Na2S Enhancement of Pyrrhotite/Sulfur Fixed Bed Reactors: Synergistic Denitrification Mechanism and Microbial Community Restructuring for Low-Alkalinity Advanced Nitrogen Removal

Advanced nitrogen removal from secondary effluent of municipal wastewater treatment plants (WWTPs) faces substantial technical challenges. This study investigated the denitrification performance and underlying mechanisms of a pyrrhotite/sulfur-coupled autotrophic denitrifying biological filter (PS-CFBR) enhanced by Na2S addition. This study investigated the denitrification performance of aPS-CFBR enhanced by Na2S addition, operated at a controlled hydraulic retention time (HRT) of 6 h with varying alkalinity dosages (149–446 mg/L as CaCO3) and influent sulfide-to-nitrogen (S/N) ratios (0.54–1.62). The results indicated that Na2S addition shortened the PS-CFBR start-up period by 10 days. At an HRT of 6 h and an alkalinity dosage of 149 mg/L as CaCO3, the TN removal efficiency in the Na2S-supplemented reactor (R1) was 22.87% higher than that in the control (CK). The corresponding first-order rate constant (k) in R1 was 2.65-fold greater than in CK. Under low-alkalinity conditions (149 mg/L as CaCO3), the effective influent S/N ratio was determined to be 0.54–1.08. The TN removal efficiency and rate constant (k) in R1 were 22.87% and 2.65-fold higher than those in CK at an S/N ratio of 0.54, respectively. The effluent pH remained stable at 7.30, SO42− production was only 5.16 mg/L higher than that in CK, and alkalinity consumption per mg of N removed was 2.15 mg/L lower than that in CK. X-ray photoelectron spectroscopy (XPS) and microbial community analyses revealed that Na2S promoted Sₙ2− formation on the pyrrhotite surface and enriched denitrifying (Herbaspirillum, Flavobacterium, Sulfurimicrobium), iron-oxidizing (Pseudoxanthomonas), and iron-reducing (Clostridium) bacteria. RT-qPCR further indicated that Na2S addition increased the abundances of denitrification functional genes (narG, nirS, nirK, norB, and nosZ). These findings provide valuable insights into the development of advanced denitrification technologies for secondary effluent from municipal wastewater treatment plants.