WASTE MANAGEMENT
PIH-76
pork industry handbook
COOPERATIVE EXTENSION SERVICE • PURDUE UNIVERSITY • WEST LAFAYETTE, INDIANA
Methane Gas From Swine Manure
Authors
John M. Sweeten, Texas A & M University
Charles Fulhage, University of Missouri
Frank J. Flumenik, North Carolina State University
Reviewers
James C, Converse, University of Wisconsin
Elbert C. Dickey, University of Nebraska
A. G. Hashimoto, USDA/SEA, Clay Center, Nebraska
Don D. Jones, Purdue University
Richard K. White, Ohio State University
The concept of methane production has appeal-to many swine producers as a means of reducing escalating fuel bills. From the estimated 9 million tons of swine manure (dry basis) produced annually in the United States, roughly 75 billion cu. ft. of methane could be produced every year. For this potential to be realized, however, economical methane production units must be developed and demonstrated.

Anaerobic digestion, the process used to produce methane, also stabilizes the swine manure, thereby reducing its odor and fly breeding potential. Essentially all the original nutrients in raw manure are present in the digested slurry and can be recovered by land fertilization. Another possibility for nutrient recovery is centrifugation of the digested slurry to reclaim edible protein feedstuffs.

There are drawbacks, too, however. Since commercial prototype digesters are not widely available, farmers and their consultants may have to design their own methane production systems. Methane is explosive at concentrations of 5-15% in air. Mechanical aspects of manure handling (grinding, mixing, screening and pumping) and general plumbing problems (gas leakage, corrosion of pipes and valves, etc.) have been the major source of problems in methane production, rather than the chemical and biochemical processes. These problems could be overcome by adequate financing and competent engineering design.

Methane is difficult to store, requiring a pressure of 5,000 psi for liquefication, or 30 times the pressure needed to produce liquefied petroleum gas (LPG). Thus, use of methane is limited to continuous, stationary sources such as heating or operation of electric generators near the site of methane production. On a swine farm it may be difficult to satisfy energy needs through methane production alone, and a large swine feeding operation may yield more methane than can be used at that site on a continuous basis.
Sale of methane through a natural gas pipeline system will be practical only in rare circumstances. This is because it would normally take as many as 50,000 hogs to produce 100,000 cu. ft. of methane daily, which is considered the smallest marketable gas quantity. In a typical confinement swine operation, most of the gas should be usable on the farm.

Some of the principles of methane or biogas production together with yields and uses of the product gases on a swine farm are explained in this fact sheet.

Basic Process
Methane (ChU) is the primary component of natural gas. Like natural gas, pure methane has an energy content of 1,000 BTUs per cu. ft. Bacterial degradation of manure under anaerobic conditions (without oxygen) releases a mixture of gases (biogas) which usually consists of 50-60% methane, 40-50% carbon dioxide, and about 1 % by volume of hydrogen sulfide, ammonia and other trace gases. Raw biogas has an energy content of only 500-600 BTUs per cu. ft. Carbon dioxide, trace gases and water vapor can be removed by chemical means, yielding pipeline quality gas.

In anaerobic digestion of manure, bacteria degrade organic solids into organic acids and then into methane, carbon dioxide and water. Changes in the manure loading rate, temperature fluctuations and the presence of oxygen or toxic elements frequently cause problems in the functioning of the digester. The methane-producing bacteria are most sensitive to improper operating conditions, and when they are inhibited, organic acids will accumulate.

The amount of methane obtainable from swine manure varies depending upon the feed ration, manure collection procedures, digester design and operating conditions. The amount of wet manure (feces and urine) produced daily by swine amounts to 4-7% of their body weight. For a 150 lb.