U.S. wastewater treatment plants are responsible for about 47 million metric tons of carbon dioxide equivalent emissions each year, according to a new study led by researchers from Northwestern University and the University of Illinois Urbana-Champaign. The research, published in Nature Water on October 8, found that methane and nitrous oxide emissions from these facilities are 41% higher than previous government estimates.
Jennifer Dunn, senior author of the study and professor at Northwestern’s McCormick School of Engineering, said, “To see it plain as day, how methane and nitrous oxide emissions are the dominant players, that was pretty striking.” She added that identifying the sources of these greenhouse gases will help target decarbonization efforts more effectively.
The team analyzed data from over 15,000 wastewater treatment facilities across the United States. Their findings show that methane makes up 41% of total emissions—about 16 million metric tons of CO2 equivalent—while nitrous oxide accounts for 24%, or around 12 million metric tons.
The main source of methane is anaerobic digestion, a process where microorganisms break down sewage sludge and produce biogas. While this biogas can be used as renewable energy, leaks from anaerobic digesters can release significant amounts of methane into the atmosphere. Dunn noted, “The trouble with anaerobic digesters is that they can leak, and some anaerobic digesters leak a lot.” She also pointed out that fixing these leaks is an achievable way to reduce emissions: “It's something that's addressable. So that’s an example of a low-hanging-fruit opportunity that we saw right away.”
Nitrous oxide is released during nitrogen removal processes in wastewater treatment. Many plants use nitrification-denitrification to remove excess nitrogen before discharging water back into the environment. This process emits nitrous oxide and harmless nitrogen gas. According to Dunn, developing methods to capture nitrogen directly from wastewater for use in products like fertilizer could further reduce both energy use and greenhouse gas emissions. “If we could just avoid putting that nitrogen back into the air and use it while we have it to make a product like feed or fertilizer, then we're moving towards a circular nitrogen economy,” she said.
The researchers are now collaborating with treatment facilities to gather more detailed data and improve an open-source modeling tool designed to help plants understand their own emission profiles. Dunn explained their goal: “A lot of municipalities have climate action plans, and so they want to do things to reduce their greenhouse gas emissions. One of our hopes was that this study would help treatment plants.”
Although this analysis focused on U.S. facilities, Dunn said their methods can be applied globally if sufficient data is available. With increasing global population and expanding public sanitation services, reducing emissions from wastewater treatment will be important for meeting climate goals worldwide.
“It’s a substantial sector, and it does need attention,” Dunn said.
The study was supported by the U.S. Department of Energy Industrial Technologies Office with contributions from the National Renewable Energy Laboratory.