A new Swedish study utilizing drones equipped with custom sensors says greenhouse gas emissions from wastewater treatment plants could be more than twice as high as previously estimated.
According to the new research from Linköping University, the drones measured methane (CH4) and nitrous oxide (N2O) emissions at Swedish facilities and found levels roughly 2.5 times higher than international models predict.
The drones were equipped with sensors for CH4, N2O, temperature, pressure, relative humidity, and an anemometer for wind and 3D positioning.
The findings, published in Environmental Science & Technology, challenge longstanding estimates by the UN Intergovernmental Panel on Climate Change (IPCC), which attributes about 5% of global human-caused methane and nitrous oxide emissions to wastewater treatment.
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Nitrous oxide is a powerful greenhouse gas which has a climate impact almost 300 times higher than carbon dioxide per kilogram and is emitted by all secondary wastewater treatment plants.
“We show that the climate impact from nitrous oxide emissions from sludge storage is as great as that from methane, and this wasn’t known before. So it’s a major extra source to keep an eye on,” announced Magnus Gålfalk, an environmental scientist at Linköping University, who led the study. “Now that we know more about these emissions, we also know more about how they can be reduced.”
The IPCC currently calculates wastewater emissions using standardized emission factors based on population size rather than direct measurements. Gålfalk, a docent at Tema M – Environmental Change at Linköping, said this model is too rigid and fails to reflect progress made by municipalities investing in cleaner operations.
“It would be better if the emissions reported were based on actual measurements,” he said. “This would make it easier for municipalities to show the benefits of investments to mitigate the emissions.”
The research team, including Linköping Professor David Bastviken, conducted drone-based measurements at 12 Swedish wastewater plants using anaerobic digestion to treat sludge. Most of the excess emissions occurred after digestion, when sludge is stored to reduce harmful microorganisms before being reused as fertilizer.
The study revealed that methane releases during sludge storage were significantly underestimated. It also found large quantities of N2O emissions, a powerful but fairly unknown greenhouse gas which has a climate impact almost 300 times higher than carbon dioxide per kilogram.
The research was supported by the European Research Council, Horizon 2020, the Swedish Research Council, Formas, and Svenskt Vatten Utveckling.
The full study — “In Situ Observations Reveal Underestimated Greenhouse Gas Emissions from Wastewater Treatment with Anaerobic Digestion – Sludge Was a Major Source for Both CH4 and N2O” — appears in Environmental Science & Technology (Vol. 59, Issue 34, 2025).
