Revisit the role of hydroxylamine in sulfur-driven autotrophic denitrification

Through dedicated batch tests using the enriched sludge dominated by sulfur-oxidizing bacteria (SOB), the potential transformation of hydroxylamine (NH₂OH) by SOB and the effects of NH₂OH on the rate-limiting sequential reduction processes of sulfur-driven autotrophic denitrification (SDAD) were systematically explored in this study. The results indicated that NH₂OH might be first converted to NO by SOB and then participate in the SDAD process, thus accelerating the utilization of S^2- and contributing to the formation of N₂O. Up to 3.5 mg-N/L NH₂OH didn't affect the NO₃^- or NO₂^- reduction of SDAD, during which no significant changes were observed for the NH₂OH concentration. Comparatively, even though NH₂OH had no direct impact on the N₂O reduction of SDAD, it could be consumed and therefore affect the depletion of N₂O indirectly by regulating the toxic effect and electron supply of S^2-. These findings provide novel implications for applying NH₂OH to SDAD-based integrated processes for biological nitrogen removal.