Nitrous oxide (N₂O) is a powerful greenhouse gas that can be emitted during wastewater nitrification and denitrification. While much has been learned about N₂O emissions from suspended growth processes, little is known about emissions from biofilm, which are of increasing interest for wastewater treatment. Since biofilms exhibit complex substrate gradients and microbial stratification, it very difficult to predict their behavior.
This research uses mathematical modeling to address the mechanisms of emissions from nitrifying biofilms, denitrifying biofilms, and combined nitrifying and denitrifying (SND) biofilms. For the nitrifying biofilm, the model included N₂O formation by ammonia-oxidizing bacteria (AOB) via the hydroxylamine (NH₂OH) and the nitrifier denitrification pathways. N₂O emissions from AOB biofilms of different thicknesses were explored as a function of bulk dissolved oxygen (D.O.). The results suggested that N₂O emissions from thicker nitrifying biofilms were greater than those from suspended growth systems. The main cause is the diffusion of NH₂OH, an AOB nitrification intermediate, from the outer, aerobic regions of biofilm to the inner, anoxic regions. The presence of NOB also enhanced N₂O emissions. This was due to an increased D.O. gradient in the biofilm.
The denitrifying biofilm model included N₂O formation by heterotrophs via conventional denitrification pathway. The denitrifying model showed that, in presence of excess of electron donor, denitrifying biofilms have two distinct layers of activity: an outer layer with net production of NO₂- and N₂O, and an inner layer with a net consumption. The biofilm thickness affects N₂O emissions, and rates are influenced by the electron availability within the biofilm. The presence of D.O. in the bulk led to a spike of N₂O formation in the outer biofilm, due to diffusion of NO₂- from the inner biofilm and its high affinity for reduced electron carriers relative to the other nitrogen oxides in the denitrification pathway. The model results were compared to those of an experimental system. Also, a model of N₂O emissions in a denitrifying filter was developed and used to explore emissions in a biofilm reactor.