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

This study presented a novel strategy employing sulfide (NaS) as the external electron donor and enzyme activity inhibitor to enhance nitrogen removal and enable nitrous oxide (NO) recovery in low carbon-to-nitrogen (C/N) ratio wastewater treatment. With the addition of NaS, the system achieved a maximum total nitrogen (TN) removal of 100.0 % while accumulating nitrogen oxide (NO) and elemental sulfur (S) at an average yield of 60.5 % and 69.1 %, respectively. Systematic investigations under varying NaS 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 NaS concentration had significant effects on the nitrogen removal performance and microbial community; high concentration of NaS (225.0 mg-S/L) would inhibit nitrogen removal efficiency, and with the decrease of C/N ratio and increase of NaS concentration, the microbial community structure gradually shifted from heterotrophic bacteria to autotrophic bacteria. Transcriptomic analysis revealed that NaS addition suppressed nitrous oxide reductase (NOR) 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 NO accumulation. By integrating autotrophic denitrification with targeted NO and S 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.