Adding plant material to manure digesters can turn biogas production from a break-even proposition into a profitable one for U.S. dairy farms, according to research published in a peer-reviewed agricultural journal.
The finding matters because manure storage and processing is one of the largest sources of agricultural greenhouse gas emissions in the United States. Dairy operations produce methane — a gas roughly 28 times more potent than carbon dioxide over a 100-year period, according to the U.S. Environmental Protection Agency — when manure decomposes in open lagoons and holding ponds.
Biogas systems capture that methane and burn it to generate electricity or refine it into renewable natural gas. But digesters that process manure alone generate too little gas to cover their construction and operating costs, which run into the millions of dollars per facility. Mixing in crop residue, food waste, or other plant material — a process called co-digestion — raises gas output enough to change the economics.
The U.S. has roughly 340 operational anaerobic digesters on livestock farms, according to EPA data, a fraction of the more than 20,000 large dairy and hog operations nationwide. High upfront capital costs and uncertain revenue have limited adoption despite federal incentives.
Those incentives include the USDA's Rural Energy for America Program, which offers grants covering up to 25% of project costs, and the federal Renewable Fuel Standard, which assigns credits to biogas-derived natural gas. Utilities in states including California and Vermont also pay premium rates for renewable natural gas injected into their pipelines.
Co-digestion introduces its own complications. Plant material must be sourced, transported, and tested for contaminants, and digesters designed for manure can struggle with the acidity of certain feedstocks. Farms that accept food waste from municipalities often must secure permits and manage odor complaints from neighbors.
Still, the potential revenue streams are stacking up. Renewable natural gas credits traded under California's Low Carbon Fuel Standard have at times exceeded $100 per credit, and major energy companies including Chevron and BP have invested in dairy biogas projects in recent years.
The research adds to a growing body of evidence that the economics of farm-scale biogas depend heavily on what goes into the tank — not just how the system is built.
