Exposure of sulfur-driven autotrophic denitrification to hydroxylamine/hydrazine: Underlying mechanisms and implications for promoting partial denitrification and N2O recovery

Hydroxylamine (NH2OH) and hydrazine (N2H4) are crucial intermediates in biological nitrogen removal processes, but their potential effects on the nitrogen-related metabolic pathway during the sulfur-driven autotrophic denitrification (SDAD) process remain unclear and were thus explored in this work. The results of dedicated batch tests indicated that the accumulation ratio of NO2− was positively correlated with the NH2OH concentration, reaching 86.2 ± 2.6% at the NH2OH concentration of 2.0 mg-N/L, while it was barely affected by the N2H4 concentration. Furthermore, the joint inhibition of NH2OH and free nitrous acid led to the substantial accumulation of N2O (up to 32.9 ± 1.3%) during the NO2− reduction phase. In contrast, N2H4 exhibited great potential for facilitating N2O accumulation in both the NO3− reduction and NO2− reduction phases with an accumulation ratio of 13.0 ± 0.3% and 27.5 ± 1.5%, respectively, at the N2H4 concentration of 2.0 mg-N/L. Total enzyme activity analyses revealed that the presence of NH2OH inhibited the activity expression of all denitrifying enzymes, with the downstream enzymes (such as NO2− reductase and N2O reductase) being more susceptible to NH2OH. The addition of N2H4 did not cause a significant decrease in the activity of NO3− reductase and NO2− reductase, but notably curbed the activity of N2O reductase. Based on the responses of the SDAD-related nitrogen metabolic pathway to NH2OH/N2H4, this work provided effective technical means for i) the promotion of NO2− accumulation which might enable the coupling of SDAD with Anammox and ii) the efficient recovery of N2O via the SDAD process.