights for economy-driven wastewater management.