Calcium-modified siderite-driven optimization of sulfur autotrophic denitrification systems: Synergistic nitrogen/phosphorus removal and multidimensional mechanism insights.

Sulfur autotrophic denitrification (SAD) is a promising technology for secondary treatment. However, existing methods struggle with coordinating sulfur oxidation-acidification, limiting efficient nitrogen and phosphorus removal from medium to high- strength wastewater. This study proposes a calcium-modified siderite-sulfur autotrophic denitrification (SCAD) process. Through long-term (90 d) continuous-flow comparisons of SAD, sulfur-siderite denitrification, and SCAD, the study evaluates SCAD's advantages in N/P removal, sulfate suppression, and pH buffering. The Fe2+/Ca2+ slow-release properties of Siderite-Ca stabilize pH and enhance denitrification. Phosphorus removal occurred via Fe-P and Hydroxyapatite precipitates. Microbial community analysis reveals that increased biomass, alongside the functional co-enrichment of Thiobacillus, Sulfurimona, and Ferritrophicum, drives enhanced pollutant removal. The Fe2+/S0 co-electron donor mechanism upregulates key denitrification (narG, nirK) and sulfur oxidation (sqr) genes. This study establishes SCAD as a novel strategy for concurrent nitrogen-phosphorus elimination with pH stability, while elucidating microbial community evolution and contaminant degradation mechanisms.