New drone-based measurements reveal that wastewater treatment plants emit several times more greenhouse gases than previously estimated, highlighting sludge storage as a significant yet overlooked climate risk.
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In a recent article published in Environmental Science & Technology, researchers investigated the greenhouse gas emissions associated with wastewater treatment plants (WWTPs), emphasizing the significance of emissions arising from sludge storage and handling processes that are often overlooked or underestimated in current assessments.
Background
Wastewater treatment plants (WWTPs) play a crucial role in removing organic material, nitrogen, phosphorus, and other pollutants from incoming water before it is released back into the environment. Many large WWTPs now incorporate anaerobic digestion to process sludge, producing biogas - mainly methane - as a renewable energy source. This approach supports energy recovery and reduces reliance on fossil fuels. However, it also generates potent greenhouse gases (GHGs).
Traditionally, emissions from WWTPs have been estimated using standardized methods, such as those based on Intergovernmental Panel on Climate Change (IPCC) guidelines. These rely on generalized emission factors, often derived from lab-scale studies or small systems. As a result, they do not fully capture the complexity of real-world conditions and tend to underestimate emissions - especially from sludge storage, which has emerged as a significant yet largely overlooked source of both methane (CH4) and nitrous oxide (N2O).
Drone-Based Assessment of Wastewater Treatment Plant Emissions
This study introduced a drone-based measurement system designed specifically for large-scale monitoring of GHG emissions from WWTPs. Two advanced drone platforms, each equipped with multi-gas sensors capable of detecting CH4 and N2O, were used to assess emissions across various facility components, including sludge storage areas and treatment units.
Flying at low altitudes - around one to one and a half meters above ground - the drones performed crosswind sampling using a systematic back-and-forth flight pattern. This approach supported a mass balance method to quantify emissions by comparing gas concentrations upwind and downwind of emission sources, while factoring in wind speed and direction.
The drones’ sensors were calibrated for real-time CH4 and N2O detection and included onboard wind measurement tools. Data were collected across a wide temperature range (–1 °C to +27 °C), reflecting real operational conditions. Post-flight processing yielded gas flux calculations, allowing for accurate estimates of emission rates from full-scale sludge piles and treatment steps.
The study covered 13 facilities, including complete WWTPs and large sludge storage sites, ensuring a broad representation of treatment configurations and environmental conditions. Additional techniques - such as manual air sampling and spectral mapping - were used to validate the drone data, strengthening the reliability of the findings.
Key Findings on Methane and Nitrous Oxide Emissions
The study found that CH4 and N2O emissions were significantly higher than previously estimated using standard emission factors. In WWTPs with anaerobic digestion, actual emissions were 2.9 to 6.3 times greater than expected. Methane emissions from sludge storage alone accounted for as much as 78 % of a facility’s total methane output, highlighting this often-overlooked phase as a major contributor.
Nitrous oxide emissions from sludge piles - frequently excluded in conventional assessments - were also surprisingly high. In some cases, their climate impact rivaled that of methane when viewed through the lens of global warming potential.
Several factors influenced emission levels. Younger sludge piles tended to release more gas, likely due to ongoing biological activity. Methane emissions were also sensitive to temperature, with higher levels recorded during warmer months (above 5 °C), supporting existing knowledge that microbial activity increases in warmer conditions.
These findings suggest that conventional methods, which rely on static, average emission factors, substantially underreport real emissions. The discrepancy becomes even more critical given the global rise in anaerobic digestion infrastructure. Accurate, real-world measurements are essential for developing effective climate strategies.
Implications for Climate Policy and Wastewater Management
This study clearly shows that greenhouse gas emissions from WWTPs using anaerobic digestion are significantly underestimated in existing emissions inventories. The use of drone-based monitoring enabled detailed, real-time measurement of CH4 and N2O across full-scale facilities, revealing emission levels several times higher than previously assumed.
As the wastewater sector continues to adopt energy recovery technologies, there is an urgent need to reassess its environmental footprint. Reliable, site-specific emissions data will be essential for informing policies and practices that align with climate targets. Moving forward, integrating advanced measurement tools like drone-based systems can play a key role in managing and mitigating the GHG impact of wastewater treatment.
Journal Reference
Gålfalk M., Bastviken D. et al. (2025). In situ observations reveal underestimated greenhouse gas emissions from wastewater treatment with anaerobic digestion. Environmental Science & Technology, 59, 18146−18155. DOI: 10.1021/acs.est.5c04780, https://pubs.acs.org/doi/10.1021/acs.est.5c04780