Modeling of Nitrous Oxide Production by Autotrophic Ammonia-Oxidizing Bacteria with Multiple Production Pathways

Abstract

Autotrophic ammonia oxidizing bacteria (AOB) have been recognized as a major contributor to N₂O production in wastewater treatment systems. However, so far N₂O models have been proposed based on a single N₂O production pathway by AOB, and there is still a lack of effective approach for the integration of these models. In this work, an integrated mathematical model that considers multiple production pathways is developed to describe N₂O production by AOB. The pathways considered include the nitrifier denitrification pathway (N₂O as the final product of AOB denitrification with NO₂^– as the terminal electron acceptor) and the hydroxylamine (NH₂OH) pathway (N₂O as a byproduct of incomplete oxidation of NH₂OH to NO₂^–). In this model, the oxidation and reduction processes are modeled separately, with intracellular electron carriers introduced to link the two types of processes. The model is calibrated and validated using experimental data obtained with two independent nitrifying cultures. The model satisfactorily describes the N₂O data from both systems. The model also predicts shifts of the dominating pathway at various dissolved oxygen (DO) and nitrite levels, consistent with previous hypotheses. This unified model is expected to enhance our ability to predict N₂O production by AOB in wastewater treatment systems under varying operational conditions