Manure digesters, touted as eco-friendly solutions for managing agricultural waste and reducing greenhouse gases, have limited capacity to reduce livestock-related greenhouse gas emissions, and entail potential hazards that may outweigh their benefits, according to a new paper from researchers at the Johns Hopkins Bloomberg School of Public Health.
Manure digesters are covered lagoons or large, sealed tanks that use bacteria to break down animal manure and other organic farm waste. The process yields methane-rich “biogas,” which can be a source of energy and a nutrient-rich “digestate” that can be applied as fertilizer. Critics, including environmental nonprofits and rural community advocates, have argued that digesters worsen pollution hazards and incentivize industrial animal agriculture with no meaningful climate benefit.
The researchers evaluated the literature relevant to claims made by proponents and opponents, and found the benefits of manure digesters were overstated and risks and harms understated.
The paper was published online November 8 in Current Environmental Health Reports.
“Based on available evidence, we conclude that manure digesters should not be subsidized and promoted as a pro-environment, pro-public-health technology,” says study senior author Keeve Nachman, PhD, MHS, the Robert S. Lawrence Professor in Environmental Health and Engineering at the Bloomberg School and associate director of the Johns Hopkins Center for a Livable Future.
According to data from the Environmental Protection Agency, 460 manure digesters were in use or under construction on farms in the U.S. as of June 2024. Their rapid adoption has been supported by state and federal financing and subsidy programs.
The digesters have been promoted as a “green” technology that reduces agricultural greenhouse gas emissions and serves as a renewable source of energy and fertilizer. The technology is particularly widespread in the dairy industry in California, where the state provides grants and other incentives to support digester construction and subsidizes the sale of biogas digesters.
For the study, the researchers reviewed the literature relating to manure digesters and their impacts.
“We didn’t come to this with a bias against manure digesters,” Nachman says. “We started with what was being said about them, which is that they may be a sustainable climate solution, and we went through the claims and did our best to figure out what evidence exists to support or refute them.”
The authors concluded that manure digesters are not a climate solution. Methane emissions from untreated manure only accounted for about 11% of total U.S. agricultural greenhouse gas emissions, and even in the best case, digesters reduce just a portion of that fraction. As the scientists note, even that minor impact could be diminished by biogas leaks and increases in nitrous oxide, another potent greenhouse gas.
The authors also note that digesters may often end up “pollution swapping”—for example, reducing methane emissions while increasing ammonia emissions, toxic byproducts from biogas flaring, and other pollutant releases into the environment.
“Digesters don’t appear to be the solution for agricultural contaminants that proponents would have everyone believe,” says Brent Kim, MHS, an assistant scientist at the Johns Hopkins Center for a Livable Future and one of the study’s authors.
The researchers observed that any sustained economic viability of digesters is likely possible only in large livestock operations. Promotion of digester technology, including with government subsidies, effectively incentivizes industrial-scale agriculture, which has been widely demonstrated in prior studies to pose elevanted risks from air and water pollution, animal-to-human disease transmission, and other public health hazards to rural populations.
The authors also note that promoting manure digesters may represent a missed opportunity to protect the environment and public health.
“When we adopt a measure to address a problem, even an ineffective measure, society and policymakers tend to move on as if the problem has been solved,” says Kim. “Evaluating the state of scientific evidence surrounding newly purported ‘solutions’ is a critical step when we’re dealing with complex, systemic problems for which silver bullets rarely exist.”
“Deconstructing the Livestock Manure Digester and Biogas Controversy” was written by co-first authors Allie Wainer, David Love, and Brent Kim; and by Jamie Harding, Qinfan Lyu, D’Ann Williams, Christopher Heaney, Benjamin Hobbs, and Keeve Nachman.
Funding for the study was provided by the Silicon Valley Community Foundation.
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Journal
Current Environmental Health Reports