Biogas & Anaerobic Digesters

Anaerobic digestion is the process of bacteria and other microorganisms breaking down organic materials in the absence of oxygen. As they break down the materials, the bacteria and microorganisms give off a mixture of methane and carbon dioxide called biogas.

Anaerobic digesters, or “biodigesters,” are a technology that use this process to intentionally convert waste like manure, crop residue, and food processing waste into biogas and reusable byproducts. The biogas can be used as a renewable source of energy in a number of ways, including producing heat, electricity and transportation fuel. The byproducts can be used as high-quality fertilizer and animal bedding.

Biodigesters also have benefits such as reducing odors from manure storage facilities, improving air and water quality, and reducing greenhouse gas emissions. Wisconsin currently leads the nation in energy production from anaerobic digestion and is ideally positioned for the use of biodigesters because of the size of its dairy and food processing industries.

Key Points

  • Anaerobic digesters allow farmers and others to turn waste streams into energy.
  • Digester systems can help farmers to become energy independent, save money on energy bills, cut pollution, and keep waste out of our lakes and streams.
  • Digester systems can also be the solution for multiple waste management and disposal problems for municipalities, food processors and other industries.
  • Wisconsin has agricultural resources with the potential to generate significant amounts of clean energy from digesters.
Clean Wisconsin's Work

Digesters are a great opportunity for generating clean energy from waste, especially with Wisconsin’s large agricultural sector. However, there currently are a number of barriers that make it difficult for farmers and business to realize the benefits of this technology. Clean Wisconsin is working to put policies in place that will break down these barriers, so Wisconsin farmers and businesses can invest in making their own clean energy and help bolster their bottom lines while stabilizing costs.

For more information and specific examples, see our new report, “Down on the Farm.”

Clean Wisconsin Media and Materials

Questions and Answers

What is biogas?

“Biogas” is a gas mixture produced when manure and other organic materials decompose anaerobically (meaning without oxygen). It is typically composed of 55-65% methane (CH4), 35-45% carbon dioxide (CO2), and trace amounts of other inert gases.[1]

What can biogas be used for? 

The most common large-scale uses for biogas are the production of electricity and heat. The gas is either burned directly to create heat, or fed into an engine to create electricity. Some co-generation systems do both and increase efficiency by capturing the heat created from the engine to warm water or heat buildings. Biogas can also be “upgraded” to be identical to natural gas, so that it can be sold into the natural gas market or compressed for other uses, like vehicle fuels.[2]

How does anaerobic digestion help mitigate greenhouse gas emissions? 

Any time organic materials decompose, they produce greenhouse gases such as carbon dioxide, methane and nitrous oxide. Digesters put organic waste to work, capturing the emissions as biogas. This biogas can then be used in place of other forms of fuel, like coal or natural gas. When this happens, it displaces the potential new emissions from those other fuels with emissions that would have occurred anyway as a result of decomposition. An additional benefit of digesters is that when the biogas is burned, all the methane that would have been produced through decomposition is converted into carbon dioxide, a much less potent greenhouse gas.[3]

What are some other benefits of the anaerobic digestion process?

Besides electricity and heat generation, there are several other benefits that arise from the use of anaerobic digestion. These include:

  • Reduced odors: Acids that cause odor are consumed by bacteria during anaerobic digestion, reducing odor and improving air quality in the process.
  • High-quality fertilizer: Nitrogen found in manure is converted into ammonium, which is the primary component of commercial fertilizer. Heat from the digestion process eliminates many pathogens and the waste is an easier-to-handle, higher-quality fertilizer.
  • Improved waste management: Proper control and disposal of manure can be a significant expense for farmers. The digestion process converts manure into digestate that can be much easier to manage.
  • Reduced surface and groundwater contamination: Anaerobic digestion destroys disease-causing bacteria and when applied responsibly, the reduced application of the waste fertilizer has less potential for runoff.[2]
  • Phosphorus removal: Anaerobic digesters can also facilitate the removal of phosphorus from waste streams through a number of different processes.

Is anaerobic digestion different than composting?

Yes. Composting breaks down organic materials through a number of processes involving microorganisms and other creatures, like earthworms, that require oxygen. In anaerobic digestion, biogas is formed solely through the activity of bacteria in the digestion process in an anaerobic (oxygen-free) environment.[4]

Quick Facts
  • As of September 2012, it was estimated that manure digesters in the U.S. produced the equivalent of approximately 586 million kilowatt hours (kWh) of usable energy per year;[5] enough to supply all the energy needs of more than 21,000 average American homes.[6]
  • 82% of manure digesters in the nation are located on dairy farms, and 12% are on swine farms. The remaining 6% are poultry, beef or mixed farms.[5]
  • In 2011, manure digester systems in the U.S. directly reduced methane emissions by 55,000 metric tons, or 1.2 million metric tons carbon dioxide equivalent (CO2e), and avoided 301,000 metric tons of CO2e by displacing fossil fuels with captured methane.[7]  These reductions are equivalent to:[6]
    • Removing about 303,000 passenger vehicles from the road
    • Reducing oil consumption by almost 3.4 million barrels
    • Reducing gasoline consumption by over 163 million gallons
    • The total amount of carbon taken up by nearly 1.2 million acres of forestland, more than 2.5 times the size of all Wisconsin’s state forests combined.
  • Wisconsin leads the nation in the number of operating manure digesters, as well as the total capacity and energy produced by those digesters. As of September 2012, there were 29 operational on-farm digesters in the state, with a combined electrical power capacity of 18,968 kW. Taken together, they are estimated to produce the equivalent of 120,982 MWh of electrical generation per year.[5]
  • In addition to on-farm digesters, Wisconsin also has around 28 digesters in industrial settings, and 60 in municipal settings.[8]
  • The AgSTAR Program projects that the U.S. could have upward of 8,000 anaerobic manure digesters producing biogas on farms across the land. The total generating capacity would be around 1,500 MW, or about 2 percent of all electricity.[9]
  • Germany is the world leader in biogas, with 7,215 plants installed as of 2011, accounting for 2,904 MW of electrical power capacity and 54,000 jobs for the nation.[9]
Wisconsin Data, Trends, and Legislation

Wisconsin leads the nation in the number of anaerobic manure digestion systems, as well as the total energy production from those systems. With Wisconsin’s sizable dairy industry, anaerobic digestion provides excellent economic and operational opportunity for large-scale dairies.[1] Furthermore, domestic energy production from cleaner sources, like anaerobic digestion, can reduce Wisconsin’s dependence on energy from polluting and imported sources.

  • Wisconsin’s 29 operating anaerobic manure digesters have a combined electrical power capacity of 18,968 kW and produce the equivalent of 120,982 MWh of electrical generation per year.[5]
  • Wisconsin’s 28 industrial and 60 municipal-owned biogas plants represent 50 MW of total electrical capacity.[8]
  • Wisconsin dairy cattle produce 23 million tons of manure each year. If all this manure was run through digesters and converted to natural gas, it could produce nearly 4.4 percent of the state’s natural gas needs.[9]
  • Wisconsin digesters average approximately 70 tons per week of digested solids and 130,000 cubic feet per day of biogas.[9]
  • Wisconsin is home to nearly 1,000 food processing facilities; waste from these facilities is a prime fuel for the anaerobic digestion process.[9]
  • Existing on-farm biogas systems have the potential to offset $2.2 million of coal or $15 million in natural gas if upgraded to pipeline-quality gas.[11]
  • Adding digesters to just 250 farms could have $250 million in economic benefits in Wisconsin.[12]
  • Wisconsin dairy farmers spend an estimated $48.5 million on manure management each year.[11]
  • All but one of Wisconsin’s 29 anaerobic digesters are located on dairies with 800 or more cows.[5] However, smaller farms may represent an even larger opportunity: there are over 13,000 farms with less than 200 cows, as compared to the 250 large dairies (those with more than 500 cows) in the state.[13]

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Please cite this resource as: Clean Wisconsin, Inc. “Biogas and Anaerobic Digesters.” Clean Wisconsin Enviropedia. Retrieved from www.cleanwisconsin.org/enviropedia.