Enhancing Sulfide and Methane Control in Sewers: Unveiling the Impact and Mechanisms of Free Nitrous Acid Pre-Exposure Coupled with Calcium Peroxide Dosing

Although biocides have proven highly efficient in controlling hazardous emissions and mitigating organic carbon loss in sewers, the resistance of biofilm and extracellular polymeric substances (EPS) to biocides has emerged as a significant impediment to addressing these challenges. In this study, a promising strategy of calcium peroxide (CaO2) dosing assisted by free nitrous acid (FNA) pre-exposure overcame these limitations. Prior to a 6-hour exposure to 0.1 % (w/v) CaO2, a 2-hour pre-exposure to a low dose of 0.26 mg-N/L FNA demonstrated a significant enhancement in sulfide and methane control efficiency by 267.14 % and 390.47 %, respectively, compared to direct exposure to 0.2 % CaO2 for 12 h. Moreover, there was a notable increase of 40.05 % in soluble COD level in the effluent. FT-IR and XPS analyses discovered the disruption, migration, and dissolution of biofilm, accompanied by a substantial increase in membrane permeability (up to 2.89 folds). Additional analysis revealed that the synergistic effects of various reactive nitrogen/oxygen species destabilized the structure of EPS, e.g., declining total EPS yield, disintegrating humic acid-like and tryptophan- and protein-like substances, reducing hydrogen bond in β-sheet of proteins. Further down-regulation of node genes associated with sulfidogenesis and methanogenesis verified the toxic mechanism at the genetic level. Microbial community analysis revealed a significant reduction in the abundance of sulfate-reducing bacteria and methanogenic archaea, leading to an extended recovery period for sulfide and methane production in sewer biofilm. This study provides an effective strategy and in-depth mechanisms for biofilm control, applicable beyond sewer biofilm.