A new study from Linköping University using custom drones reveals that wastewater treatment plants may be emitting 2.5 times more greenhouse gases than previously thought.
The unexpected spike in methane and powerful nitrous oxide comes primarily from sludge storage
Researchers from Linköping University in Sweden have found that greenhouse gas emissions from many wastewater treatment plants (WWTPs) may be more than twice as large as current reporting models suggest. Researchers utilised a custom-built drone equipped with specialised sensors to measure the potent greenhouse gases methane (CH4) and nitrous oxide (N2O) at several Swedish facilities.
The findings, published in the journal Environmental Science & Technology, highlight a significant, previously unknown source of emissions and call into question the current global reporting methodology.
Discrepancy in global emission estimates
Globally, WWTPs that process household and industrial sewage are estimated by the UN’s Intergovernmental Panel on Climate Change (IPCC) to account for roughly five percent of human-induced methane and nitrous oxide emissions. The IPCC arrives at this figure using emission factors linked to the number of households connected to a plant, which generates an estimate rather than a number based on direct measurement.
Led by Docent Magnus Gålfalk, researchers sought to compare these estimations with real-world data. Their measurements at twelve Swedish treatment plants that use anaerobic digestion for sludge treatment revealed that methane and nitrous oxide emissions were significantly higher—approximately 2.5 times the levels indicated by the IPCC’s calculation models.
The sludge storage surprise
The study identified the primary source of the elevated emissions: the sludge storage stage that follows anaerobic digestion. After digestion, the sludge is stored to reduce harmful microorganisms before being utilised as, for instance, fertiliser. The researchers found that the amount of methane (CH4) released during this storage phase had been significantly underestimated.
Perhaps more critically, the drone measurements also detected large amounts of nitrous oxide (N2O) Nitrous oxide is a particularly powerful, yet often overlooked, greenhouse gas, possessing a climate impact nearly 300 times greater per kilogram than carbon dioxide (CO2).
“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,” Gålfalk stated. “So it’s a major extra source to keep an eye on.”
Call for measurement-based reporting
The researchers emphasise that wastewater treatment plants continuously work to reduce emissions, but the current IPCC model does not reflect these efforts, meaning reported emissions remain static regardless of mitigation success.
Gålfalk argues that an overhaul of the reporting system is necessary. “It would be better if the emissions reported were based on actual measurements,” he suggests. “This would make it easier for municipalities to show the benefits of investments to mitigate the emissions.” The new data provides a clearer roadmap for targeted emission reduction strategies, particularly
