Optimization of key operational parameters in a pyrite driven mixotrophic denitrification system

With the intensification of water eutrophication, traditional nitrogen and phosphorus removal processes face challenges such as low efficiency and high cost under low-carbon conditions. In this study, a pyrite driven mixotrophic denitrification(PMD) system was constructed in batch reactors. The effects of pyrite dosage, C/N ratio,pH and temperature on nitrogen and phosphorus removal were systematically investigated. Results showed that optimal performance was achieved at pyrite dosage of 60-150 g/L and C/N ratio of 3, with total nitrogen(TN) and total phosphorus(TP) removal rates reaching 91.73% and 99.85%, respectively. Excessive pyrite addition induced passivation and nitrite accumulation, inhibiting denitrification, while overly high C/N ratios suppressed autotrophic denitrification and reduced phosphorus removal. Neutral pH(pH=7) and moderate temperatures(25-35 ℃) provided the best synergistic performance. Acidic conditions inhibited enzyme activity and induced sludge hydrolysis, whereas alkaline conditions hindered electron transfer and triggered dissimilatory nitrate reduction. Low temperatures reduced enzymatic activity, while high temperatures damaged enzyme stability, both leading to efficiency loss. Sludge analysis revealed that increased pyrite raised sludge concentration but excessive dosage caused inorganic accumulation,reducing mass transfer and system stability. This study clarified key operational parameters for PMD and provided theoretical support for its engineering application in simultaneous nitrogen and phosphorus removal.