Exceptional nitrate and phosphate removals from low carbon-to-nitrogen ratio wastewater with a novel pyrite-sulfur composite filler

This study developed a pyrite-coupled sulfur autotrophic (PSAD) with a novel sulfur/pyrite composite filler to remove nitrogen and phosphorus at exceptional rates from wastewater of a low C/N ratio. The composite filler (radius 2.5 mm) was made by mixing the pyrite and sulfur powders at a volume ratio of 2:1, followed by melting with rapid stirring at 170℃ and cooling naturally. Three PSAD reactors with different sulfur/pyrite packings were tested: R1, which applied distinct layers of sulfur and pyrite particles; R2, which mixed the sulfur and pyrite particles in the filler; and R3, which employed the yielded pyrite-sulfur composite filler. The R3 demonstrated superior nutrient removals than R1 and R2, reaching 98.8 % total nitrogen and 96.2 % total phosphorus at a steady-state test of a hydraulic retention time of 12 h. The sulfate generated by R3 was 18.2 % less than the theoretical value of the sulfur autotrophic denitrification. The tested pyrite-sulfur composite filler provides more attachment sites, increasing the abundances of functional microorganisms for nitrogen, sulfur, and iron cycling, including Thiobacillus (73.7 % in R3 than 62.1 % in R1 and 64.5 % in R2), Ferritrophicum (11.7 % in R3 than 7.74 % in R1 and 8.66 %), and Geothrix (3.04 in R3 than 2.22 % in R1 and 2.47 % in R2). These microbes were claimed to collaboratively employ sulfur-based and pyrite-driven metabolic routes to achieve denitrification while promoting iron redox cycling (Fe²⁺/Fe³⁺ transformations) that facilitated the synergistic removal of phosphorus through combined adsorption and potential precipitation processes.