Emit What You Eat: An Investigation of the Link Between Bovine Diet and Their Excreta Methane Emissions
This morning I found myself a pair of shorts to wear and went downstairs to have a juicy, cherry-flavored Popsicle. Outside, however, the harsh winter wind was forcing snowflakes onto my window, tackling tree branches, and biting the noses of unfortunate pedestrians. The discrepancy between the pedestrians‘ Eskimo-style attire and my light, summery clothing made me better appreciate the comforts brought to me by natural gas. Thanks to Canada‘s abundant natural resources, we Canadians—faced with harsh conditions and extreme temperatures—can survive and create comfortable living environments amidst an unforgiving climate. The idea of my expedition, sparked by the painful realization that these sources of heat are non-renewable and rapidly depleting, was that the guarantee for future availability of natural gas for heating might in fact lie in our backyard.Having recently read that livestock manure contains a portion of volatile (organic) solids—such as fats, carbohydrates, proteins, and other nutrients, available as food and energy for the growth and reproduction of anaerobic bacteria—and that methane gas is the main product of these processes (often referred to as "bio-gas") and, when harnessed, can supply a large amount of energy, the seed was planted, and I began to do background research. I was looking for information about similar projects done in the past to evaluate the idea‘s feasibility.
Natural gas is a mixture of light hydrocarbon molecules, predominantly methane (CH4). It exists in a gaseous state at ordinary temperatures and pressures. Because it is a clean-burning fuel, virtually benign to the environment, it is becoming the fuel of choice for generating electricity, heating and cooling homes, powering automobiles, and fueling barbeques. Natural gas is conventionally extracted from wells in the earth, but due to its finiteness, its price will rise with scarcity, and when depleted we will be faced with a dire crisis.
Anecdotal evidence suggests that one of the first uses of bio-gas was for heating bathwater in Assyria during the 10th century BC, and Persia during the 16th century. Later, in 1808, Sir Humphrey Davy established methane as a product of the anaerobic digestion of cow manure (Lusk). Presently, the anaerobic digestion process is widely used not only as an energy source, but also for waste management, and for the use of its co-products (pure and odor-free, with the consistency of sawdust) as animal bedding or fertilizer. A good example of how this process has been successfully put into practice may be seen at the dairy barns and milking parlors on Fairgrove Farms in Sturgis, Michigan. Here, the manure is collected and conveyed to a plug-flow bio-digester. This bio-digester is in the form of a horizontal concrete tank, with a four-inch concrete cover to collect the bio-gas produced in the process. The bio-gas thus created fuels a Caterpillar generator that drives an induction generator, producing about $3,200 (U.S.) worth of electricity per month. The owners of the farm are very satisfied with the results, which, in addition to the electrical savings, include odor and fly control, and an added monthly revenue of $3,000 (U.S.) from animal bedding sold to nearby farms.
The method and feasibility was already established; I wanted to prove it for myself, and at the same time add a twist. To broaden the horizons of the existing topic, my purpose was to determine the effect of bovine diet on their excreta emissions by comparing the methane emissions of the cow manure from three farms, each administering different diets. With such information, farmers would have the choice to either minimize methane emissions (to reduce ozone depletion), or to maximize the methane emissions for the purpose of harnessing the fuel. Results from a study in 1997 (Johnson) found that an average dairy cow produces about 109 to 126 kilograms of methane annually (either as a by-product of ruminant digestion or as emissions from excreta). It listed some factors that reduce CH4 emissions, including high-quality feed, using legumes in grazing rotations, a high grain-to-forage ratio, the grinding and pelleting of feed, and not fermenting the feed.
Before I could begin any part of the experiment, I needed to design an apparatus to collect the gas. Since a full-scale bio-digester wasn‘t available for me to experiment with, I had to come up with a smaller-scale method of collecting my data. The ideas for the set-up seemed to flow from me with ease—how to maintain a constant temperature for the fermentation process, how to contain the slurry (mixture of water and manure), etc., but the idea causing the most concern was how to collect and measure the gas. At first I thought of connecting a syringe to the plastic tubing running from the slurry containers, but the stopper in the syringe was too sticky and did not slide along the tube with any ease. I thought of using the clean bag used in hospitals to hold IV liquid, but the idea was too impractical. Finally, after discussing certain proposals with one of my advisors, the idea of the downward displacement of water technique seemed to be the ideal solution.
I visited three farms in the Kingston, Ontario, area (O‘Connor, Maclean, and Doublejay) to collect the samples. Due to the limited selection of farms in my area, I couldn‘t find three farms with identical purposes (i.e. three milking farms or three beef farms). Thus, while the Maclean and Doublejay farms were dairy farms, the O‘Connor farm was a hobby farm (the cows served no specific purpose—only provided joy for their owners). The predominant feed constituents in the diets were: grass and hay (O‘Connor), fermented corn (Maclean), and oats and barley (Doublejay).Based on the factors inhibiting methane production (mentioned above) and the known feed on the farms in question, I predicted that Doublejay farms, which fed their cows dry, ground feed with no fermentation, and had a high grain-to-forage ratio, would produce the least amount of methane. I also predicted the O‘Connor farm would have the highest total methane production because of the lack of grain in the diet. If my predictions proved right, the O‘Connor diet would be the most desirable for the harnessing and utilizing of the fuel.
I collected approximately two kilograms of manure from each farm in separate buckets and added an equal amount of water to each in order to increase the viscosity (Figure 1). I poured each bucket of slurry into three labeled, two-liter pop bottles (nine pop bottles total) and sealed the openings with rubber stoppers (Figure 3). To collect the gas, I used the downward displacement of water technique, with an aluminium loaf pan filled with water, an inverted 100-milliliter graduated cylinder, a retort stand holding the cylinder‘s mouth under the surface of the water, and a tube leading from the pop bottle into the graduated cylinder to transport the gas (Figure 2). After recording the date and time of commencement, I placed the samples in a temperature-controlled environment (water bath) at 95°F for 88 hours. At regular time intervals of 16 hours and eight hours, the gas production was recorded for each of the nine samples by reading the water level of the graduated cylinders (Table 1). To my surprise, the gas produced had filled virtually all of the graduated cylinders by the second reading! This unforeseen circumstance meant that the cylinders had to be refilled with water after each reading in order to obtain acceptable readings.The results obtained during the expedition provided interesting data as to rates of methane production and the factors that affect it. The total methane production is graphed in Figures 4–7. I postulated that the samples from the Doublejay farm would produce the least amount of methane due to dry feed, a high grain-to-forage ratio, and ground grain in the diet. The results verified this prediction. The samples from the Doublejay farm produced 4.9 percent less methane than the Maclean farm and 3.2 percent less than the O‘Connor farm.
Although I predicted that the samples from the Maclean farm would yield the second highest amount of methane, they actually produced the most. The reasoning behind the hypothesis was that although fermented corn (a methane-production enhancer) was a constituent in the diet, there was also a high grain-to-forage ratio (a methane-production inhibitor). The results indicated that fermented food rations in the diet have a strong effect on methane production and override the reduction factor provided by a high grain-to-forage ratio.
Although the overall experiment seemed to run smoothly, there were some errors in the method that I would like to mention. Perhaps the most significant error was the underestimated volume of methane emitted. Because of this error, the capacity of the graduated cylinders used was much too small and contributed to inaccurate measurements (because once the water was displaced completely out of the cylinder, no further gas emissions could be measured until the next reading). For a repeat of this experiment, I would use graduated cylinders with a greater capacity. The loaf pans, used as a container for the water in the downward displacement of water technique, leaked due to excessive handling, as they were refilled at every reading. This allowed the water to escape the container and release the water held within the graduated cylinders. Luckily, this happened only at the conclusion of the experiment after the last readings were taken. To avoid this problem in the future, more durable containers could be used.
The final error in this experiment that I will discuss pertains to the composition of the gas collected. When I was first designing the experiment, I searched for experts in my specific topic and came across Paul Harris, from the University of Adelaide in Australia. He was kind enough to give me some pointers for my method and also mentioned that the gas collected from the samples would be a mixture mainly of methane and carbon dioxide, with traces of other gases. Apart from carrying out a flame test to assess the gas‘s flammability, I did not conduct any further analysis of the gas content. This would be a goal of mine for future investigation, but the combustion of the gas was satisfaction enough for the moment.
There are many applications of the results obtained from this experiment. Presently in North America, the process of anaerobic digestion has been limited to private agricultural use. The reason seems to be because of the lack of public demand. Because of North America‘s abundance of natural resources, the threat of scarcity seems a distant nightmare and, for the present, the public is satisfied to rely on them. Fortunately, with time the process will be refined, and when the need arises we will have a backup source of energy. In third-world countries, however, where electricity and heat are sparse and biological waste is profuse, the anaerobic digestion process could be the light at the end of the tunnel. Farmers could consider the factors I found to maximize methane emissions and incorporate them into their cattle‘s diet. thus taking advantage of waste-managing, bedding-providing, electricity-supplying cow manure. Whoever imagined how attractive this repulsive matter could eventually become? Possibly in 100 years‘ time, my descendents will wake up, find some shorts to wear and a Popsicle to eat in the house, look out the window at the heart of winter, and be able to thank cow manure for providing them with their comfort.
Acknowledgments: There are many people who graciously contributed to this expedition. I greatly appreciate the assistance from my advisors: Mr. Tim Dowling, Mr. Erich Kuehnle, and Ms. Alex Nuttall, who helped me to collect the samples, verify the measurements, and discuss ideas/problems. Acknowledgements go to my teachers: Mrs. Delvecchio and Mr. Chittick especially, for their support and help. I would also like to recognise Mr. Paul Harris, an expert in his field (anaerobic digestion processes), who offered many pointers for which I am very grateful. I would also like to thank my parents and younger sister for their consistent motivation and enthusiasm.
American Society of Agricultural Engineers. Livestock Waste: A Renewable Resource. St. Joseph, Michigan: American Society of Agricultural Engineers, 1981.
Goldstein, J. "Anaerobic Digestion Advances." BioCycle 41, February 2000, pp. 30–32.
Harris, Paul. "Methane Digesters." University of Adelaide, Australia. Retrieved from the World Wide Web on November 20, 2001: http://www.webconx.com/Methane.htm
Johnson, Matthew T. "Analysis of Bovine Methane Emissions." Retrieved from the World Wide Web on November 19, 2001: http://www.meteor.iastate.edu/gcp/studentpapers/1997/chem/johnson1.html
Lusk, P. Methane Recovery From Animal Manures: A Current Opportunities Casebook. Golden, Colorado: National Renewable Energy Laboratory, 1998.
Copyright © 2002 American Museum of Natural History.
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Supplement a study of alternative energy sources with an activity drawn from this winning student essay.
- Send students to this online article, or print copies of the essay for them to read.
- Have them research the energy possibilities of methane further, reporting on what they learn. If desired, you can have them start with the references cited at the end of the essay.
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