Wastewater treatment releases nearly double the methane that was previously thought

By using lasers that measure airborne methane, a research team has calculated that municipal wastewater treatment plants in the US emit almost two times more of the greenhouse gas than currently estimated. According to the researchers, the unaccounted-for methane emissions would have the same effect on climate as 5.1 million metric tons of carbon dioxide or the addition of approximately 1 million more cars on the road (Environ. Sci. Technol. 2023, DOI: 10.1021/acs.est.2c05373).

The researchers say this large discrepancy stems from the US Environmental Protection Agency basing its estimates on limited data: measurements taken from just 14 wastewater facilities located in different parts of the globe. In contrast, the Princeton team measured methane emissions from four times as many plants—a total of 63 facilities located across California and the Mid-Atlantic region, representing about 2% of the wastewater treated in the US. “The goal of our work was to try and get a more representative sample of [methane] emissions from the sector,” says Mark Zondlo, an environmental engineer at Princeton University and one of the authors of the study.

Along with sampling more plants, Zondlo and his team also took a different approach when measuring methane from each of the treatment facilities. According to Daniel Moore, a graduate student in Zondlo’s group at Princeton, the emission data used today by the EPA and the Intergovernmental Panel on Climate Change (IPCC) come from sampling individual sources of methane within treatment plants and adding those emissions up. But “who’s to say there’s not a leak somewhere that they’re not measuring?” Moore says.

Instead, for this study, Moore and Zondlo used a laser-based gas sensor attached to the roof of an electric vehicle to measure the plume of methane coming of each facility. Using this mobile laboratory, the team’s measurements are far more likely to get an accurate read of the plant’s total methane footprint, Moore says. It also meant the researchers could take measurements more often because they could take measurements from a public road and didn’t need access to the treatment plants.

“What they’re showing is that it’s relatively simple to estimate what’s been emitted from your wastewater plants,” says Euan Nisbet, an earth scientist at Royal Holloway, University of London, who was not involved in the study. The next step, he says, is to ask, “How can we bring that down?”

That’s one question Zondlo hopes to answer in future research. He notes that although this study provides an improved estimate of methane emissions from wastewater treatment facilities, “it doesn’t tell us how to reduce the emissions per se.” Future work will also test whether treatment plants sampled in this study are representative of facilities across the US and in other countries.

Francesca Hopkins, a climate scientist at the University of California, Riverside notes that the analysis could not detect certain potentially relevant parameters such as seasonality or differences across plant types. These could have an effect on emissions, she adds. Hopkins will be collaborating with Zondlo on future projects to gain insight into some of these outstanding unknowns but was not involved in the new study.

In the meantime, Zondlo says that the results from this study can help improve the existing EPA and IPCC guidelines on wastewater treatment. At the very least, Hopkins says, the works highlights a “need to better understand the uncertainties in emission inventory estimates of wastewater treatment plants . . . as we try to reduce methane emissions,” she says.