Digestate Utilization In The U.S.

digestate as bedding material

Farmers are finding economic benefit to generating their own cow bedding and quick release fertilizer.

Ron Alexander
BioCycle January 2012, Vol. 53, No. 1, p. 56


Digestate is the liquid or solid material generated after the anaerobic digestion process. In the U.S., digestate is usually separated to create a liquid and a solids product. The liquid is typically applied onto farmland as a low analysis fertilizer and the solids (usually around 30% total solids after separation) are used “as is” for animal bedding, or composted for use in horticultural applications. Some farmers who use the digested fibers as bedding will age or compost it to reduce its moisture content, making it easier to handle and less messy, while also reducing its odor and ammonia content. Data from a recent Innovation Center for US Dairy project, completed by R. Alexander Associates, Inc., found that farmers are finding economic benefit to generating their own cow bedding and quick release fertilizer (digestate liquid instead of aged manure) by managing their manure through anaerobic digestion.

Wet or low solids digestion is primarily used on farm-based manure management projects and in wastewater treatment facilities. In these processes, the digestion is done in aqueous conditions (around 5% total solids). Dry digestion (or high solids digestion), on the other hand, processes feedstock resembling a compostable mix of organics (35-40% total solids, but stackable). This technology has been used to a great degree in Europe for biowaste (a mixture of food waste and yard trimmings), and to a much lesser extent to process manure. This would also likely be the case in the U.S. market.

Digestate solids from dry digestion are likely to be composted whereas with wet digesters (especially those treating primarily manure), other options exist, and will be driven by the expected end use. In wet digestion, soluble nutrients (e.g., ammonia nitrogen and potassium) from the feedstocks will go into solution, thereby reducing the nutrient content of the separated solids. In dry digestion, the nutrients are concentrated within the solids, as little liquid is actually generated. Like in composting, as dry digestion continues (along with outdoor aging of the digested solids), carbon is converted (into methane and carbon dioxide) and water is lost through evaporation and heating. Therefore, the nutrients not used by the microbes are further concentrated within the solids.

Whether wet or dry digestion is used, the digested feedstock may or may not contain enough volatile solids or porosity to properly compost (or reach pasteurization temperatures) following digestion. This must be studied and considered during the planning stages of the project. Observations in the U.S. suggest that separated manure solids, although very wet, can often be properly composted without the addition of bulking agents. Data from Europe suggests that the volatile solids content after dry digestion varies, therefore, pasteurization temperatures required in the U.S. may not always be met. In these cases, fresh yard trimmings are simply blended into the digested material to allow for proper composting.

Experience in the Marketplace

Relatively little digestate has hit the marketplace thus far in the U.S., primarily because much of the manure-based solid digestate is reused by farmers as cow bedding. The cost of wood-based animal bedding is getting more expensive as these materials are burned in biomass plants. As a result, sand and separated manure solids are the only inexpensive bedding materials currently available. And sand is only inexpensive if its source is relatively close to the farmer’s location. Again, based on data generated through the Innovation Center for US Dairy project, sand costs anywhere from $6 to $13/ton, delivered (one ton of sand equals approximately 0.8 cy). Wood-based animal bedding products (e.g., sawdust, shavings, etc.) can cost $10 to $20/cy, delivered. For this reason, AD fiber in a fresh or composted form is an attractive bedding material.

Manure based fiber, primarily after processing (e.g., aging, composting, biodrying) is also making its way into the horticultural marketplace. It has been used as a lightweight compost (soil amendment), as well as a nursery media component to replace peat, coir and bark. It is currently sold on a wholesale basis for $6 to $16/cy (picked up) for use in these applications, and double that price on a retail basis. It is likely that even higher values are attainable because of the fiber’s physical characteristics (e.g., lightweight, therefore highly shippable). As shown in the data in Table 1, the digester fiber can be easily stabilized through composting, making a better horticultural component.

Liquid manure digestate is primarily being land applied onto farmland as a replacement to chemical fertilization. This liquid contains substantial amounts of ammonia-based nitrogen, which allows for fast vegetative growth. However, that also means it possesses a substantial “burn potential” (see ammonia and electrical conductivity content in Table 2). Therefore, it may require watering (over the top irrigation) to get it off of leaf surfaces. Concentrating the liquid into a higher grade liquid fertilizer is also being investigated. If economically feasible, it will allow the product to be shipped over longer distances, expanding its geographical market.

Digestate that is primarily food waste derived is not commonplace in the U.S. thus far, but it appears to be coming, based on interest in increasing food waste collection. (This includes use of old wastewater digesters at treatment plants (wet digestion).) However, if this feedstock is “dry digested” along with yard trimmings, it should be composted to produce a high quality product. If food waste is processed as the primary feedstock through wet digestion, a lot of processing will be required to yield a marketable product.

Regardless of the planned anaerobic digestion feedstocks and technology, the prospective processor must plan ahead and have an idea of what they are going to do with the resulting products. Without an upfront and understood outlet, unforeseen costs may be incurred by the facility. For that reason, the AD industry should learn an important lesson already learned by the U.S. composting industry…. invest in determining your markets and in product and market development. The sale of the resulting product can generate substantial revenue.

Ron Alexander is president of R. Alexander Associates, Inc. (Apex, North Carolina, 919-367-8350, www.alexassoc.net), a company specializing in market development and research for organic recycled products. Alexander is a horticulturalist with over 27 years of experience working with compost and related organic recycled products. He has also authored “The Practical Guide to Compost Marketing and Sales”, 2nd Edition (The JG Press, Inc., 2010) and is a US Composting Council Board Member.

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