Understanding the climate cost of cleaning our water
Analysis of more than 15,000 U.S. treatment plants offers a roadmap to cutting their emissions
Each year, U.S. wastewater treatment plants clean trillions of gallons of water, from what we flush down the toilet to drain down the sink.
In a new study, a team led by researchers from Northwestern University and University of Illinois Urbana-Champaign analyzed data from more than 15,000 wastewater treatment facilities to understand the climate costs hidden within all this cleaning.
The study, published Oct. 8 in the journal Nature Water, estimates that U.S. wastewater treatment plants emit the equivalent of approximately 47 million metric tons of carbon dioxide (CO2) each year. Of these emissions, two greenhouse gases, methane and nitrous oxide, play a larger role than previously understood, exceeding current government estimates by 41%.
“To see it plain as day, how methane and nitrous oxide emissions are the dominant players, that was pretty striking,” said Northwestern’s Jennifer Dunn, the study’s senior author.
Dunn is a professor of chemical and biological engineering at Northwestern’s McCormick School of Engineering and director of the Center for Engineering Sustainability and Resilience.
A clearer picture of wastewater treatment’s contribution to climate change will help pinpoint where decarbonization efforts can be most effective, according to Dunn.
“We found that there are relatively low-hanging-fruit opportunities to reduce greenhouse gas emissions from wastewater treatment plants,” she said. “We also observed that there is a need to develop new technologies to push those reductions even further.”
Measuring the cost of cleaning
To capture the full climate cost of wastewater treatment, the researchers tallied not only emissions from each plant’s treatment process, but also those from producing the energy and chemicals the facilities require to operate and from the disposal of solid waste after treatment.
They found methane and nitrous oxide, potent greenhouse gases that arise from the on-site processes that plants use to clean wastewater, were the biggest contributors. Methane accounted for 41% of total emissions, the equivalent of 16 million metric tons of CO2. And nitrous oxide made up 24%, the equivalent of 12 million metric tons of CO2.
Methane: Sewage sludge’s dirty secret
At treatment plants, wastewater and the solids within it undergo multiple stages of processing to render them clean enough to return to the environment.
One common way plants break down wastewater solids, or sewage sludge, is through anaerobic digestion. Inside an anaerobic digester, microorganisms feed on waste and create biogas, which is majority methane.
“The trouble with anaerobic digesters is that they can leak, and some anaerobic digesters leak a lot,” Dunn said.
Indeed, while treatment plants can use biogas as a renewable source of energy, methane emissions can outweigh the climate gains.
But the good news, Dunn said, is that a leak can be fixed.
“It's something that's addressable,” she said. “So that’s an example of a low-hanging-fruit opportunity that we saw right away.”
A need for innovation
Before wastewater is clean enough to be released back into surface or groundwater, treatment plants often have to remove excess nitrogen, which could otherwise lead to increased algal growth and decreased oxygen in the water, harming aquatic life.
To strip nitrogen from wastewater, many plants use a process called nitrification-denitrification. Along the way, it releases nitrous oxide — a potent greenhouse gas — and harmless nitrogen into the air.
Meanwhile, other technologies then recover nitrogen from the air for use in products such as fertilizer. These technologies are often energy-intensive, so developing a way to harvest nitrogen directly from wastewater could save energy and 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,” Dunn said.
What’s next
The researchers are currently working with wastewater treatment facilities to collect more detailed data and refining an open-source modeling tool that plants can use to understand their emissions.
“A lot of municipalities have climate action plans, and so they want to do things to reduce their greenhouse gas emissions,” Dunn said. “One of our hopes was that this study would help treatment plants.”
Though their analysis included only U.S. plants, their methods can work for any plant with appropriate data, she said.
With world population continuing to grow and public sanitary services expanding to serve more people than ever before, decarbonization of wastewater treatment worldwide will be important for reaching climate targets.
“It’s a substantial sector, and it does need attention,” Dunn said.
Journal
Nature Water
Article Title
Benchmarking greenhouse gas emissions from US wastewater treatment for targeted reduction
Article Publication Date
8-Oct-2025
Wastewater plants produce twice as much greenhouse gases as officially estimated
Researchers from Princeton Engineering measured gas emissions from 96 plants across the U.S.
image:
Researchers from Princeton engineering, shown during a stop in Pennsylvania, measured emissions from plants across the country.
view moreCredit: Nathan Li/Princeton University
Wastewater plants emit about twice as much greenhouse gas as previously believed, according to Princeton engineers who used a mobile lab to check plants across the country.
In an article published Oct. 8 in the journal Nature Water, a research team led by professors Mark Zondlo and Z. Jason Ren, in collaboration with Prof. Francesca Hopkins of UC-Riverside, reported that collectively sewer plants produced 1.9 times the nitrous oxide gas estimated by the Environmental Protection Agency and 2.4 times the methane.
Based on the new measurements, wastewater plants contribute 2.5 percent of U.S. methane emissions and 8.1 percent of nitrous oxide. Both methane and nitrous oxide are potent greenhouse gases, collectively responsible for about 22 percent of global warming since 1850, the researchers said.
The researchers said the good news is that much of the emissions are produced by a small number of wastewater plants. This means that taking steps to improve relatively few plants would have an outsized impact on overall pollution.
Zondlo, a professor of civil and environmental engineering, said the work highlights the importance of greenhouse gas emissions from wastewater systems, which he said play a key role in public and environmental health.
“We want clean water,” Zondlo said. “But there is another side of the issue, and air emissions have not received the same attention that water does.”
To do this, the team used a mobile laboratory called the “Princeton Atmospheric Chemistry Experiment” – an electric vehicle equipped with laser-based systems developed by Zondlo and colleagues, along with commercial gas and meteorological sensors. The Princeton team took direct measurements from 96 wastewater plants that together treat 9% of U.S. wastewater. Graduate students Daniel Moore and Nathan Li drove the mobile lab quarterly from the East Coast to California and monitored emissions from plants each season over the 14-month campaign.
“It was a lot of miles,” said Moore, who estimated their total travel at about 52,000 miles. Moore is now a data scientist with the environmental group WattTime.
To capture readings, the team drove the mobile lab on public roads around the plants’ perimeters and recorded emerging gas plumes. Typically, the researchers would drive by a facility about 10 times to gather a sample, and they sampled many facilities multiple times under different weather conditions and times of day (37 plants were sampled quarterly.) Individual emissions were analyzed for each plant to build both sector-level emissions as well as understand the operations of each plant. Data for each facility were reported anonymously as the goal of the study was to not report publicly on any specific plant but rather capture trends in the industry as a whole.
“We wanted to figure out how things were in the real world, not just under ideal conditions,” Moore said.
Wastewater plants usually rely on microbes to consume waste as part of the cleaning process, and the microbes produce different gases such as methane and nitrous oxide as byproducts. Due to the complexity of wastewater composition, the treatment technologies and operational strategies vary across facilities, leading to different biological reactions and, consequently, large variations in greenhouse gas emissions. Because it is a biological process, many environmental factors influence how much gas the microbes are producing.
Other factors also play a role. Heavy rains can affect sewage content, and temperature and the season can impact emissions.
“One time, we were invited into a facility and found high concentrations of nitrous oxide around one aeration tank. We came back a week later, and there was nothing,” Moore said.
The unpredictable nature of wastewater plant emissions was a factor in the challenge of estimating national emission levels, the researchers said. Zondlo said previous estimates were based on good science, but much of that rested on very small parts of a very small number of plants that were extrapolated to a national basis. One part of any individual plant has difficulty in reflecting emissions from other known sources, capturing operations that may not be optimized, or identifying unknown sources on these large facilities.
“Wastewater plants are often complex. Many plants were built in the 70’s and have been added to with changing technologies,” he said. “Our approach says let’s look at the entire facility, let’s look at a lot of facilities and let’s look at different times.”
Most wastewater plants in the U.S. are operated by municipalities or local government authorities, the researchers said. Ren, a professor of civil and environmental engineering and the Andlinger Center for Energy and the Environment, said that there is not a lot of information available to guide plant operators.
“They know they have emissions. In many cases, they don’t know how high they are,” Ren said. “Most of the utilities are focused on making sure the water is clean. Air emissions haven’t been their top priority, though many are becoming increasingly interested in understanding them better.”
Zondlo said that the next step toward minimizing emissions would be to work with plant operators to get a better understanding of what is happening inside the wastewater plants. Because most plants are large and relatively complex, it is likely that certain processes or treatments emit more gas than others. Maintenance and equipment age could also play a role.
“If we can get more information on the plants, we can make it easier to get a handle on the air emissions as well as the water,” he said.
Ren said controlling emissions might also provide a financial incentive for utilities. If there is an economic way to recapture methane or nitrogen gas, they might become a revenue stream for the wastewater operators.
“Methane, for example, is a greenhouse gas and it is not good for the environment. But it is also a valuable renewable energy source,” he said.
The article, Comprehensive assessment of the contribution of wastewater treatment to urban greenhouse gas and ammonia emissions, was published Oct. 8 in Nature Water. Besides Ren, Zondlo, Moore and Li, authors include Jun-Jie Zhu, Hongming Yi, Lei Tao, James McSpiritt, Vladislav Sevostianov, and Lars Wendt, of Princeton University; Nidia Rojas Robles and Francesca Hopkins of the University of California-Riverside. Support for the project was provided in part by the Alfred P. Sloan Foundation in the Energy and Environment Program.
The researchers attached an array of sensors to the roof of an electric vehicle, which they drove to plants across the U.S.
Credit
Princeton University
Journal
Nature Water
Method of Research
Experimental study
Subject of Research
Not applicable
Article Title
Comprehensive assessment of the contribution of wastewater treatment to urban greenhouse gas and ammonia emissions
Article Publication Date
8-Oct-2025
No comments:
Post a Comment