Hydroxylamine diffusion can enhance N₂O emissions in nitrifying biofilms: a modeling study

Area of Science:

• Environmental Microbiology
• Environmental Engineering
• Biogeochemical Cycles

Background:

• Wastewater treatment plants (WWTPs) are significant sources of nitrous oxide (N2O), a potent greenhouse gas.
• N2O emissions from WWTPs are primarily linked to nitrification and denitrification processes.
• Limited understanding exists regarding N2O emissions specifically from biofilm processes within WWTPs.

Purpose of the Study:

• To adapt a suspended-growth mathematical model to investigate N2O emissions from nitrifying biofilms.
• To elucidate the mechanisms of N2O formation and release from biofilms, focusing on ammonia-oxidizing bacteria (AOB).
• To assess the influence of operational parameters on N2O emissions in biofilm systems.

Main Methods:

• Adaptation of a suspended-growth mathematical model to simulate nitrifying biofilms.
• Inclusion of N2O production pathways: hydroxylamine oxidation and nitrifier denitrification by AOB.
• Simulation of N2O emissions under varying dissolved oxygen (DO) concentrations, ammonia levels, and biofilm thicknesses.

Main Results:

• Modeled N2O emissions from nitrifying biofilms were significantly higher than from suspended growth systems.
• Hydroxylamine diffusion from aerobic to anoxic zones within the biofilm was identified as a key driver of elevated N2O emissions.
• Intermediate DO concentrations, higher bulk ammonia, and increased biofilm thickness enhanced N2O emissions.
• The model accurately predicted N2O emissions in a pilot-scale reactor, except when minimal NH2OH diffusion was assumed.

Conclusions:

• Hydroxylamine plays a critical, previously underestimated role in N2O emissions from nitrifying biofilms.
• Biofilm structure and operational conditions (DO, ammonia) significantly influence N2O production.
• Findings highlight the need to consider biofilm processes for accurate greenhouse gas accounting in WWTPs.