In Situ Observations Reveal Underestimated Greenhouse Gas Emissions from Wastewater Treatment with Anaerobic Digestion – Sludge Was a Major Source for Both CH₄ and N₂O

Wastewater treatment plants (WWTPs) receive large and increasing organic carbon and nitrogen flows through societies. Consequently, WWTPs emit greenhouse gases (GHGs), including methane (CH₄) and nitrous oxide (N₂O). However, large uncertainties remain, as direct measurements of WWTP emissions have been challenging, and emission estimates frequently depend on uncertain emission factors and activity data. Using drone-based measurements, we here show that the combined CO₂-equivalent emissions of CH₄ and N₂O from WWTPs with anaerobic digestion (AD) and sludge storage were 2.4-fold higher than IPCC-recommended emission-factor-based estimates. N₂O emissions from sludge, presently assumed to be zero, were 9% of the CH₄ emissions by weight and contributed to half of the total CO₂-equivalent sludge emissions. Hence, with the necessary increase in AD in WWTPs to recover energy and reduce fossil fuel use, emission mitigation is needed, and adequate tools facilitating flux observations by WWTP managers are key for setting effective mitigation priorities.