Start-up phase optimization of pyrite-intensified hybrid sequencing batch biofilm reactor (PIHSBBR): Mixotrophic denitrification performance and mechanism.

Pyrite-based autotrophic denitrification (PAD) is an emerging biological process to diminish nitrate pollution, but the relatively low NO3--N removal rate limits its practical application. In this research, a pyrite-intensified hybrid sequencing batch biofilm reactor (PIHSBBR) was designed to treat low C/N ratio domestic wastewater. The results showed that PIHSBBR could achieve optimal removal of COD, NH4+-N, and TN under the aeration rate of 1.0 L/L∙min and the hydraulic retention time (HRT) of 8 h, with removal rates of 69.67 ± 4.37%, 77.04 ± 4.84%, and 63.92 ± 6.66%, respectively. The PAD efficiency in PIHSBBR during the stable operation was not high (13.05-31.01%), and the main nitrogen removal pathway in PIHSBBR, especially in the aerobic zone, was simultaneous nitrification and denitrification (SND). High-throughput sequencing analysis unraveled that Planctomycetota (3.65%) had a high abundance in the anoxic zone of PIHSBBR, implying that anaerobic ammonium oxidation (anammox) might have occurred in the anoxic zone. In addition, the nitrogen cycle function gene with the highest abundance was nirBD, indicating the possible presence of dissimilatory nitrate reduction to ammonium (DNRA) within the system (aerobic and anoxic zones). Our research can provide useful information for the improvement and future application of PIHSBBR.