Reporting Oct. 8 in Nature Water, researchers led by Professors Mark Zondlo and Z. Jason Ren, working with UC Riverside's Francesca Hopkins, found sectorwide emissions equal to 2.5 percent of U.S. methane and 8.1 percent of nitrous oxide. Those gases together account for about 22 percent of historical warming since 1850.
The team's electric-vehicle lab, outfitted with laser-based instruments and meteorological sensors, circled plant perimeters to capture plume signatures under varying weather and operating conditions. Graduate students Daniel Moore and Nathan Li logged about 52,000 miles over 14 months, sampling 96 facilities that collectively treat 9 percent of U.S. wastewater, with 37 plants revisited each season.
"We want clean water," said Zondlo. "But there is another side of the issue, and air emissions have not received the same attention that water does." The study indicates a relatively small subset of plants drives a disproportionate share of emissions, pointing to targeted mitigation opportunities.
Plant biology and operations explain much of the variability. Microbial processes generate methane and nitrous oxide as byproducts, and emissions shift with temperature, rainfall, and treatment steps. "We wanted to figure out how things were in the real world, not just under ideal conditions," said Moore, who noted readings could swing dramatically even within a week at the same facility.
Past national estimates, the authors said, were grounded in limited, component-level measurements extrapolated to entire systems. The new approach scanned whole facilities repeatedly to capture overlooked or nonoptimized sources across complex plants, many expanded over decades with mixed technologies.
Utilities largely prioritize effluent quality, and detailed air-emissions guidance is sparse. "They know they have emissions. In many cases, they don't know how high they are," Ren said. He added that better diagnostics could enable targeted fixes and even turn liabilities into value if plants can economically recover methane or nitrogen streams.
The researchers plan to collaborate with operators to link measured plumes to specific unit processes, maintenance practices, and equipment age, enabling practical controls that reduce emissions without compromising water treatment.
Research Report:Comprehensive assessment of the contribution of wastewater treatment to urban greenhouse gas and ammonia emissions
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