Electron competition (EC) and pH stress are two key factors influencing NO 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 NO production by characterizing the denitrifying kinetics in batch tests. The model exhibits good fitting results on NO accumulation across a wide NO/NO ratio range from 0 to 9, demonstrating its excellent ability in describing the EC effect and determining rate constants for NO and NO reductions. Therefore, accurate pH-dependent relationships of NO and NO reduction rates in a pH range of 5.0-10.6 are demonstrated using this model. The results show that EC dominates NO 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 NO/NO ratio greater than 0.1 or a COD/NO ratio smaller than 3 favors NO production due to the preferential supply of electrons to NO 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.
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