Pyrite-based mixotrophic denitrification promoted nitrogen removal from actual secondary effluent in natural aerobic...

Implementing advanced nitrogen removal in wastewater treatment plants has emerged as a critical strategy to comply with tightening global nitrogen discharge mandates. To achieve advanced nitrogen removal under low-carbon treatment conditions, pyrite-based mixotrophic denitrification (PMD) demonstrates promising application prospects. However, existing research predominantly focuses on synthetic wastewater in laboratory settings, with insufficient attention to practical applications using complex real wastewater. This study investigated PMD treating actual secondary effluent (TN: 16-25 mg/L) from a full-scale wastewater treatment plant under natural aerobic conditions. Through optimized pyrite particle size (1.2-2.4 mm) and carbon-to-nitrogen ratio (C/N = 3:1), PMD achieved 90.8 % TN removal efficiency with effluent TN < 2 mg/L, meeting stringent discharge standards. Under optimized conditions, oxygen boosted S²⁻/Fe²⁺ release from pyrite. The 1.2-2.4 mm particles created micro-aerobic and micro-anaerobic niches, which facilitated synergistic symbiosis between autotrophic and heterotrophic denitrifiers. This microbial collaboration endowed the system with enhanced denitrification performance and improved filtration characteristics. The dominant bacterial genera in optimal conditions were unclassified_f_Rhodocyclaceae (22.4 %), unclassified_f_Comamonadaceae (16.3 %), and Thauera (5.9 %), while nirK-type denitrifying bacteria dominated the denitrification in this system. This research offers valuable recommendations for developing an energy-efficient and low-carbon advanced nitrogen removal process in wastewater treatment facilities.