Anaerobic Digestion Found to Produce Renewable Energy

As the nation looks to agriculture for renewable fuels from crops and other sources, University of Florida (UF) researchers have developed a manure management system for livestock farms that produces energy, saves valuable nutrients for fertilizer, cuts greenhouse gas emissions and stops offensive odors.

“It’s an environmentally friendly solution for an unpleasant housekeeping task,” said Ann Wilkie, an associate research professor with UF’s Institute of Food and Agricultural Sciences. “It’s not often that one technology can solve several major problems, but our innovative animal manure management system is a sustainable option for dairies and other livestock operations that produces renewable energy and protects the environment.” She said the growing number of big dairy and swine livestock farms — along with urban sprawl in rural areas — has resulted in greater awareness and concern about the proper storage, treatment and utilization of manure. Without proper management, animal manure can get into groundwater supplies, and odor problems can irk nearby residents. “The key to our waste management system is a natural biological process called anaerobic digestion that relies on microorganisms to transform animal manure into methane gas,” Wilkie said. “Anaerobic digesters, which process waste under oxygen-free conditions, are different than conventional aerobic systems that use oxygen to treat the waste.” She said anaerobic digesters can process five to 10 times more waste than aerobic systems. Because the waste is enclosed to keep oxygen out, anaerobic digestion keeps odors in. Odors, flies and pathogens are reduced by as much as 95 percent. With anaerobic digestion, the methane produced can be used to heat water or generate electricity, eliminating greenhouse gas emissions that contribute to global warming. Nutrients such as nitrogen and phosphorus can be recovered and used to fertilize crops. To demonstrate the technology at a working dairy farm, a large-scale anaerobic digester at UF’s 500-cow Dairy Research Unit in Hague is now generating biogas from manure flushed from animal barns and milking parlors. The patented waste treatment technology is being made available for licensing by UF’s Office of Technology Licensing. About 40 cubic feet of methane per day can be produced from the waste of each dairy cow, Wilkie said. Each cubic foot of methane has about l,000 BTUs, which adds up to a huge amount of usable energy. A British Thermal Unit is the amount of heat needed to raise the temperature of water by one degree Fahrenheit. Art Darling, executive director of Sunshine State Milk Producers Inc. in Orlando, said although methane technology is not cheap, it can solve important energy and environmental problems on Florida dairy farms. Darling said the UF system takes advantage of the fact that it is less expensive to move liquid containing manure than moving dry manure solids. The anaerobic digester processes manure from the large volumes of water used to flush waste from animal holding areas at the dairy. Because manure flushed from these areas is so diluted by water, only two types of anaerobic digesters are practical for Florida dairies — covered lagoons and fixed-film digesters, Wilkie said. Covered lagoons require large land areas, gas-tight covers and careful sealing to prevent nutrients from leaching into groundwater. By contrast, the fixed-film anaerobic digester at Hague is a 100,000-gallon tank that has a relatively small footprint, which can be a real plus when local land-planning issues are a concern, she said. “In covered lagoons, which are less efficient than fixed-film anaerobic digesters, the digestive bacteria float around, making only random contact with the manure particles,” Wilkie said. “In fixed-film digesters, the bacterial growth occurs on the surfaces of the internal media that the waste must flow over, thereby assuring frequent contact. In this way, higher volumes of wastewater can be processed.” She said a fixed-film digester can process flushed manure in two to three days compared to 30 to 40 days for a covered lagoon. Generally, the fixed-film design is suitable for any livestock manure that is diluted with water for transport or processing, such as dairy and swine waste. The byproducts of anaerobic digestion — liquid fertilizer and compost — reduce the need for synthetic fertilizers and soil conditioners that are produced using less sustainable methods, providing environmental benefits as well as cost savings, Wilkie said. Anaerobic digestion reduces the potential for global warming in two ways, she said. First, by capturing biogas, anaerobic digestion can reduce natural emissions of methane, a potent greenhouse gas. Second, when anaerobic digestion produces renewable fuel to replace fossil fuels such as coal, oil and natural gas, production of carbon dioxide from burning those fossil fuels is avoided. Another advantage of anaerobic digestion is that it produces very little sludge, which requires further processing and disposal. With aerobic treatment, up to 50 percent of the organic matter from the waste is converted to sludge. The anaerobic digester also lowers the levels of pathogens; starvation and competition with other microorganisms help kill pathogens that might be in the manure, Wilkie said.
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