Biofuels and Bioenergy

Small-scale animal farmers face unique challenges in bringing products to local markets. To stay profitable amid changing market conditions, on-farm systems that integrate inputs and outputs to keep nutrient and energy flows within the farm are essential. Anaerobic digestion (AD) of wastes is a powerful technology that serves this function, as it transforms manure into two valuable onfarm byproducts: Renewable biogas that can replace fossil fuels and nutrient-rich digestate that can be used as an organic replacement of petrochemical fertilizers. However, small-scale digesters that function passively (and therefore cost-effectively) in cold climates previously did not exist. In the last decade, such digesters that passively maintain operating temperatures through the greenhouse effect and thermal mass have been developed and implemented for under $300 in material costs (Figure-1). These digesters have the potential for low-income farmers in cold climates to manage manure more efficiently while creating a yield-boosting organic fertilizer and a fossil fuel substitute that can be used for heating greenhouses, value-added process heating or electricity generation. As food systems become more localized and distributed, such a technology can be a keystone to small farm viability. Researchers at Appalachian State University in the mountains of North Carolina have been designing, constructing and testing multiple prototype systems for cold temperature anaerobic digestion. A solar thermal system that uses thermo-siphoning has proven to be effective in maintaining digester temperatures and gas production through Boone, North Carolina winters that have an average of Soil Loss and Available Water Content to Assess the Sustainability of temperature between 0 and 5oC. We also have a Taiwanese type digester that is buried, insulated and heated through heat exchangers in-line with a static compost pile that has been able to maintain proper temperatures during periods of cold weather. We are also experimenting with a super-insulated digester building that should require very little energy for temperature maintenance.