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| Fig. 1: Overview of the circular economy of biogas production from waste (Source: Wikimedia Commons) |
More than 70 million tons of organic waste is produced in the United States each year. [1] The majority of this waste consists of food waste, livestock manure, agricultural wastes, waste water, and inedible food wastes. A significant risk to the environment and public health arises when the pathogens, chemicals, and excess nutrients within the waste are not properly managed but instead contaminate water sources, cause algal blooms, and harm wildlife. An opportunity comes from the fact that the decomposition of organic waste generates large amounts of methane a potent greenhouse gas that has more than 80 times the warming power as carbon dioxide. [1] However, methane produced from organic waste (biogenic methane) is derived from atmospheric carbon (instead of being pulled from the earth like methane from fossil fuels) and is recycled as a part of the biogenic carbon cycle. [2] Manure management alone from agriculture accounted for roughly 4% of total anthropogenic methane emissions in 2010. [3]
Methanes relatively short atmospheric lifespan of 12.4 years (carbon dioxide can linger around for hundreds to thousands of years) means that methane only warms the planet for about 12 years and is considered a short-lived climate pollutant. It is possible for the amount of methane being emitted by organic waste to equal the amount of methane being destroyed through oxidation, thus making atmospheric warming neutral. [2] Anaerobic digestion technology presents an opportunity to capture and harness biogas the gas that is naturally produced as organic matter decomposes and to extract the biogenic methane from it. Biogenic methane can be used as an energy source, which can create a cooling effect in the atmosphere, since there would be more methane being destroyed than emitted. The biogenic methane that AD systems recover can also help displace fossil fuels, reduce emissions, improve air and water quality, stimulate rural economic development, and promote sustainable environmental practices.
Biogas is produced from the fermentation of organic matter by microorganisms under anaerobic conditions. The process of breaking down plant and animal products in an oxygen-free environment known as methanization occurs spontaneously in natural ecosystems, landfills, and some manure management systems but can be optimized in anaerobic digesters and used to harness energy and valuable soil products. The biogas produced in an anaerobic digester typically contains 50 to 70% methane, 30 to 40% carbon dioxide, and small traces of other gases, but only the methane is used as an energy source. [1] The liquid and solid residue produced, known as digestate, is often used as a soil amendment.
Today in the United States, there are 2,200 operating biogas systems with the potential to add over 13,500 new systems. [1] Biogas can be harnessed for the production of heat and electricity, or can be further purified to be nearly identical to natural gas and serve as vehicle fuel or be injected into natural gas pipelines. Stored biogas can act as a clean, reliable source of baseload power in place of coal or natural gas and reduce the dependence on fossil fuels as well as limit the amount of methane released into the atmosphere. According to the Environmental and Energy Study Institute, tapping all the potential biogas in the United States would be the equivalent of removing the amount of emissions produced by 800,000 to 11,000,000 passenger vehicles. [1] The anaerobic digestion process would also reduce the odors and environmental contamination posed by waste management pollution. The use of digestate, additionally, reduces the use of chemical fertilizers. Fig. 1 depicts a general overview of how organic waste can produce value when processed in anaerobic digesters. Lastly, building the 13,500 new biogas systems could help local economies by adding around 335,000 temporary construction jobs and 23,000 permanent jobs. [1]
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| Fig. 2: Livestock in agricultural setting. (Source: Wikimedia Commons) |
Despite the numerous environmental and economic benefits of biogas, the United States today has only tapped into 20% of the total biogas potential that could be extracted from food waste, landfill gas, livestock waste, wastewater, and crop residues. [1] Food waste makes up 21% of U.S landfills with only 5% of that waste currently being exploited as fertilizer; the majority of the waste decomposes in landfills and produces methane. [1] Approximately 66.5 million tons of food waste was produced in the U.S. in 2010 alone, which when coupled with anaerobic digestion, could provide enough energy to power 1.5 to 2.5 million homes each year. Landfills are the third largest source of methane emissions in the United States and naturally contain the same anaerobic bacteria that exist in digesters, meaning that landfill gas could be captured and used as energy instead of allowing it to escape into the atmosphere. Currently, landfill gas projects generate nearly 17 billion kilowatt-hours of electricity and supply 98 billion cubic feet of natural gas to users annually. [1] As a comparison, livestock manure contributed to about 10% of all methane emission in 2015 in the U.S., but only 3% of livestock waste is exploited for biogas production.
Using livestock manure in digesters to produce biogas could not only reduce up to 99% of manure pathogens but generate over 13 million megawatt-hours of energy each year. For reference, the average U.S. home in 2015 used about 10,812 kilowatt-hours of electricity. In wastewater treatment plants, sewage sludge from the treatment process is often already treated in anaerobic digesters, but the biogas produced is usually flared. Only about 860 of the 1,269 wastewater treatment plants in the U.S. currently use their biogas. [1] If all 1,269 facilities installed energy recovery systems, treating over 5,000,000 gallons of wastewater daily could reduce annual carbon dioxide emissions by 2.3 million metric tons which is the same as removing about 430,000 cars from the road. Lastly, with a current estimated surplus of 104 million tons of crop residues, removing these residues would support sustainable harvesting and the crops could be co-digested with other organic waste to improve processing efficiency. [1]
All organic matter, whether it is of plant or animal origin, can be methanized to biogas by anaerobic digestion (biological oxidation in the absence of oxygen). The production of methane by anaerobic digestion requires two main groups of anaerobic bacteria: the acid formers which convert organic matter into organic acids and the methane formers which convert the organic acids into methane and carbon dioxide. [4] Conventional digestion units operate in the mesophilic range of 90-110°F but recent research shows potential promise in the thermophilic range of 120-140°F. The production of biogas can be compared to the controlled combustion of wood to produce charcoal in an air-limited environment to produce a more valuable product. Digesters are airtight so that the organic matter can be insulated, heated and stirred. As stated earlier, only about 65% of the biogas produced is methane, which is used as an energy source and further combusted. A primary limitation of adding livestock waste to digesters is their high nitrogen content relative to their carbon content. Crop residues, which are low in nitrogen but high in carbon, are often mixed with livestock waste to help achieve a carbon to nitrogen ratio of 20:1, which is the optimal ratio for methane production.
The production potential of methane, often expressed in terms of cubic feet of gas produced per pound of volatile solids destroyed, is optimized when the digester is at 95°F, has neutral acidity which indicates the bacterial populations are in balance, is uniformly loaded, and has a 20:1 C:N ratio. [4] Since temperature is crucial to methane generation, digester insulation can be achieved by mounding the soil around the tank or burying the tank in soil, and high temperatures can be produced year round with an efficient heat exchanger. Stirring with a mechanical mixer, compressor or pump is also necessary to promote adequate contact between the bacteria and the waste and help remove the gas from the liquid contents. Since acid-forming bacteria thrive under a wider range of conditions than the methane formers, less than optimum conditions can result in digester upset or acidic conditions, which can be temporarily managed by adding an alkaline substance to the mix. [4]
Although the digestion of organic waste has the potential to supply methane, reduce odors (Fig. 2 depicts livestock from which foul odors are made as a result of the uncontrolled decomposition of their manure), and produce digestate, there are several drawbacks of biogas production that must be considered. Insulating, heating, and keeping methane digesters air tight is very expensive since a conventional digester is 15-20 times the size of the daily waste volume produced. [4] Digesters require a high level of maintenance and are sensitive to environmental changes. A biological upset, which may take months to correct, means that methane generation can completely cease or be very low. The start of the methane generation process, which is the most crucial part of the process, is difficult since bacteria generally grow very slowly and it can take weeks for a sufficient bacterial population to grow. Methane is also difficult to store at room temperature without special, expensive equipment and can be explosive if exposed to air. [4] Lastly, the price of biogas is not yet competitive with fossil fuels, as it takes 235 cubic feet of digester gas to equal one dollars worth of propane in terms of energy content.
The Renewable Fuel Standard, a part of the 2005 Energy Policy Act, requires the U.S. transportation fuel supply to incorporate renewable fuels and has been an important driver of investment in the biogas industry. The Renewable Fuel Standard has a variety of fuel categories of which certain fuel volumes for each must be met. [1] Biogas is approved as a renewable fuel within the cellulosic biofuel category, and producers of renewable natural gas can make $40/MMBtu by using renewable identification numbers that can be traded. The EPA also updated the Renewable Fuel Standard to allow biogas-derived electricity to count as vehicle fuel that can qualify for renewable identification numbers. In addition, the Farm Bill's Energy Title IX has created programs that have greatly stimulated the growth of the biogas industry. For instance, the Rural Energy for America Program (REAP) supplies grants and loan guarantees to agricultural producers and rural small businesses to promote the development of renewable energy and energy efficiency. Pennwood farms received over $500,000 in REAP grants and loans and used the money to install an anaerobic digester in 2011, which has since saved them $60,000 a year. The farm uses the digestate produced as bedding and the waste from the farms 600 cows generates more electricity than the farm needs. [1]
Biogas systems pose a huge opportunity for the United States and the rest of the world to turn the millions of tons of organic waste into renewable energy, heat, or vehicle fuel. Waste management can also become a revenue stream for Americas agricultural industries and can help reduce the dependence on fossil fuels, reduce emissions, improve environmental quality, and create thousands of jobs. Recycling nutrients in the food supply would also reduce the need for chemical fertilizers. Significant political support and the continuance of funding from the Farm Bill and Renewable Fuel Standard will be required for biogas systems to reach their full potential. Furthermore, additional research on methanization in digesters will be crucial for making the process more efficient and thus making biofuel prices more competitive. To reduce waste and increase clean energy supplies, global governments should strongly consider the potential applications and benefits of biogas.
© Amir Kader. 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] "Biogas: Converting Waste to Energy," Environmental and Energy Study Institute, October 2017.
[2] S. Liu, J. Proudman, and F. M. Mitloehner, "Rethinking Methane From Animal Agriculture," CABI Agric. Biosci. 2,22 (2021).
[3] "Agricultural Methane: Reducing Emissions, Advancing Recovery and Use Opportunities," Global Methane Initiative, September 2011.
[4] D. D. Jones, J. C. Nye, and A. C. Dale, "Methane Generation from Livestock Waste," Purdue University Cooperative Extension Service, September 2015.
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