Enhanced and sustainable advanced nitrogen removal in mixotrophic systems using pyrite and solid carbon source

Utilizing widespread minerals/solid wastes as electron donors for denitrification is conducive to sustainable wastewater treatment. The current denitrification technologies based on single pyrite/solid carbon sources have problems of limited removal efficiency or unstable carbon release. In this study, two continuous biofilters, pyrite-corncob mixotrophic system (R_PCM) and pyrite-polybutylene succinate mixotrophic system (R_PPM), were conducted and operated steadily for a long period (>326 d). The mixotrophic systems achieved advanced removal of NO₃^--N (18 mg L^-1) and a small amount of NH₄⁺-N (2.5 mg L^-1), with stabilized effluent TIN less than 2 mg L^-1 at HRT of 4 h. Additionally, the systems demonstrated several distinct advantages, including no additional alkalinity requirement and a low risk of secondary contamination. R_PCM could achieve advanced nitrogen removal at a higher nitrogen loading rate (93.6 mg L^-1 d^-1) but demanded periodic replenishment of corncob. In contrast, the organic matter release and nitrogen removal performance of R_PPM exhibited stability throughout the operation. The increased abundance of functional microorganisms related to C, N, S, and Fe metabolism was essential for advanced nitrogen removal through synergistic effects. This study will provide implications for developing novel wastewater treatment processes emphasizing both nitrogen removal and waste valorization.