Effect of calcinated pyrite on simultaneous ammonia, nitrate and phosphorus removal in the BAF system and the Fe2+ regulatory mechanisms: Electron transfer and biofilm properties

To efficiently remove nitrogen and phosphorus from secondary effluent with low values of COD/TN, a novel biological aerated filter (BAF) utilizing calcined pyrite with a large specific surface area (SSA) and pore diameter (PD) was designed to address this challenge. From the perspective of nutrients removal performance, and the corresponding effluent total nitrogen (TN) and PO43--P in the calcined pyrite autotrophic denitrification (CPAD) process decreased from 40.21 to 1.07 mg/L to 1.22 and 0.14 mg/L, respectively. Furthermore, the nutrients removal kinetics analysis showed that the CPAD and pyrite autotrophic denitrification (PAD) processes could be fitted with Half-order and Zero-order reactions via kinetics analysis, respectively, indicating that the TN removal performance of CPAD processes was better than that of the PAD process. Moreover, CPAD combined with sulfur autotrophic denitrification (SAD) processes was fitted by First-order reaction, and the TN removal performance was further enhanced over the CPAD process. From the perspective of microregulation, Fe2+ production in the PAD and CPAD processes could accelerate the electron transfer rate by increasing electron transport system activity (ETSA) and reducing electrochemical impedance spectroscopy (EIS). Moreover, Fe2+ stimulated microbes to produce more proteins (PN) and C-10-HSL, which improved biofilm stability and interspecific communication processes. Notably, nitrifiers and autotrophic denitrifiers were simultaneously enriched via detection of high-throughput sequencing of 16 S rRNA genes, which verified the feasibility of simultaneous nitrification and autotrophic denitrification. Therefore, BAF with calcined pyrite and sulfur as composite fillers have a considerable advantage in nutrients removal.