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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 published 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, reports 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% of U.S. methane emissions and 8.1% of nitrous oxide. Both methane and nitrous oxide are potent greenhouse gases, collectively responsible for about 22% 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 not to 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, it 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.