Predicting N₂O emissions from nitrifying and denitrifying biofilms: a modeling study.

Area of Science:

• Environmental Microbiology
• Environmental Engineering
• Greenhouse Gas Emissions

Background:

• Wastewater treatment plants are significant sources of nitrous oxide (N₂O), a potent greenhouse gas.
• While N₂O emissions from suspended-growth systems are well-studied, those from biofilm processes remain less understood.
• Biofilm structure, with substrate gradients and microbial stratification, may influence N₂O production differently than suspended-growth systems.

Purpose of the Study:

• To explore the mechanisms of N₂O emissions from nitrifying and denitrifying biofilms using mathematical modeling.
• To compare N₂O emissions from biofilm processes with those from suspended-growth systems.

Main Methods:

• Development and application of mathematical models for ammonia-oxidizing bacteria (AOB) and denitrifying biofilms.
• Simulation of N₂O production and diffusion within biofilm structures.
• Analysis of the influence of key intermediates (hydroxylamine, nitrite) and environmental factors on emissions.

Main Results:

• N₂O emissions from AOB biofilms can be significantly greater than from suspended-growth systems, driven by hydroxylamine diffusion.
• The presence of nitrite-oxidizing bacteria further enhances N₂O emissions in nitrifying biofilms.
• Denitrifying biofilm emissions are generally higher than suspended-growth systems, with magnitude dependent on dissolved oxygen, COD, nitrate, and biofilm thickness.

Conclusions:

• The accumulation and diffusion of intermediates like hydroxylamine and nitrite are key factors distinguishing N₂O emissions from biofilms.
• Mechanisms of N₂O emissions from biofilms are more complex than from suspended-growth systems.
• Biofilm processes in wastewater treatment may lead to higher N₂O emissions in many operational scenarios.