Using sensors mounted on drones, climate researchers have determined how much greenhouse gases are released by sewage treatment plants. They discovered unexpectedly high emissions: The systems emit more than twice as many greenhouse gases in the form of methane and nitrous oxide than previously thought. This knowledge could be used to optimize the cleaning processes in sewage treatment plants in the future and reduce emissions.
When it comes to greenhouse gases, carbon dioxide (CO2) the speech. Because it accounts for the majority of all greenhouse gases that we release into the earth’s atmosphere. But other emissions also contribute to climate change, including nitrous oxide (N2O) and methane (CH4). Both occur in lower concentrations in the atmosphere than CO2. However, methane contributes around 200 times more to global warming and nitrous oxide even around 300 times more.
Both gases arise from natural sources such as moors, permafrost or bodies of water, but also from man-made sources. These include, for example, agriculture, animal husbandry, the chemical industry and the energy sector, but also sewage treatment plants. The Intergovernmental Panel on Climate Change (IPCC) estimates that the latter release around four to nine percent of man-made methane and around 2.5 percent of man-made nitrous oxide. This estimate is based on isolated measurements and calculations of how much organic material ends up in a wastewater treatment plant on average, providing a global approximation.
Emissions significantly underestimated
But how much methane and nitrous oxide are really released from sewage treatment plants? This is what Magnus Gålfalk and David Bastviken from Linköping University in Sweden have discovered. To do this, the two researchers used drones, which they equipped with sensors for methane and nitrous oxide, among other things. Using these drones, they analyzed the gas concentrations in the airspace above twelve standard Swedish wastewater treatment plants. In the plants, the sewage sludge resulting from wastewater treatment is stored for one to six months in the final process step before it is reused as fertilizer, for example. During storage, the sludge rots, which naturally reduces the amount of microorganisms that are potentially harmful to the environment or health. However, this digestion also releases greenhouse gases from the sewage sludge piles.
The drone analyzes now showed: The real measured values for nitrous oxide and methane are around three and five times higher than the values estimated by the Climate Council. Converted into CO2 equivalents, sewage treatment plants release at least 2.4 times more of these greenhouse gases annually than previously thought. At higher temperatures and different weather conditions than in the measurements in southern Sweden, there could be even more emissions. More important than the environmental conditions, however, is the length of time the sludge is stored, the data showed.
Opportunity for future emissions reductions
Overall, the results show that emissions from wastewater treatment plants were greatly underestimated, especially with regard to nitrous oxide, the team concludes. “We show that the climate impact of nitrous oxide emissions from sludge storage is just as great as that of methane. This was previously unknown,” says Gålfalk. “It is about [bei Lachgas aus Kläranlagen] “So it’s an important additional source that you have to keep an eye on,” explains the climate researcher.
With knowledge of the exact greenhouse gas emissions from wastewater treatment plants, operators can now develop new measures to reduce them. To date, only part of the methane produced in sewage treatment plants has been captured and reused as biogas. In the future, methane and nitrous oxide from all process steps could be captured and recycled. However, the prerequisite for this is to measure exactly when and where how much of the gases are produced. The researchers therefore advocate continuing to precisely measure emissions of methane and nitrous oxide using drones or similar techniques. “It would be better if the reported emissions were based on actual measurements. This would make it easier for municipalities to demonstrate the benefits of investments to reduce emissions,” says Gålfalk.
Source: Linköping University; Specialist article: Environmental Science & Technology, doi: 10.1021/acs.est.5c04780