Effect of Reflux Ratio on Performance of Pyrite-Based Mixotrophic Sequencing Batch Biofilm Reactor Treating Real Domestic Wastewater

The influent of municipal wastewater treatment plants (WWTPs) in China is generally facing the problem of insufficient carbon sources. Pyrite-based autotrophic denitrification (PAD), which uses pyrite as an electron donor, has good advantages of less acid production, low sulfate generation, and efficient simultaneous nitrogen and phosphorus removal from wastewater. In this study, a novel type of mixotrophic sequencing batch biofilm reactor (SBBR) was constructed by combining PAD with SBBR, and the effect of reflux ratio on the treatment efficiency of pyrite-based SBBR was studied. The research showed that the addition of reflux improved the removal capability of nitrogen and phosphorus, and the optimal reflux ratio for the pyrite-based SBBR was 200% when the removal rates of CODCr, NH4+-N, TN, and PO43−-P reached 71.07%±5.33%, 80.94%±7.23%, 53.94%±3.34%, and 49.68%±11.85%, respectively in the anoxic environment with pH 7–8. The variation of pollutants in the typical operating cycle showed that most of the organic pollutants were removed in the aerobic stage. The analysis of extracellular polymeric substances (EPS) further verified that the addition of reflux was beneficial to the removal of nitrogen and phosphorus.