Drone measurements reveal significant greenhouse gas emissions from sludge

Greenhouse gas emissions from many Swedish wastewater treatment plants (WWTPs) may be more than twice as high as previously estimated. A new study from Linköping University shows that sludge storage releases large amounts of methane and nitrous oxide that have not been accounted for in current reporting.

Researchers at the Linköping University used drones with specially built sensors to measure emissions at twelve Swedish treatment plants. The results indicate that total emissions of methane and nitrous oxide are around 2.5 times higher than the figures calculated by the Intergovernmental Panel on Climate Change (IPCC).

Green house gas emissions overlooked

“We show that certain greenhouse gas emissions from wastewater treatment plants have been overlooked. Now that we know more about where they occur, we can also see how to reduce them,” says Magnus Gålfalk, associate professor at Linköping University, who led the study published in Environmental Science & Technology.

According to the IPCC, wastewater treatment plants that handle domestic and industrial wastewater account for about five percent of global human-caused methane and nitrous oxide emissions. These estimates are based on calculation models rather than direct measurements. The researchers argue that this approach hides the progress many plants have made in reducing emissions. “It would be much better if reported emissions were based on real measurements. That would make it easier for municipalities to justify investments in new technology,” says Gålfalk.

Sludge storage a key source

Most emissions occur after the digestion process, when the remaining sludge is stored to reduce microorganisms before it is used as fertilizer. The study shows that methane emissions during storage have been underestimated, and that significant amounts of nitrous oxide are also released. Nitrous oxide is a particularly potent greenhouse gas with a climate impact nearly 300 times higher than carbon dioxide per kilogram. “We show that the climate impact from nitrous oxide emissions during sludge storage is as large as that from methane. That was not known before,” says Gålfalk.

Call for better monitoring

The researchers conclude that current knowledge of greenhouse gas emissions from wastewater treatment plants with anaerobic digestion is incomplete. They call for a reassessment of how emissions are calculated and for better monitoring methods. “Until we have accurate data showing where, when, and how much is being emitted, it will be difficult to develop effective reduction strategies,” the authors write. The study was funded by the European Research Council (Horizon 2020), the Swedish Research Council, Formas, and Swedish Water Development (SVU).

This article was published first by the Swedish platform for water professionals cirkulation.se