Deciphering nitrogen removal enhancement mechanism in anammox reactor under various pyrite doses: Competition-driven multi-pathway nitrogen removal and enrichment synergy

Anammox is an energy-efficient nitrogen removal process, but its practical application is constrained by limited bacterial activity, while its enhancement mechanism by pyrite remains poorly understood. This study systematically evaluated the effects of pyrite at dosages of 0–2.0 g/L on anammox reactor performance, and deciphered the underlying mechanism. Results demonstrated that pyrite exerted dose-dependent enhancement effects on anammox systems by iron–sulfur utilization competition drives differential enrichment of bacteria and differentiation of nitrogen removal synergistic modes. Low dosage (0–0.4 g/L) pyrite activate sulfur autotrophic denitrification partially replace anammox in nitrogen removal. Optimal dosage (0.8–1.2 g/L) pyrite achieving a 16.9 ± 1.4% nitrogen removal rate (NRR) elevation through synchronous enrichment and synergy of nitrate-dependent ferrous oxidation (NDFO) and anammox bacteria. High dosage (1.6–2.0 g/L) pyrite only yielded 7.8 ± 1.3% NRR increase via oxidative stress induces endogenous denitrification. This study clarifies the dose-response mechanism of pyrite-enhanced anammox, providing a regulation strategy for efficient operation.