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| Fig. 1: Manure digesters near Waunakee, Wisconsin (Source: Wikimedia Commons) |
On all animal farms across the world, there is always one constant: manure. Because of its abundance of organic matter and nutrients, this manure is often made into fertilizer to sustain a multitude of crops. In addition to these nutrients, however, the anaerobic digestion of manure by bacteria produces a significant amount of carbon dioxide and methane, a principal component of natural gas known both as a great fuel and a potent greenhouse gas. As scientists study the impacts of greenhouse gases on our climate, it has become increasingly apparent that the methane emanating from lagoon and pit manure management systems is proving harmful to Earth's atmosphere. Although these systems provide a useful way to store manure (for future uses such as fertilizer etc.), they create an environment where these harmful greenhouse gases are rampantly emitted through anaerobic digestion. According to Key and Sneeringer, the U.S. agricultural sector was responsible for 6.1 percent of total U.S. greenhouse gas emissions in 2018, with methane emissions from manure management responsible for about 10.5 percent of these agricultural emissions. [1]
Methane digesters (see Fig. 1), also known as anaerobic digesters or manure digesters, can be a viable method to remedy this problem. These devices can be used to capture and burn methane from lagoon or pit manure-storage facilities before sending the biogas to a boiler or electricity generator. Thus, this technology not only limits the amount of methane in our atmosphere, but also supplies farmers with a renewable source of electricity that can be used on-site or sold to the electricity distribution grid.
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| Table 1: Total United States manure production by main livestock and percent reduction of volatile solids by anaerobic digesting bacteria. [2-4] |
In 2017, the USDA recorded the livestock inventory of the United States to be about 31.7 million beef cattle for slaughter, 9.6 million dairy cows, 72.3 million swine (hogs & pigs), and 1.6 billion broiler chickens. [2] On average, annually, a dairy cow will produce around 20,000 kg of manure, a beef cow will produce around 4,500 kg of manure, and a typical hog will excrete around 550 kg of manure. [3] A broiler chicken will produce around 5 kg of manure in the seven weeks that it is fed (about 37 kg annually accounting for the cycling of chickens). [3] The total amount of manure produced annually for each of these livestock sectors is shown in Table 1.
The amount of this manure that is digestable, however, depends on the amount of volatile solids present in the manure. For example, cattle produce 5 kg of volatile solids per day, pigs produce 0.4 kg of volatile solids per day, and chickens produce 0.02 kg of volatile solids per day. [4] Therefore, about 40% of cow manure, 30% of hog manure, and 20% of chicken manure are volatile solids.
Of these volatile solids, however, only a fraction contained in the raw manure are actually broken down or destroyed by the bacteria. This is due to the composition of the volatile solids of the waste which includes both readily degradable organic compounds including lipids, proteins, and carbohydrates, as well as more refractory organics which may include lignocellulosic materials, complex lipopolysacharides, structural proteins (keratin) and other refractory organics. Thus, all volatile solids are not equal and, therefore, exhibit different rates and extents of biodegradation during anaerobic digestion. For instance, the non degradable portion of volatile solids tends to be high in fibrous solids. [5] The percent reduction of these volatile solids (fraction of the volatile solids that is broken down anaerobically by bacteria) is shown in Table 1.
As of April 2017, there were 250 anaerobic digesters in operation at commercial livestock facilities in the United States. The full potential to provide renewable energy is much greater: an estimated 8,100 U.S. dairy and swine operations could support biogas recovery systems. [6] Installing digesters at dairy and swine operations where it is feasible could reduce their methane emissions by about 85 percent or nearly 2 billion kilograms per year. [6]
These digesters aren't just great for reducing emissions.The biogas they produce is great for energy generation. In fact, if these digesters were installed, they would produce biogas that could generate over 1.11 × 1017 joules of energy per year. [6]
Although this technology seems ideal on the surface, there are some reasons why there are so few digesters in the United States. The issues with adopting such technology primarily concern a lack of economic incentive. A significant factor that affects the economic incentive to build these digesters is farm size. For example, a typical Northeastern dairy farm with 200 cows spent nearly $29,000 on electricity, propane, and natural gas expenses in 2005, and that expense rose to $63,000 on farms with 500 cows, and $114,000 on farms with 1,000 cows. [3] It becomes obvious that larger farms have a much stronger incentive to seek out investments that will allow them to replace purchased electricity, while small farms with digesters would need a market outlet for their electricity. When it comes to farms of a given size, however, expenses can vary widely, and so can the incentives for digester adoption, with differences in farm production practices and location. [3] Location matters of course because electricity prices vary across the country and variations in climate affect heating and cooling demand. Farms with relatively great electricty costs are heavily incentivized to invest in methane digesters to avoid local cost of electricity. Farm production practices matter as well because, for example, farms that milk three times a day will use more electricity and fuel than those of similar size that milk two times a day. Other practices such as extensive crop growing, where heifers are raised, etc. also contribute greatly to how much electricity a farm must purchase. For instance, if a typical 500-cow Northeastern dairy spent about $63,000 on those expenses in 2007, a dairy with 500 cows but with production practices more common for Western operations would spend about $28,000. Moreover, a Western operation with 1,000 cows would spend about $51,000, still well below expenses at a 500-cow Northeastern dairy. [3] Thus, a typical Northeastern dairy would be more incentivized to build a digester to avoid paying these relatively enormous energy costs each year.
Because anaerobic digesters have so many benefits attributed to them, especially for the environment, however, several federal entities have been working with an aim to provide incentives to build these projects. Probably the most significant of these incentives, is one that allows producers to be paid for reductions in greenhouse gas emissions. [1] This incentive, and others like it will hopefully encourage farms of all sizes and location to adopt these useful devices.
In all, methane digesters seem to be some of the most promising technology in terms of renewable energy generation. Not only are these devices proficient at generating energy, they help significantly reduce our agricultural greenhouse gas emissions. Currently, there remains some incentive issues that prevent methane digesters from widespread adoption, however, with federal intervention we should start seeing more digesters across the U.S. agricultural sector being put into use.
© Amir Davis. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] N. Key and S. Sneeringer, "Climate Change Policy and the Adoption of Methane Digesters on Livestock Operations," U.S. Department of Agriculture, Economic Research Report No. 111, February 2011.
[2] "2017 Census of Agriculture," U.S. Department of Agriculture, AC-17-A-51, April 2019.
[3] J. M. MacDonald et al., "Manure Use for Fertilizer and for Energy," U.S. Department of Agriculture, June 2009.
[4] I. M. Nasir, T. I. Mohd Ghazi, and R. Omar, "Anaerobic Digestion Technology in Livestock Manure Treatment For Biogas Production: A Review," Eng. Life Sci. 12, 358 (2012).
[5] A. C. Wilkie, "Anaerobic Digestion of Flushed Dairy Manure," University of Florida, 2003.
[6] "Market Opportunities For Biogas Recovery Systems at U.S. Livestock Facilities," U.S. Environmental Protection Agency, EPA-430-R-18-006, June 2018.
