This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: The U.S. dairy industry is a major contributor to agricultural greenhouse gas emissions, with manure storage and handling representing a significant source of methane. Anaerobic digestion of cow manure—a biological process in which microorganisms break down biodegradable organic matter—has been widely studied as a strategy to mitigate manure-related emissions while producing renewable energy.
That energy comes in the form of biogas that can be used to generate electricity or make renewable natural gas. But adoption of the strategy has been constrained by costs. Now, a team led by researchers at Penn State may have found a way to make the process profitable.
The researchers, who recently published their findings in Biomass Conversion and Biorefinery, reported that mixing manure with plant biomass can increase methane production—which is necessary for biogas—and improve the economic feasibility of farm-scale anaerobic digestion. They found that winter rye, a tough, cold-hardy cereal grain planted in the fall to protect and enrich the soil over the winter months, is particularly useful for this application. But anaerobic digester projects remain highly dependent on financial incentives, so they carry substantial investment risk for farmers, noted study senior author Juliana Vasco-Correa, assistant professor of agricultural and biological engineering in Penn State's College of Agricultural Sciences.
The problem, she explained, is that building and operating a digester is expensive, while the revenue from selling electricity or gas may not be high enough. "Anaerobic digestion can help dairy farms manage manure while producing renewable energy, but many projects struggle economically," she said. "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." In the study, the researchers modeled a 1,000-cow dairy farm in the Northeast and compared manure-only anaerobic digestion with co-digestion using switchgrass, winter rye and corn stover—a strategy called grass-to-gas. They included switchgrass because it is a perennial grass that is good for the soil and can grow on marginal land; winter rye because it can grow between cash crops, so it does not need additional land; and corn stover because it is an agricultural residue. The team evaluated both electricity generation, which was the most common use of the biogas for many years, and renewable natural gas, which is the most common new use for biogas due to its eligibility for federal compliance credits as a transportation fuel.
Winter rye was the most promising crop, they found, because it yielded more biogas. Instead of looking at one "best-case" economic scenario, the researchers combined detailed process modeling with engineering design, performance metrics and financial realities to evaluate whether the technology could succeed at farm scale. They evaluated how feedstock choice, methane yield, system scale, biomass cost and market conditions interacted, and found that co-digestion can substantially improve project economics.
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