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Methane emissions from wastewater treatment may be nearly double previous estimates

Photo credit: Dana.S., stock.adobe.com

After measuring and analyzing emissions from 63 U.S. municipal wastewater treatment plants, two new studies by researchers at Princeton University show that the process used to quantify methane emissions may be  substantially underestimating the extent of the methane released.

Researchers at Princeton’s Andlinger Center for Energy and the Environment suggest that guidelines developed by the Intergovernmental Panel on Climate Change (IPCC) are based on limited measurements at a relatively small number of wastewater treatment plants. When the researchers used the Princeton Atmospheric Chemistry Experiment (PACE) Mobile Laboratory to estimate methane emissions, they were nearly double the estimates derived from the IPCC method.

Using open-path portable gas sensors with near-infrared (IR) and mid-IR lasers as light sources, the PACE tests yielded methane estimates 1.9 times greater than emissions estimates that use existing IPCC and U.S. Environmental Protection Agency guidelines. The results could mean that current guidelines underestimate methane emissions equivalent to 5.3 million metric tons of carbon dioxide, according to the new studies.

Mark Zondlo, professor of civil and environmental engineering and associated faculty at the Andlinger Center for Energy and the Environment, said that the waste sector is one of the largest anthropogenic sources of methane in the world.

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“As cities continue to urbanize and develop net-zero plans, they can’t ignore the liquid wastewater treatment sector,” Zondlo said in an announcement from the university.

Daniel Moore, first author of the direct measurement study, and a graduate student in civil and environmental engineering, suggested that the IPCC guidelines assume a “certain level of efficiency in these wastewater treatment systems that may not exist on a plant-to-plant basis.” Moore pointed to leaks and inefficient equipment that may go undetected at wastewater treatment plants, yet could lead to significant greenhouse gas emissions.

Professor of civil and environmental engineering, Zhiyong Jason Ren, said his team of researchers, in a second study, utilized machine learning methods to analyze published literature data from methane monitoring studies of various wastewater collection and treatment processes around the globe.

“Not many people have studied the methane emissions associated with wastewater infrastructure, even though we know that it’s a hotspot for methane production,” Ren said in an announcement from the university. “We were able to show, using two different approaches, that methane emissions are a much bigger issue for the wastewater sector than previously thought,” Ren added.

Independent studies by Princeton researchers Cuihong Song and Daniel P. Moore concur that methane emissions from municipal wastewater in the United States are actually closer to double than formerly calculated. Photo credit: Bumper DeJesus, Princeton University

Cuihong Song, first author of the study and a postdoctoral researcher in civil and environmental engineering at Princeton, said the research determined that treatment plants equipped with inefficient anaerobic digesters were among the biggest methane leakers due to issues such as pressure buildups. The research showed that these plants emitted more than triple the methane released by plants without digesters.

“If the digester is not gas-tight, you can end up with high methane emissions,” Song said.

The study notes that wastewater treatment plants equipped with anaerobic digesters account for less than 10% of all treatment plants in the U.S. However, most of these plants are large facilities, and they treat around 55% of the wastewater in the country.

While the researchers say that estimations of methane emissions may be skewed, Ren added that the methane produced from processes like anaerobic digestion also serves as a valuable energy source, and by “identifying and mitigating fugitive methane emissions, we would see double benefits” through recovery.

Along with anaerobic digesters, researchers found that methane emissions from sewer systems contribute significantly to nationwide methane emissions. However, current guidelines largely do not account for fugitive methane emissions from sewers, which the researchers said are important to account for in future greenhouse gas inventories.

“We have more than a million miles of sewers in the U.S. filled with rich organic matter that may be causing methane emissions, but we have very little understanding of their scope,” Ren said.

The researchers are now working with partners to build an inventory and methodology that would allow managers to easily monitor their methane emissions.

This article appears in ES&E Magazine’s April 2023 issue:

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