Digested Solids – Forms, Markets and Trends

Are Digested Solids a Viable Product?

Anaerobic digesters for U.S. livestock operations are becoming more complex. A study of livestock-based digesters in 2003 found they were built largely to meet on-farm needs for power or gas. Digester residuals were mostly land applied as nutrients for crop production. A few used fibrous solids as animal bedding (King, 2003). In recent years, more livestock-based digester projects have been built by third-party developer/managers. Projects increasingly employ a systems approach, where individual product streams are managed in concert for greatest profit by the project manager. This approach holds the promise that digestate residuals, especially fiber solids, will no longer be neglected, but instead play a larger role in offsetting weak performance in energy revenues.

What did we do?

Looking closely at dairy-based digesters, the solids recovered after separation from the digester eflluent have unique characteristics. Most notably, these solids tend to be fibrous with high cellulose, hemicellulose, and lignin content. Digestion also reduces pathogenic contaminants, volatile solids, odor, and viable weed seeds (MacConnell, 2010). These qualities can be influenced by the makeup of an animal’s feed and the use of co-digestion feedstocks, such as municipal or industrial wastes or other agricultural manures or byproducts

Table 1 shows the characteristics of dairy AD solids compared to raw manure and raw separated solids (MacConnell, 2010).

Table 1. Fiber Characteristics

Table 1.

As is. In bulk. Sold to a wholesale buyer—this is the easiest way to sell digested dairy fiber. Through a combination of literature search and expert interviews, this presentation looks at the methods project managers might use to add more value to their digested fiber.

What have we learned?

Composting. Perhaps the most basic way to add value to digested dairy fiber is simply to apply basic compost processing methods—aerating the material under controlled conditions for sufficient time to reduce odor and stabilize the organic matter. While already low in pathogens, hot composting practices can give additional assurance of pathogen reduction. In co-digestion situations, screening the material to remove contaminants and assure consistency and uniformity is desired. Even wholesale buyers will pay more for material that is already composted (King 2003)

Processing to compete – replacing peat. Because of its physical similarity, researchers have explored using digested dairy fiber as a direct replacement for peat moss in nursery and horticulture mixes. WSU was an early source of research and growth trials on such uses. Their research showed that with minimal post-digestion treatment, amended digested dairy fiber performed as well or better than peat in soilless mixes. (MacConnell, 2007, and Kruger, 2008) In 2007, Organix, a Washington company, announced the first shipments of RePeat, using their patent-pending FibreRite production system. Since then several new varieties of these peat replacements have hit the market nationwide, under such brands as Magic Dirt, EnerGro, and MooFiber.

Organic certification. Organic gardening and food production is growing rapidly in Washington state and around the nation. Getting an organic certification for organic matter and nutrients that have been digested and composted will add significant value to the final product (King, 2003).

Branding and marketing for retail. Moving away from bulk and wholesale are the next steps in moving material up the value chain. However, putting product in bags and selling into retail markets requires significant investments in packaging, branding, marketing and sales. This is like adding an additional business onto the back end of a digester project and demands its own feasibility analysis.

Vermicomposting. Using earthworms, especially redworms, to further process fiber solids and excrete earthworm castings, produces another specialty soil product. Vermicomposts and earthworm castings are well-known and appreciated in some growers in some markets. They are often used as a small additive in specialty soil mixes to allow the use of “earthworm castings” on the list of ingredients. Two commercial examples of vermicompost production lie on opposite coasts—Sonoma Valley Worm Farm in California and Worm Power in New York. Sonoma Valley Worm Farm direct markets high-quality vermicompost to a variety of growers throughout their region, with special emphasis on vineyards. Worm Power topped 2 million pounds of production in 2012 and signed an agreement with Rochester, NY-based Harris Seeds to market its vermicompost products regionally.

Specialty products produced from the separated fiber materials are another area of interest. Perhaps the best known of such products are the biodegradable planting nursery pots sold as Cow Pots by the Fruend Dairy Farm in Connecticut.

Biochar. This is another specialty product from a fledgling industry that fits in niche markets. It could be used to process digested fiber. It has received a strong research focus in the Pacific Northwest. The value of biochar in landscape or agricultural uses is still being studied, though at present it appears to have less to do with agronomic benefit, than on measured benefits for carbon sequestration and the value given to these benefits through carbon credits or other mechanisms (Galinato, 2011). On the other hand, replacing biochar for conventional forms of activated carbon for filtering stormwater or wastewaters shows some promising results and is getting a lot of attention.

Future Plans      

We will continue to evaluate methods to add value and publish the full results in a Anaerobic Digestion technology brief on this topic.


Jim Jensen, Sr. Bioenergy & Alt Fuel Specialist, Washington State University Energy Program jensenj@energy.wsu.edu

Craig Frear, Chad Kruger, and Georgine Yorgey, Center for Sustaining Agriculture and Natural Resources, Washington State University

Additional information  


Galinato, S., Yoder, J., Granatstein, D., 2011. The economic value of biochar in crop production. Energy Policy.

King, 2003. Study to Evaluate the Price and Markets for Residual Solids from a Dairy Cow Manure Anaerobic Digester—Final Report, King County Solid Waste, Seattle, WA.

Kruger, Chad, et.al., 2008. High-quality fiber and fertilizer as co-products from anaerobic digestion. Journal of Soil and Water Conservation.

MacConnell, C.B., Collins, H.P., 2007. Utilization of re-processed anaerobically digested fiber from dairy manure as a container media substrate. Proceedings of the International Symposium on Growing Media, Nottingham, UK.

MacConnell, C., Frear, C., Liao W., 2010. Pretreatment of AD-treated fibrous solids for value-added container media market, Center for Sustaining Agriculture and Natural Resources, Pullman, WA.


This research was supported by funding from USDA National Institute of Food and Agriculture, Contract #2012-6800219814; Biomass Research Funds from the Washington State University Agricultural Research Center; and the Washington State Department of Ecology, Waste 2 Resources Program.

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