A pair of recent studies from researchers at Princeton University have found that wastewater collection and treatment systems emit nearly twice as much methane as previously estimated. Because methane is a significantly more potent greenhouse gas than carbon dioxide, the findings raise the prospect that the wastewater sector will be called on to make dramatic reductions in its methane emissions as part of efforts to address climate change.

Double current estimates

Published February 27 on the website of the journal Environmental Science & Technology, the article titled “Underestimation of Sector-Wide Methane Emissions from United States Wastewater Treatment,” by Moore et al., reported that methane emissions from U.S. municipal wastewater treatment operations could be nearly twice as much as that of the current estimates by the U.S. Environmental Protection Agency.

The findings are based on what the researchers say was the largest study of methane emissions from U.S. water resource recovery facilities to date. Using a mobile laboratory, the researchers measured methane emissions from 63 WRRFs having treatment capacities ranging from <0.1 to 193 mgd. Located in the mid-Atlantic region and California, the facilities treat the equivalent of 2% of the 62.5 billion gal. treated per day nationally.

“If the results from those 63 plants are representative, actual methane emissions from wastewater treatment facilities across the U.S. would be about 1.9 times greater than emissions estimates that use existing [Intergovernmental Panel on Climate Change] and EPA guidelines, meaning that those guidelines underestimate methane emissions equivalent to 5.3 million metric tons of carbon dioxide,” according to a February 28 news release from Princeton.

Similar findings

The findings track closely with those of a second, independent study also conducted by Princeton researchers. 

In an article titled “Methane Emissions from Municipal Wastewater Collection and Treatment Systems” that was published February 3 on the website of the journal Environmental Science & Technology, Song et al. describe the results of what they say was the first quantitative analysis of methane emissions from wastewater collection and treatment systems. 

For their study, the authors used a literature mining method to collect all available published literature concerning methane emissions from wastewater. They then identified the main sources of methane emissions from wastewater operations and estimated nationwide methane emissions from the U.S. wastewater sector.

All told, the researchers estimated annual emissions from the U.S. wastewater sector to be approximately 10.9 ± 7.0 million metric tonnes of carbon dioxide equivalents per year. This figure is approximately twice the IPCC’s previous estimate of 4.3 to 6.1 MMT CO2-eq/yr.

Facilities with anaerobic digestion had methane emissions three times higher than those without the treatment process, according to the February 3 article. Emissions from such facilities were “mainly caused by biogas leakage from anaerobic digesters and incomplete flaring of excess methane,” the article states. Storage and dewatering also were found to be “major sources of methane emissions.”

Approximately 79% of total methane emissions result from WRRFs having anaerobic digestion or stabilization ponds, “indicating they should be the priorities in monitoring and mitigation efforts,” according to the article. 

In their article, Song et al. suggest various ways for reducing methane emissions from WRRFs. “Several approaches may help abate fugitive emissions from [anaerobic digestion], such as covering the storage facility, installing vacuum degassing, switching digester feeding from in parallel to in series, or directly feeding the upgraded biogas (or biomethane) into the local gas grid instead of doing combined heat and power which is known to have minor [methane] slippage due to incomplete combustion,” the authors wrote.

Greater study, funding needed

Although they have their limitations, the studies ideally will help to spark greater interest among wastewater sector participants in efforts to reduce methane emissions, says Z. Jason Ren, a professor of civil and environmental engineering at Princeton University and a co-author on both articles.

“These studies are still limited on sample size, duration, and representation on the size of the facilities and sewer lines,” Ren says. “Compared with research on other emission sources, such as oil and gas wells and pipelines, or even landfills, many more vigorous studies are needed to have more accurate information on carbon emissions associated with the wastewater sector.”

“We hope such research can receive more support from the wastewater sector, and by working together we can get the true quantification of this sector's GHG emissions, because without that we can't develop reliable net-zero plans,” Ren says.

For its part, the National Association of Clean Water Agencies seconds the call for additional research to further quantify and identify the sources of fugitive methane in wastewater facilities, says Cynthia Finley, NACWA’s director of regulatory affairs. The results of such studies are “very important and could lead to some good results in the end in reducing greenhouse gas emissions and increasing energy production,” Finley says.

NACWA would like to see the EPA help pay for methane-reduction projects at wastewater facilities using funding from its Greenhouse Gas Reduction Fund program, Finley says. Created by the Inflation Reduction Act, the GGRF has $27 billion in funding to be awarded in a competitive fashion as part of two grant programs. 

In mid-February, the EPA said that it expects to release Notice of Funding Opportunities for the two programs in early summer 2023.