Shift in activated sludge microbiomes associated with nitrite accumulation and high nitrous oxide emissions

Nitrous oxide (N₂O) emissions can constitute over half of the carbon footprint of a wastewater treatment plant (WWTP), and emission peaks frequently correlate with nitrite (NO₂⁻) concentrations. However, connections between the microbiome and high N₂O and NO₂⁻ levels are not well-documented. Here, we characterize the microbiomes in several parallel lines of a WWTP during massive N₂O emissions (20 % of influent nitrogen load) with prolonged NO₂⁻ accumulation in most lines, aiming to identify key differences between communities in lines with high and low NO₂⁻ concentrations. The abundance of nitrite-oxidizing bacteria (NOB) was extremely low in the lines with NO₂⁻ accumulation, which also had slightly lower abundances of ammonia-oxidizing bacteria (AOB). Some incomplete denitrifiers were more abundant in the lines with NO₂⁻ accumulation. Lines without NO₂⁻ had a higher relative abundance of filamentous bacteria and better floc formation. These findings confirmed our hypothesis that loss of NOB caused NO₂⁻ accumulation, inducing increased N₂O emissions. AOB are suspected to be the main source of N₂O during the studied period, with a likely contribution from heterotrophic denitrifiers. A few species were identified as interesting candidates for further study regarding their potential role in increased N₂O emission from WWTPs. Long-term microbiome monitoring is necessary to understand the changes in the microbiome that might initiate NO₂⁻ accumulation and high N₂O emissions.