Combining plant biomass with dairy manure significantly improved anaerobic digester economics, making renewable energy projects more practical for farmers.
Penn State researchers have found that adding plant biomass to dairy manure during anaerobic digestion could improve renewable energy production and reduce the financial barriers associated with farm-scale projects.
The findings suggest winter rye could be a particularly valuable feedstock for dairy farms seeking to manage manure, reduce methane emissions, and generate renewable energy.
According to Jeff Mulhollem of Penn State College of Agricultural Sciences, anaerobic digestion converts organic materials into biogas that can be used to generate electricity or processed into renewable natural gas.
Despite its environmental benefits, the technology remains financially challenging for many dairy producers because of substantial construction and operating expenses.
“Anaerobic digestion can help dairy farms manage manure while producing renewable energy, but many projects struggle economically,” said study senior author Juliana Vasco-Correa, an Assistant Professor of Agricultural and Biological Engineering at Penn State.
“A key question is whether adding crop residues or dedicated biomass to manure can make these systems more financially viable. The results of our study demonstrate that co-digestion with plant material can enhance the economic performance of manure-based anaerobic digestion,” Vasco-Correa added.
Published in Biomass Conversion and Biorefinery, the study modeled a 1,000-cow dairy operation in the northeastern United States. Researchers compared manure-only digestion with systems that combine manure with switchgrass, winter rye, or corn stover. They also evaluated the financial performance of producing electricity versus renewable natural gas.
Each biomass option offered distinct advantages. Switchgrass can grow on marginal land and contribute to soil health, while corn stover provides an opportunity to use crop residues.
Winter rye stood out because it can be planted between cash crops without requiring additional dedicated farmland. Its higher biogas yield also made it the strongest candidate among the evaluated feedstocks.
The economic analysis indicated that co-digestion could significantly lower the financial assistance needed to make projects economically viable.
The estimated capital support required to reach the break-even point dropped from approximately 47% for manure-only systems to as little as 4% under the best-performing co-digestion scenario. Such assistance can include grants and loan guarantees.
Study first author Camila Valderrama, who recently earned a master's degree in agricultural and biological engineering from Penn State, emphasized that system design must reflect the intended energy product.
“Larger systems improved the economics of renewable natural gas but could make electricity production less attractive, showing that scale and end use need to be considered together,” Valderrama said.
Vasco-Correa added that the modeling framework can be adapted to different farm sizes, feedstocks, locations, and market conditions. Researchers have also made their models, code, and datasets publicly available to support additional studies.
The findings highlight the importance of evaluating biomass availability, energy markets, system capacity, and policy incentives together. By selecting suitable feedstocks and carefully planning projects, dairy producers could improve renewable energy economics while addressing manure management and agricultural emissions.
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