Wastewater treatment plants can be significant sources of nitrous oxide (N2O), a potent greenhouse gas. While our understanding of N2O emissions from suspended-growth processes has advanced significantly, less is known about emissions from biofilm processes. Biofilms may behave differently due to their substrate gradients and microbial stratification. In this study, we used mathematical modeling to explore the mechanisms of N2O emissions from nitrifying and denitrifying biofilms. Our ammonia-oxidizing bacteria biofilm model suggests that N2O emissions from biofilm can be significantly greater than from suspended-growth systems. The driving factor is the diffusion of hydroxylamine, a nitrification intermediate, from the aerobic to the anoxic regions of the biofilm. The presence of nitrite-oxidizing bacteria further increased emissions. For denitrifying biofilms, our results suggest that emissions are generally greater than for suspended-growth systems. However, the magnitude of the difference depends on the bulk dissolved oxygen, chemical oxygen demand, and nitrate concentrations, as well as the biofilm thickness. Overall, the accumulation and diffusion of key intermediates, i.e. hydroxylamine and nitrite, distinguish biofilms from suspended-growth systems. Our research suggests that the mechanisms of N2O emissions from biofilms are much more complex than suspended-growth systems, and that emissions may be higher in many cases.@en

Nitrous oxide (N2O) is a potent greenhouse gas that can be formed in wastewater treatment processes by ammonium oxidizing and denitrifying microorganisms. While N2O emissions from suspended growth systems have been extensively studied, and some recent studies have addressed emissions from nitrifying biofilms, much less is known about N2O emissions from denitrifying biofilm processes. This research used modeling to evaluate the mechanisms of N2O formation and reduction in denitrifying biofilms. The kinetic model included formation and consumption of key denitrification species, including nitrate (NO3 -), nitrite (NO3 -), nitric oxide (NO), and N2O. The model showed that, in presence of excess of electron donor, denitrifying biofilms have two distinct layers of activity: an outer layer where there is net production of N2O and an inner layer where there is net consumption. The presence of oxygen (O2) had an important effect on N2O emission from suspended growth systems, but a smaller effect on biofilm systems. The effects of NO3 - and O2 differed significantly based on the biofilm thickness. Overall, the effects of biofilm thickness and bulk substrate concentrations on N2O emissions are complex and not always intuitive. A key mechanism for denitrifying biofilms is the diffusion of N2O and other intermediates from one zone of the biofilm to another. This leads to zones of N2O formation or consumption transformations that would not exist in suspended growth systems.@en