Modeling electron competition and pH factor interactions improve cognitions of nitrite accumulation during...

Electron competition (EC) and pH stress are two key factors influencing NO₂^- production during denitrification, but their dominance and interactions in the full pH range are poorly understood. In this study, we propose a concise Electron Competition Inhibition (ECI) model to clarify the contribution of EC and pH stress to NO₂^- production by characterizing the denitrifying kinetics in batch tests. The model exhibits good fitting results on NO₂^- accumulation across a wide NO₃^-/NO₂^- ratio range from 0 to 9, demonstrating its excellent ability in describing the EC effect and determining rate constants for NO₃^- and NO₂^- reductions. Therefore, accurate pH-dependent relationships of NO₃^- and NO₂^- reduction rates in a pH range of 5.0-10.6 are demonstrated using this model. The results show that EC dominates NO₂^- production in a near-neutral pH range from 6.6-9.0, while pH stress plays a dominant role at pH 5.8-6.8 and 9.1-10.1. Within near-neutral range, maintaining a NO₃^-/NO₂^- ratio greater than 0.1 or a COD/NO₃^- ratio smaller than 3 favors NO₂^- production due to the preferential supply of electrons to NO₃^- reduction. This provides a kinetics kinetic strategy to shorten reaction time and a stoichiometric strategy to control the addition of carbon sources for maintaining partial denitrification in biological nitrogen removal processes.