J. Anim Sci.
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J. Anim Sci. 1979. 48:202-217.
© 1979 American Society of Animal Science
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Experimental Methane Production from Animal Excreta in Pilot-scale and Farm-size Units1,2,

R. J. Smith, M. E. Hein and T. H. Greiner3

Iowa State University, Ames 50011

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.

Footnotes

1 Paper presented as part of the Symposium on Alternatives in Animal Waste Utilization at Annual Meeting of the American Society of Animal Science, University of Wisconsin, Madison, July 23 to 27, 1977.

2 Journal Paper No. J-8917 of the Iowa Agriculture and Home Economics Experiment Station, Ames, Iowa. Preparation of the material in this paper has been supported by Iowa project 2126, a contributing project to the North Central Regional Research Project NC93, Animal Waste Management Systems for the 1980's.

3 The authors are associate professor, graduate research assistant, and pre-doctoral associate in the Agricultural Engineering Department.






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Copyright © 1979 by the American Society of Animal Science.