Effect of H₂S on N₂O Reduction and Accumulation during Denitrification by Methanol Utilizing Denitrifiers

Sulfide is produced in sewer networks, and previous studies suggest that sulfide in sewage could alter the activity of heterotrophic denitrification and lead to N₂O accumulation during biological wastewater treatment. However, the details of this phenomenon are poorly understood. In this study, the potential inhibitory effects of sulfide on nitrate, nitrite, and N₂O reduction were assessed with a methanol-utilizing denitrifying culture both prior to and after its exposure and adaptation to sulfide. Hydrogen sulfide was found to be strongly inhibitory to N₂O reduction, with 50% inhibition observed at H₂S concentrations of 0.04 mg H₂S–S/L and 0.1 mg H₂S–S/L for the unadapted and adapted cultures, respectively. In comparison, both nitrate and nitrite reduction was more tolerant to H₂S. A 50% inhibition of nitrite reduction was observed at approximately 2.0 mg H₂S–S/L for both unadapted and adapted cultures, while no inhibition of nitrate reduction occurred at the highest H₂S concentrations applied (2.0 mg H₂S–S/L) to either culture. N₂O accumulation was observed during nitrate and nitrite reduction by the adapted culture when H₂S concentrations were above 0.5 and 0.2 mg H₂S–S/L, respectively. Additionally, we reveal that hydrogen sulfide (H₂S), rather than sulfide, was likely the true inhibitor of N₂O reduction, and the inhibitory effect was reversible. These findings suggest that sulfide management in sewers could potentially have a significant impact on N₂O emission from wastewater treatment plants