Oil and gas systems, livestock and landfills top the global list of large methane emitters. However, new research has shown that urban areas release significantly more methane than previously thought—and untreated wastewater could be a major source of those additional emissions.
Published in March 2023 in Environmental Research Letters, University of Wisconsin-Madison engineers found that urban methane emissions are three to four times higher than previously estimated. The researchers studied methane emissions from 61 cities around the world, revealing that increased methane emissions correlates with the amount of untreated wastewater per person.
Scaled up to the 385 cities worldwide with more than 2 million citizens each, the research suggests these urban areas could account for up to 22% of global methane emissions.
James Schauer
James Schauer, a professor of civil and environmental engineering at UW-Madison, says he and his collaborators noticed higher-than-expected methane levels while studying atmospheric plumes over the city of Dhaka in Bangladesh. They used tools like the TROPOspheric Monitoring Instrument (TROPOMI) aboard the Sentinel 5 Precursor satellite to measure methane and other air pollutants around the world.
“We really started trying to understand, in the context of air quality, why the methane levels were so high,” Schauer says. “Then we started connecting across cities in the region, and then around the world, trying to understand this observation.”
Organic material in untreated wastewater breaks down anaerobically after it’s discharged into water bodies, creating methane emissions that bubble up into the atmosphere. Methane emissions, along with carbon dioxide, are one of the main drivers of human-caused climate change. Though methane only remains in the atmosphere for about 12 years—compared to centuries for carbon dioxide—it’s about 25 times more potent than carbon dioxide at trapping heat. That means methane is a significant driver of short-term climate change, and any efforts to reduce our impact on the environment should include methane emission reductions.
In their research, Schauer and his colleagues examined various sources to explain the variability of methane emissions in the 61 urban areas they studied. Within that group, 33 cities had medium to high levels of untreated water; reducing methane emissions in those cities, for example, to the mean level of emissions of cities with little or no untreated wastewater could reduce global methane emissions by 2%. Additionally, reducing all 61 cities to the emission level of the lowest city in the study could result in a reduction of almost 6% in worldwide methane emissions.
Reducing emissions in cities around the world to the level of those with complete wastewater treatment could lower global methane emissions by as much as 10%, according to Benjamin de Foy, the paper’s lead author and a professor of earth and atmospheric sciences at Saint Louis University.
“Our estimates of methane emissions suggest that there is methane formation in the environment as a result of the release of untreated wastewater which is much larger than the estimates in current inventories,” de Foy says. “Some urban areas could reduce their emissions 50 percent or more by fully treating all their wastewater.”
Schauer says that while leaks from natural gas use and landfills contribute to methane emissions, it became clear that these sources alone couldn’t account for heightened methane concentrations in the atmosphere. “Researchers understood that there were emissions of greenhouse gases from water treatment facilities but the impact of untreated wastewater emissions of methane has not been fully appreciated,” he says. “There have always been a lot of questions about why we’re unable to fully predict global methane emissions and develop a complete picture of global methane sources. The current study helps to address a long-standing question that is important to developing strategies to mitigate the increase of greenhouse gases in the atmosphere.
This research indicates that untreated wastewater may be a much larger methane contributor than expected and that knowledge may help communities focus on goals that can tangibly help in the fight against climate change, in addition to co-benefits for human and ecological health. One hundred fifty countries have committed to reducing their methane emissions by 30% by 2030 relative to 2020 as part of the Global Methane Pledge. Improved wastewater treatment could make a significant contribution to this goal.
The researchers say there’s more work to be done to create a more refined global methane emission inventory and to understand why emissions vary so much from city to city, even within the same country. During the course of the research, they found that Milwaukee, Wisconsin, has a large methane enhancement, for example, while Minneapolis does not. Such differences also are present from region to region on a global scale. For example, cities in Europe and China tended to have much lower methane emissions than those in North America or other parts of Asia like Dhaka. Such differences could stem from how, or how much, different cities treat their wastewater.
“China has relatively low emissions because it has focused on comprehensive wastewater treatment due to its high population and for ecological protection,” Schauer says. “China has a bad history with pathogens and wastewater, so the country has put a lot of effort into making sure it’s treating all of its wastewater. As an added benefit, it keeps methane emissions low.”
Schauer is the William C. Boyle Professor of Environmental Engineering at UW-Madison and director of the Wisconsin State Laboratory of Hygiene. De Foy is the Banpu Chair in Sustainability at SLU. Alba Lorenete, a research scientist at the Netherlands Institute for Space Research (now at the Environmental Defense Fund); and Tobias Borsdorff, a research scientist at the Netherlands Institute for Space Research, are also authors on the paper.
Featured image caption: Banani Lake in Dhaka, Bangladesh has struggled due to pollution resulting from waste flowing in from the surrounding area. New research shows that untreated wastewater may be contributing to methane emissions that are warming the planet. Photo courtesy of Benjamin de Foy/Saint Louis University.