According to engineers from Princeton, wastewater treatment facilities release approximately double the amount of greenhouse gases than previously thought, based on assessments conducted using a mobile laboratory across the nation.
In a study published on October 8 in the journal Nature Water, a team of researchers led by professors Mark Zondlo and Z. Jason Ren, alongside Professor Francesca Hopkins from the University of California, Riverside, revealed that sewer plants emitted 1.9 times more nitrous oxide and 2.4 times more methane than the Environmental Protection Agency had estimated.
The new findings indicate that wastewater facilities are responsible for 2.5% of methane emissions and 8.1% of nitrous oxide emissions in the United States. Both gases are significant contributors to climate change, accounting for roughly 22% of global warming since 1850, according to the research team.
The researchers noted a positive aspect: a small number of wastewater plants are responsible for the majority of emissions. This suggests that enhancing operations at just a few facilities could significantly reduce overall pollution levels.
Zondlo, a professor in civil and environmental engineering, emphasized the critical role of greenhouse gas emissions from wastewater systems in both public health and environmental sustainability. “We desire clean water,” he stated. “However, the issue of air emissions has not received the same level of focus as water quality.”
To investigate this, the team utilized a mobile lab known as the “Princeton Atmospheric Chemistry Experiment,” an electric vehicle outfitted with laser-based technology developed by Zondlo and his team, in addition to commercial gas and meteorological sensors. The Princeton group conducted direct measurements at 96 wastewater facilities that collectively process 9% of the nation’s wastewater. Graduate students Daniel Moore and Nathan Li drove the mobile lab quarterly from the East Coast to California, monitoring emissions at various plants throughout a 14-month study.
According to the new findings, wastewater facilities account for 2.5% of U.S. methane emissions and 8.1% of nitrous oxide emissions. “We covered a lot of ground,” Moore remarked, estimating their total travel distance at around 52,000 miles. He is currently a data scientist with the environmental organization WattTime.
To gather data, the team navigated the mobile lab along public roads surrounding the plants and recorded gas emissions. Typically, they would drive past a facility about ten times to collect samples, often visiting multiple plants under varying weather conditions and times of day (37 plants were sampled quarterly). Emissions from each facility were analyzed individually to create sector-wide emission profiles and gain insights into the operational dynamics of each plant. Data was reported anonymously to focus on industry trends rather than specific facilities.
“We aimed to understand the real-world scenario, not just idealized conditions,” Moore explained.
Wastewater facilities usually depend on microbes to break down waste during the treatment process, and these microbes generate gases like methane and nitrous oxide as byproducts. Due to the diverse composition of wastewater, treatment technologies and operational strategies differ among facilities, resulting in varied biological reactions and significant differences in greenhouse gas emissions. Many environmental factors also influence gas production by microbes.
Other elements can also impact emissions. For instance, heavy rainfall can alter sewage composition, while temperature and seasonal changes can affect gas output.
“Once, we were invited to a facility and detected high levels of nitrous oxide near one aeration tank. When we returned a week later, the levels were negligible,” Moore noted.
The unpredictable nature of emissions from wastewater plants complicates the task of estimating national emission levels, the researchers stated. Zondlo acknowledged that prior estimates were based on sound science, but they often relied on a limited number of plants, which may not accurately reflect emissions from other sources or capture operations that may not be optimized.
“Wastewater facilities are typically intricate. Many were constructed in the 1970s and have evolved with advancing technologies,” he explained. “Our approach is to examine entire facilities, assess numerous plants, and study various timeframes.”
Most wastewater treatment plants in the U.S. are managed by municipal or local government entities, the researchers noted. Ren, a professor of civil and environmental engineering and director at the Andlinger Center for Energy and the Environment, remarked that there is limited guidance available for plant operators.
“They are aware of their emissions, but often lack knowledge about their magnitude,” Ren stated. “Most utilities prioritize ensuring water quality, while air emissions have not been a primary focus, although interest in understanding them is growing.”
Zondlo suggested that the next step in reducing emissions involves collaborating with plant operators to gain deeper insights into the internal workings of wastewater facilities. Given the complexity of many plants, certain processes or treatments may produce more gases than others. Maintenance and the age of equipment could also influence emissions.
“By gathering more data on the plants, we can better manage both air emissions and water quality,” he said.
Ren added that controlling emissions could also offer financial benefits for utilities. If there are economically viable methods to recapture methane or nitrogen gas, these could serve as revenue sources for wastewater operators.
“Methane is a greenhouse gas detrimental to the environment, but it also represents a valuable renewable energy resource,” he explained.
Source: Princeton University
