Biogas comes from animal manure, and is perhaps the ultimate win-win energy source, allowing farmers to produce their own electricity and reduce the water contamination, odor pollution, and global warming emissions caused by animal waste.
Biogas is produced when bacteria decompose manure anaerobically -- without the presence of oxygen -- into a gas mixture composed of about 60 to 70 percent methane. This gas mixture can be used to generate heat, hot water or electricity, and the leftover digested manure can be used as fertilizer, bedding, mulch and potting soil.
In 2011, there were about 180 operational biogas recovery systems on American commercial livestock farms, which produced enough electricity to power the equivalent of 47,000 homes. The EPA's AgSTAR program reported in 2010 that about 8,000 U.S. farms could support biogas recovery systems, providing about 1,600 megawatts of energy and reducing emissions of global warming pollution by about 1.8 million metric tons of methane -- the equivalent of taking 6.5 million cars off the road.
How Biogas Energy Works
Trends in Energy Production by Anaerobic Digesters
Source: BioCycle, "Farm Digester Industry In America" Online: http://www.biocycle.net/2012/02/farm-digester-industry-in-america
Biogas recovery systems have a few main components: a manure collection system, an anaerobic digester, usually a covered lagoon or tank, which stabilizes the manure and optimizes methane production; a biogas handling system that pipes the resulting gas to the device it will fuel, such as a generator; and a storage tank for the remaining discharge.
There are a few types of digesters: Covered anaerobic lagoons are pond-like basins, often earthen, that are covered to retain the biogas. Lagoons are the simplest and most popular biodigesters, but they are limited to warmer climates -- colder temperatures can suppress methane production. Plug-flow digesters are long, narrow, heated tanks, often installed partially underground to retain the heat. These units work only with dairy manure. Complete-mix digesters are heated tanks made of reinforced concrete or steel with a mechanical, hydraulic or gas-powered mixing system. They generally require a diluted manure mix, such as manure mixed with process water.
Where Biogas Energy Is Used
Because methane is one of the most potent greenhouse gases, farmers who use biodigesters can get paid by carbon offset companies
Biogas is most commonly used on the farm where it's produced, mostly for electricity or as boiler fuel for space and water heating. Any excess electricity can often be sold back to the grid.
Large dairy and swine operations have the greatest potential to produce cost-effective biogas. Biogas recovery systems can handle liquid, slurry, or semisolid manure, and these operations can collect and store enough of this manure to produce large amounts of biogas.
AgSTAR, a voluntary program administered jointly by the U.S. Environmental Protection Agency, Department of Agriculture, and Department of Energy, promotes biogas recovery systems at concentrated animal feeding operations (CAFOs) across the country.
A single CAFO can generate as much waste as a small to midsized city, and can affect the water supply and air quality of an area that extends far beyond the operation itself. There is very little government oversight of these operations -- the EPA does not have a definitive national list -- and they pose very real risks to air and water quality. By capturing manure, biogas recovery systems can drastically reduce or eliminate untreated waste runoff from CAFOs, but in the end, strong environmental protections and government oversight of CAFOs are needed to ensure that the risks of polluted runoff are mitigated.
How Much Biogas Energy Costs
In 2011, biodigesters on U.S. farms produced enough electricity to power the equivalent of 47,000 U.S. homes
The profitability of a biogas digester depends on the size of the operation, the method of manure management and local energy costs. According to AgSTAR, biogas recovery can be profitable and most effective at existing operations of at least 500 cows or 2,000 swine. Manure should be collected frequently (at least once a week) in a liquid, slurry or semi-solid state. Any electricity that is not used on-site can usually be sold to the local utility.
Because methane is one of the most potent greenhouse gases -- 21 times as powerful as carbon dioxide in trapping heat -- farmers who use a biodigestion system can arrange to be paid by a carbon offset company. These firms have emerged in recent years in response to the growing demand from companies and individuals who want to reduce their environmental impact by "offsetting" the emissions they produce in their everyday activities. Carbon offset companies sell "carbon credits" to interested parties, and use the proceeds to pay others to reduce their global warming pollution.
Advantages of Biogas Energy
Biodigesters have the potential to slash methane emissions by 1.8 million metric tons -- the equivalent pollution reduction of taking 6.5 million cars off the road
- Air quality improves significantly because the biodigester reduces the smell of manure, turning its volatile organic compounds (VOCs) into odorless methane and carbon dioxide. Hydrogen sulfide, the source of the "rotten egg" odor, is captured in the biogas and destroyed during combustion.
- Cleaner water is an important benefit: biodigesters reduce bacteria levels in animal waste, which means that any runoff to surface waters will be less harmful. Digesters also reduce biochemical oxygen demand (BOD), a measure of the ability of organic wastes to remove oxygen from water. Aquatic species depend on dissolved oxygen in water for survival, so farms that reduce BOD help protect aquatic ecosystems.
- Greenhouse gas reduction is critical in mitigating climate change. Seven percent of methane emissions in the United States come from livestock and poultry manure, most of which in turn comes from swine and dairy operations. Biodigesters eliminate nearly all methane emissions, and as a renewable source of energy they reduce our reliance on fossil fuels.
What's Around the Corner for Biogas Energy
- Biodigesters are increasingly popular for a number of reasons, including farmers' interest in selling carbon credits and utilities' interest in meeting the renewable portfolio standards that more than 30 states have adopted. A renewable portfolio standard is a government mandate, usually at the state level, for electricity supply companies to produce a specified fraction of their electricity from renewable energy sources. Certified renewable energy generators earn certificates (known as renewable energy credits) for every unit of electricity they produce, and can sell these along with their electricity to supply companies.
- Organic (food and yard) waste biodigestion is now being considered as part of a broader push to keep organic waste out of landfills and capture the energy and ecological benefits of doing so. University of Wisconsin - Oshkosh is piloting the nation's first dry biodigester, which will convert campus food and yard waste into heat and electricity.
- The USDA Rural Business-Cooperative Service (RBS) is accepting applications for grants and loan guarantees from agricultural producers and rural small businesses that want to purchase renewable energy systems and/or improve their energy efficiency. Eligible systems include anaerobic digesters that use animal waste and other substrates to produce thermal or electrical energy.
- New digesters with gas-fired engines will be subject to emission limits for nitrogen oxides, carbon monoxide, and volatile organic compounds (VOCs). For more information, visit the EPA's AgSTAR newsroom.
- Congress should limit greenhouse gas emissions (GHG) from large manure management facilities in confined animal feeding operations, or CAFOs. According to the EPA, greenhouse gas emissions from livestock in large CAFOs account for a significant portion of agricultural GHG emissions in the United States.
- Market Opportunities for Biogas Recovery Systems
- A Guide to Identifying Candidates for On-Farm and Centralized Systems. Produced by US EPA/AgSTAR.
- Fact Sheet: CHP Biopower/Anaerobic Digestion: Have you asked all the right questions?
- The Northwest CHP (Combined Heat and Power) Application Center has a fact sheet that helps you ask all the right questions about installing a biodigester on your farm.
- Farm Digester Industry in America
- Summary on the state of the agricultural biodigester industry in the United States
-  EPA, 2010 Biodigester Update. Retreived from http://www.epa.gov/agstar/documents/2010_digester_update.pdf
-  EPA, 2008. Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006.