Modeling mechanisms of formation of nitrous oxide (N2O) in nitrifying and denitrifying biofilms

Wastewater treatment plants (WWTPs) engaging in biological nutrient removal (BNR) including nitrification and denitrification can be a significant source of nitrous oxide (N2O) emissions. Most studies on N2O emission from BNR processes have explored suspended-growth systems. Few studies have addresses biofilm processes. This study used mathematical modeling to provide insights into N2O emissions from nitrifying and denitrifying biofilms. For purely nitrifying processes, results showed higher emissions from biofilms than from suspended-growth systems under similar conditions. The key factor for the emissions was the diffusion of hydroxylamine (NH2OH) from the aerobic to the anoxic zones of the biofilm. Model findings were supported by experimental data from the literature. Preliminary results from the denitrification model suggest that N2O emissions from denitrifying biofilms also differ from those in suspended growth systems. Differences vary with concentrations of dissolved oxygen, COD, and nitrate, as well as biofilm thickness. These modeling efforts suggest that N2O emissions from nitrifying and denitrifying biofilms cannot be readily inferred from research on suspended-growth systems.