New insights into N2O emission and electron competition under different chemical oxygen demand to nitrogen ratios in a biofilm system

Nitrous oxide (N2O) is a greenhouse gas that could accumulate during the heterotrophic denitrification process. In this study, the effects of different chemical oxygen demand to nitrogen ratio (COD/N) on N2O production and electron competition was investigated. The electron competition was intensified with the decrease of electron supply, and Nos had the best electron competition ability. The model simulation results indicated that the degradation of NOx-Ns was a combination of diffusion and biological degradation. As reaction proceeding, N2O could accumulate inside biofilm. A thinner biofilm and a longer hydraulic retention time (HRT) might be an effective way to control N2O emission. The application of mathematical model is an opportunity to gain deep understanding of substrate degradation and electron competition inside biofilm. [Display omitted] •Electron competition was evaluated under different COD/N ratios.•Utilization of electron donors led to intensification of electron competition.•Nos had a dominance in electron competition during denitrification process.•A model based on ASM-ICE was calibrated to describe denitrification inside biofilm.•Accumulation of N2O was predicted in different sites under different biofilm depths.