Synergistic enhancement of nitrogen removal in partial denitrification-anammox (PD/A) systems aided by FeS and ZVI for low carbon/nitrogen wastewater treatment

The partial denitrification-anammox (PD/A) process suffers from low efficiency and poor stability in low C/N wastewater treatment. This study developed an integrated FeS-zero-valent iron (ZVI) mediated system (PD/A-FeS@ZVI) to solve these issues. Batch experiments showed PD/A-FeS@ZVI achieved 97.06 % total nitrogen removal efficiency (TNRE), 42.12 %, 24.32 %, and 252.94 % higher than PD/A-FeS, PD/A-ZVI, and control, respectively. Long-term operation enhanced autotrophic denitrification, stabilizing anammox and increasing TNRE from 79.61 ± 1.03 % to 86.27 ± 0.31 %. Interfacial analysis revealed in-situ FeS deposition on ZVI (with iron oxides) reduced charge transfer resistance (R-ct) and ΔSO₄²⁻/TN_rem. Microbial analysis showed rare-but-active sulfate-reducing bacteria (SRB) promoted ZVI sulfidation and FeS regeneration; enriched sulfur/iron-oxidizing bacteria (SOB/IOB) strengthened nitrate reduction to protect anammox bacteria (AnAOB). Mechanistically, FeS primarily drove nitrate-to-nitrite reduction (2.39 ± 0.01 mg NO x − − N / h ), while ZVI provided electrons (0.47 ± 0.01 mg NO x − − N / h ) and facilitated sulfate reduction, forming a closed S-Fe cycle. This dual mediation strategy improves PD/A’s efficiency and stability, offering a sustainable solution for low C/N wastewater treatment.