Biofilm-mediated mitigation of nitrous oxide emissions in anammox-based wastewater treatment

Abstract

Wastewater treatment is a major source of global greenhouse gas (GHG) emissions, largely driven by nitrous oxide (N2O) release during nitrogen removal processes. While anaerobic ammonium-oxidizing (anammox) bacteria have become widely adopted to optimize nitrogen removal, the pathways governing N2O emissions in anammox systems remain insufficiently characterized. Findings revealed that in aerobic environments with low dissolved oxygen, nitrifying microorganisms showed pronounced upregulation of N2O synthesis genes, confirming their pivotal role in emissions. Conversely, heterotrophic denitrification dominated N2O production in anaerobic systems. Crucially, anammox-dominated biofilms displayed substantially lower expression of N2O-related genes compared to suspended sludge, suggesting reduced emission risks in biofilm configurations. Batch experiments demonstrated a 2.78-fold lower N2O release from biofilms than suspended biomass. This mitigation was linked to restricted substrate transport within biofilms, which curbed NO2− buildup and subsequently suppressed nitrifier-derived N2O formation. Additionally, the high-density colonization of anammox bacteria in biofilms efficient scavenging of nitric oxide (NO) and hydroxylamine (NH2OH), critical intermediates in N2O synthesis. Employing an integrated multi-omics approach, this study investigates N2O production and consumption pathways in diverse anammox systems and unveils the N2O mitigation mechanisms of biofilms, offering fundamental insights for reducing N2O emissions in wastewater treatment.