1 research outputs found
Hydroxylamine Diffusion Can Enhance N<sub>2</sub>O Emissions in Nitrifying Biofilms: A Modeling Study
Wastewater treatment plants can be
significant sources of nitrous
oxide (N<sub>2</sub>O), a potent greenhouse gas. However, little is
known about N<sub>2</sub>O emissions from biofilm processes. We adapted
an existing suspended-growth mathematical model to explore N<sub>2</sub>O emissions from nitrifying biofilms. The model included N<sub>2</sub>O formation by ammonia-oxidizing bacteria (AOB) via the hydroxylamine
and the nitrifier denitrification pathways. Our model suggested that
N<sub>2</sub>O emissions from nitrifying biofilms could be significantly
greater than from suspended growth systems under similar conditions.
The main cause was the formation and diffusion of hydroxylamine, an
AOB nitrification intermediate, from the aerobic to the anoxic regions
of the biofilm. In the anoxic regions, hydroxylamine oxidation by
AOB provided reducing equivalents used solely for nitrite reduction
to N<sub>2</sub>O, since there was no competition with oxygen. For
a continuous system, very high and very low dissolved oxygen (DO)
concentrations resulted in lower emissions, while intermediate values
led to higher emissions. Higher bulk ammonia concentrations and greater
biofilm thicknesses increased emissions. The model effectively predicted
N<sub>2</sub>O emissions from an actual pilot-scale granular sludge
reactor for sidestream nitritation, but significantly underestimated
the emissions when the NH<sub>2</sub>OH diffusion coefficient was
assumed to be minimal. This numerical study suggests an unexpected
and important role of hydroxylamine in N<sub>2</sub>O emission in
biofilms