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FARM WASTE TO ENERGY: A SUSTAINABLE SOLUTION FOR SMALL-SCALE FARMS
Impact/Purpose:
Environmental problems associated with manure management in livestock agriculture, along with rising energy costs, have fueled interest in energy-producing manure treatment technologies such as anaerobic digesters (AD), which produce methane gas that can be used to generate electricity, heat and/or lamp light. Public resources to develop and implement AD on farms have been spent primarily on large-scale confined livestock operations. However, the experience of households in developing nations illustrates that AD applications for small farms exist, and our previous work illustrates that small dairy farm operators have interest in this technology. One factor limiting the economics of AD technology at small farms, especially in cold climates, is the energy required to maintain the reactor temperature at 100ºF. Potential solutions to this engineering problem are to identify alternate/additional feedstocks to the digester, which would result in greater biogas production useful for heat, or use alternate heat sources on the farm presently wasted.
Description:
Through this project, it was determined that the utilization of anaerobic digestion on small farms for the purpose of managing varying waste streams and odor issues and decreasing fossil fuel consumption and cost was feasible. The feedstocks necessary to run the digester are readily available on farms where this system would be implemented. Although quantities of feedstocks may not be exactly equivalent to that of this project’s experiments, it was proven through lab experiments that in most cases any additional organic waste added to the reactor could increase the biogas production of pure manure. Furthermore this system gives farmers a solution for any disposal issues they currently may be facing given their current waste stream and techniques for managing it.
Through the lab experiments it was determined that any of the co-digestion mixtures tested resulted in a significantly higher rate of biogas production than dairy manure alone given all other variables were constant. Furthermore, the warmer of the two reaction temperatures and the shorter of the two hydraulic residence times tested both resulted in increased biogas production rate. A separate reactor was designed to treat fibrous, high solids wastes. This leachate system proved to be a positive influence on the digester as it eliminated the need to incorporate materials with high solids content in the digester while not forfeiting the energy value of these wastes. Given these results, co-digestion, a reaction temperature of approximately 37°C, and a hydraulic residence time of 15 days proved to be the most sensible course of action.
The economic analysis demonstrated that the anaerobic digestion of farm wastes would positively influence the economics of farming on all scales. Given the regression analysis, projection of future energy prices, and estimated offset yearly energy, it was determined that the two most productive uses of the biogas produced were to generate electricity and to burn it as a substitute for propane. Given the cost of power generation equipment, the production of electricity is not a feasible option for this system as it would greatly increase the capital investment and extend the payback period. Therefore, it was decided to use the biogas produced as a substitute for propane, which for the model farm provided revenues of approximately $10,429 per year ($6,491 in profit) and would completely pay off the cost of the system in 5.2 years.The successful development of a small-scale anaerobic digester that can be efficiently operated on the many small scale dairy farms in the United States would have many benefits. The people that would benefit from this technology include both farmers and their surrounding communities. This technology would present fossil fuel savings and provide a method for small farms to treat wastes efficiently, which they were unable to do in the past. This increases prosperity as farmers and communities can see a cost savings along with the reduction in fossil fuel use. This technology also benefits the planet in that it helps treat many different organic wastes and helps prevent pollutants, including harmful pathogens, from entering the environment. The reduction in fossil fuel use helps to reduce greenhouse gas emissions as the biogas produced is carbon neutral. The treatment of dairy manure also helps in preventing methane, a potent greenhouse gas, from entering the atmosphere.