Modeling Nitrogen Oxide Formation in Biofilm Systems

Research has shown that biological nutrient removal (BNR) processes may be significant sources of nitrous oxide (N2O) and nitric oxide (NO) emissions. However, most research has focused on suspended growth systems. Little is known about emissions from biofilm systems, where substrate gradients and microbial stratifications within the biofilm can complicate predictions of aggregate behavior. This research presents a preliminary biofilm model that includes the sequential reduction of nitrate (NO3−), nitrite (NO2−), NO, and N2O by heterotrophic bacteria, as well as autotrophic nitrification and denitrification processes. The model demonstrated that biofilms are likely to produce NO and N2O when the bulk liquid is aerobic. For the simulated conditions with combined heterotrophic and autotrophic processes, a maximum of 0.78% of nitrogen removed was emitted as N2O. The results should be used with caution, as the calculated N2O and NO emissions were sensitive to bulk substrate concentrations and to several assumed kinetic parameters that were not available in the literature.