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energy production. The larger the farm the greater the economic feasibility of anaerobic digestion. Methane production has efficiencies of scale that turn positive at around 500 cow farm sizes. Anaerobically digested manure has a significantly limited odor. Most easily digested organic matter will be broken down in the anaerobic digestion process. The gas production is controlled and burned so no odors escape from the digestion process. The resulting effluent is mostly inert organics and does not develop the objectionable odors that raw manure storage produces. As the manure is anaerobically digested some of the solids are converted to methane gas, carbon dioxide gas and water. About 4% of the solids are converted reducing the solid content and raising the moisture content of the effluent about 4%. This change in addition to some breakdown of the fibers in the manure, makes the resulting effluent much easier to pump. Solid separation systems also seem to work better on digested effluent than on the raw manure. Dairy manure from 500 cows is estimated to produce about 42,000 cubic feet of biogas per day. Using a 70 kW engine and generator this could produce about 1390 kW/d of electricity and allow significant heat recovery from the engine. It may be difficult to sell the electricity and to use all the heat produced. There have been a number of anaerobic digesters installed on farms. These systems have a mixed record of success. They are more likely to get the management attention they need to work well where needed as an odor control system. Anaerobic Digestion Description Manure from the 305 foot by 360 foot free stall barn is automatically scraped into a cross alley with step dams to facilitate gravity manure flow. Ten cubic yards of kiln dried shavings are used for bedding each week. The barn is insulated below the rafters with 1.5 inches of foil faced insulation. This was done to minimize the time that manure would freeze, both to keep the alley scrapers running during the winter as well as to limit the heating requirements for the manure. The 20 hp submersible manure pump is used to pump manure into the digester once a day. This pump cost $9,000 and is used 1.5 hours a day. The digester is a plug flow concrete tank 136 feet long by 30 feet wide by 14 feet deep. The engineering design was valued at $20,000. It was sized to provide a hydraulic retention time of 20 days when the herd gets to 1000 cows. It cost $160,000 to build which includes the floating insulation, the gas containing cover, and two hot water heating circuits. Both circuits can use the heat off the engine or heat from a separate boiler if the engine is not working. One circuit in the front end of the digester is used to heat the incoming manure while the other circuit runs the length of the digester and is on a different thermostat to maintain the temperature of the manure at about 100 degrees Fahrenheit. The gas is run to a 130 kW 3306 Caterpillar engine. The engine is a diesel block with a natural gas head that can be easily converted to run on biogas. The engine runs an induction generator to produce the electricity that replaces 9 cents per kW electricity used by the farm. The extra is sold back to the utility at 2 cents per kW, which is the wholesale price. The farm averaged $3,000 per month of electricity on all their sites. They expect to reduce this to $1,000 per month with the cogeneration system. This generator will not work if the electric utility fails since the induction generator needs an input from the utility to produce electricity. For emergencies a stand by generator will need to be installed. 4PDF Image | ANAEROBIC DIGESTION AND WETLAND TREATMENT CASE STUDY: COMPARING TWO MANURE ODOR CONTROL SYSTEMS FOR DAIRY FARMS
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