Experimental Methane Production from Animal Excreta in Pilot-scale and Farm-size Units

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Abstract
The forms of anaerobic digester that have been used in municipal waste treatment are described. The expected energy output from mesophilic (35 C) digesters fed livestock manure is assessed. Daily gas production of between 1.1 and 1.5 m3/m3 day per unit liquid volume of the digester is expected. On-farm storage of digester gas is not regarded as practical, and immediate use in an engine generator is recommended. It is expected that 18% of the gas energy could be recovered as electrical energy and that 55% could be recovered as thermal energy. A digester may be integrated into a system of hydraulic manure handling because most manures will require dilution before digestion. An analysis of the energy required to heat the slurry from a cold, confinement barn for beef cattle shows that some form of influent-to-effluent heat exchanger is necessary if surplus heat is to be available during cold weather. Heat losses from above-ground digesters may be reduced to negligible proportions with 75 to 150 mm of good insulation. Mixing requirements are discussed at some length; it is concluded that intermittent pulses of high intensity are desirable. The performance of a hypothetical digester for 250 beef animals is assessed and compared with published data. A simple economic analysis of the cost of generating 1 kWh is developed by using the 250-head beef model and published construction costs. If heat production is assessed at one-eighth the value of electrical power, then a farm digester of about 100 m3 volume could produce electricity for about $.12/kWh. The paper concludes with a brief discussion of some engineering problems that may be encountered if thermophilic digestion were employed. Copyright © 1979. American Society of Animal Science . Copyright 1979 by American Society of Animal Science.

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