Evaluating four mathematical models for nitrous oxide production by autotrophic ammonia-oxidizing bacteria

There is increasing evidence showing that ammonia-oxidizing bacteria (AOB) are major contributors to N₂O emissions from wastewater treatment plants (WWTPs). Although the fundamental metabolic pathways for N₂O production by AOB are now coming to light, the mechanisms responsible for N₂O production by AOB in WWTP are not fully understood. Mathematical modeling provides a means for testing hypotheses related to mechanisms and triggers for N₂O emissions in WWTP, and can then also become a tool to support the development of mitigation strategies. This study examined the ability of four mathematical model structures to describe two distinct mechanisms of N₂O production by AOB. The production mechanisms evaluated are (1) N₂O as the final product of nitrifier denitrification with NO₂^- as the terminal electron acceptor and (2) N₂O as a byproduct of incomplete oxidation of hydroxylamine (NH₂OH) to NO₂^-. The four models were compared based on their ability to predict N₂O dynamics observed in three mixed culture studies. Short-term batch experimental data were employed to examine model assumptions related to the effects of (1) NH₄⁺ concentration variations, (2) dissolved oxygen (DO) variations, (3) NO₂^- accumulations and (4) NH₂OH as an externally provided substrate. The modeling results demonstrate that all these models can generally describe the NH₄⁺, NO₂^-, and NO₃^- data. However, none of these models were able to reproduce all measured N₂O data. The results suggest that both the denitrification and NH₂OH pathways may be involved in N₂O production and could be kinetically linked by a competition for intracellular reducing equivalents. A unified model capturing both mechanisms and their potential interactions needs to be developed with consideration of physiological complexity. © 2012 Wiley Periodicals, Inc.