Pyrite autotrophic denitrification in a biological contactor for low-strength nitrate-contaminated water treatment

A novel pyrite autotrophic denitrification biological contactor (PAD-BC) process was developed for efficient nitrogen removal in water treatment, utilizing pyrite-loaded rope packings as both biofilm carrier and electron donor. Autotrophic denitrification rates with pyrite sizes of 0.55-0.05 mm, loadings of 2.5-12.5 g/cm, and nitrate concentrations of 7-28 mg/L followed first-order kinetic model, with rate constants of 0.402-0.501, 0.078-0.487, and 0.445-0.484 /d, respectively. Increased pyrite loading and nitrate concentration enhanced nitrite accumulation and sulfate production. The PAD-BC reactor with the optimal rope configuration-pyrite particle size of 0.38-0.15 mm and a loading of 10 g/cm-achieved rapid start-up within 8 days and a high nitrogen removal efficiency of 99.0 ± 0.004 % under a nitrogen loading rate of 7.08 ± 0.11 mg N/(L·d). Microbial analysis revealed enrichment of Thiobacillus and Paenisporosarcina at the pyrite-rope biofilm interface. Functional predictions indicated higher sulfur cycling, manganese oxidation, and aromatic compound degradation within the biofilm. These findings provide valuable insights into the design and application of PAD-BC systems for treating low-strength nitrate-contaminated water.