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

Abstract

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.