Modeling effects of H2S on electron competition among nitrogen oxide reduction and N2O accumulation during denitrification

Abstract

Nitrous oxide (N₂O) accumulation during heterotrophic denitrification can be severely affected by hydrogen sulfide (H₂S) that is generated from sewer networks and introduced to the denitrification system. In this study, a new mathematical model was developed to assess nitrogen oxide dynamics during heterotrophic denitrification in the presence of different H₂S concentrations. The model describes electron competition during three-step denitrification (i.e., nitrate to nitrite, nitrite to N₂O, and N₂O and nitrogen gas) through linking nitrogen reduction and carbon oxidation processes with a pool of electron carriers and considering a non-competitive H₂S inhibition function on nitrite and N₂O reduction kinetics. The developed model well reproduced experimental nitrogen oxide data caused by the H₂S inhibition on denitrification with methanol-utilizing denitrifying cultures under different operational scenarios, indicating model validity and applicability. Modeling results revealed that N₂O accumulation could be increased with elevated H₂S levels, due to the decreased electron competition capacity of N₂O reduction compared to that of nitrate and nitrite reduction. Increasing electrons were distributed to nitrate and nitrite reduction due to their higher tolerance to H₂S. Less N₂O accumulation can be achieved by extending the feeding period of H₂S.