Modeling Denitrification as an Electric Circuit Accurately Captures Electron Competition between Individual Reductive Steps: The Activated Sludge Model–Electron Competition Model

Heterotrophic denitrification consists of the four-step sequential reduction of nitrate to dinitrogen gas over nitrite, nitric oxide, and nitrous oxide. Oxidation processes, commonly of organic compounds, provide the electrons needed for the sequential reaction steps. The intracellular electron distribution is a competitive process among the four reduction steps. In this study, a model describing organic carbon oxidation and four-step denitrification through electron competition is proposed [Activated Sludge Model–Electron Competition (ASM–EC)]. The model describes denitrification rates as an analogy to how current intensity varies through a parallel set of resistors in electric circuits. The ASM–EC model was calibrated with data from batch experiments with heterotrophic denitrifying communities, where reduction of mixtures of nitrogen oxides was monitored, while different carbon sources were supplied in excess. The carbon sources included methanol, ethanol, acetate, and their ternary mixture. The electron distribution preference and electron uptake rates varied between the carbon sources and were captured by the model structure for most of the experiments. The ASM–EC model uses fewer parameters compared to existing state-of-the-art denitrification models and performed equally well in the tested scenarios. We advocate the use of this model for denitrification in the activated sludge model, which can easily be integrated in existing model structures, because it provides a parsimonious description of electron competition during denitrification.