A quiet source of pollution has turned out to be much louder than expected.
Wastewater treatment plants across the U.S. may be releasing nearly twice as much greenhouse gas as previously believed, according to a new Princeton-led study.
The team found that actual methane and nitrous oxide emissions from these facilities were far higher than the Environmental Protection Agency’s estimates.
Researchers reported that the plants collectively produce 1.9 times the nitrous oxide and 2.4 times the methane originally calculated by the EPA.
Based on these findings, wastewater plants account for about 2.5 percent of U.S. methane and 8.1 percent of nitrous oxide emissions. These gases together have driven roughly 22 percent of global warming since 1850.
The good news: the majority of emissions come from a small number of plants. This means targeted improvements could significantly reduce overall pollution without an industry-wide overhaul.
Mark Zondlo, professor of civil and environmental engineering at Princeton, said the results highlight the overlooked environmental cost of clean water. “We want clean water,” he said. “But there is another side of the issue, and air emissions have not received the same attention that water does.”
Tracking gases on the go
To get accurate readings, the team relied on a mobile lab known as the Princeton Atmospheric Chemistry Experiment — an electric vehicle packed with laser-based systems, meteorological sensors, and commercial gas detectors.
Over 14 months, graduate students Daniel Moore and Nathan Li drove more than 52,000 miles, visiting 96 wastewater plants that collectively process about 9 percent of U.S. wastewater. They measured emissions seasonally, driving around plant perimeters to capture escaping gases under different weather conditions.
“It was a lot of miles,” Moore said. “We wanted to figure out how things were in the real world, not just under ideal conditions.”
Their fieldwork showed that plant emissions fluctuate widely depending on factors such as rainfall, temperature, and microbial activity. “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 recalled.
Old plants, new challenges
Most wastewater plants in the U.S. are decades old and municipally operated. Zondlo noted that these facilities have been repeatedly upgraded with new technologies, making them complex to assess. “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,” he said.
Z. Jason Ren, co-author and professor at Princeton’s Andlinger Center for Energy and the Environment, said operators often lack guidance. “They know they have emissions. In many cases, they don’t know how high they are,” he said.
The researchers hope their work will encourage closer collaboration with plant operators to pinpoint and mitigate emissions. Ren added that reducing greenhouse gases could also bring economic benefits. “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 findings of the study have been published in Nature Water.