Mechanisms of nitrous oxide (N₂ O) formation and reduction in denitrifying biofilms

Nitrous oxide (N₂ O) is a potent greenhouse gas that can be formed in wastewater treatment processes by ammonium oxidizing and denitrifying microorganisms. While N₂ O emissions from suspended growth systems have been extensively studied, and some recent studies have addressed emissions from nitrifying biofilms, much less is known about N₂ O emissions from denitrifying biofilm processes. This research used modeling to evaluate the mechanisms of N₂ O formation and reduction in denitrifying biofilms. The kinetic model included formation and consumption of key denitrification species, including nitrate (NO3-), nitrite (NO2-), nitric oxide (NO), and N₂ O. 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 N₂ O and an inner layer where there is net consumption. The presence of oxygen (O₂ ) had an important effect on N₂ O 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 N₂ O emissions are complex and not always intuitive. A key mechanism for denitrifying biofilms is the diffusion of N₂ O and other intermediates from one zone of the biofilm to another. This leads to zones of N₂ O formation or consumption transformations that would not exist in suspended growth systems.