USDA: Anaerobic digestion can help increase value of manure

By Erin Voegele | April 04, 2023

The USDA’s Economic Research Service in March published a report that aims to help increase the value of animal manure for farmers. The document, in part, describes existing and emerging technologies that have the potential to increase the value of manure or reduce manure management costs. It also describes current manure production, handling, storage and use data gathered through the USDA’s Agricultural Resource Management Survey.

The study found that there is substantial opportunity for increasing the use of manure as fertilizer. According to the USDA’s analysis, manure is currently applied to only 8 percent of the 240.9 million acres planted to seven major U.S. field crops. The agency said liquid-solids separation technologies; the use of manure additives, including biochar; and composting can help increase the value of manure fertilizer.

The study also addresses non-fertilizer uses for manure, including anaerobic digestion and thermochemical processes. USDA researchers site a 2018 study that examined the barriers or hurdles that U.S. dairy and swine producers face regarding the adoption of anaerobic digestion technology. That study found that greater adoption could be encouraged by lower construction costs, increased government grants and higher electricity prices.  

In its report, the ARS also discusses the impact of renewable energy credits in driving renewable natural gas (RNG) growth, including those generated through the federal Renewable Fuel Standard program and regional fuel programs, such as California’s Low Carbon Fuel Standard. RNG generated via the anaerobic digestion of manure is eligible to generate credits under such programs.

The report references the U.S. EPA’s AgSTAR data, which indicates 322 on-farm anaerobic digestion systems were operational in the U.S. as of the end of 2021, including 50 systems that began operations during that year. Roughly 78 percent of U.S. on-farm anaerobic digestion facilities are located on dairies. Financial factors currently limit the use the of anaerobic digestion systems on farms with less than 500 cows, according to the USDA. The systems are most common at concentrated animal feeding operations (CAFOs) with more than 1,000 animal units. In addition to financial limitations, other factors also impact the deployment of anaerobic digestion at smaller farm operations, such as lack of training and expertise, labor and maintenance issues. Community digesters represent a potential solution to these issues. Since 1994, community digesters have represented less than 9 percent of total U.S. anaerobic digestion facilities. They also shutdown at a higher rate than on-farm systems. Research suggests the higher rate of shutdown could result from a lack of consistent quality and cleanliness of off-farm feedstocks, such as food waste; poor revenue performance; and higher-than-expected operational, maintenance and repair costs.

Beyond anaerobic digestion, the report identifies pyrolysis, gasification and direct liquefaction as the three main thermochemical conversion processes that can be incorporated into current manure management practices and have the potential to convert feedstock into value-added products. All three processes produce a combination of volatile gases, bio-oils, and solids that can be used for heat and power generation, as transportation fuels, or as chemical feedstocks. The use of these technologies in manure management, however, face several challenges, including the diverse makeup of animal manure, high costs to meet emission limitations and high capital costs for equipment.

A full copy of the report is available on the USDA website