An anaerobic digestion system (AD system) captures the methane that may otherwise be released from conventional manure handling methods, and has the potential to reduce greenhouse gas emissions and produce clean energy. Absent such technology, confined animal feeding operations typically collect and store manure (e.g., in a waste storage facility) under anaerobic conditions, which produces the potent greenhouse gas methane. The methane is released into the atmosphere as the manure decomposes while being stored. Livestock producers handle large quantities of manure on a daily basis that emit methane. Some estimate that livestock operations produce more than 1 billion tons of manure yearly and that 7% of total U.S. methane emissions are from managed manure. Through methane capture, anaerobic digestion has the potential to reduce these emissions. In addition, anaerobic digestion of manure produces biogas—a combination of methane, carbon dioxide, and trace amounts of other gases that can be used for renewable energy purposes or flared. Recent legislation pertaining to agricultural sources of renewable energy has focused primarily on corn-based ethanol and cellulosic ethanol for liquid fuel purposes, and not biogas. The economic environment is not currently favorable to profit from the investment required to compress biogas produced from an AD system into a liquid fuel. There are, however, several successful cases where an engine-generator set has been used to generate electricity from the biogas or the biogas is burned in a boiler to produce heat. Biogas from an AD system could be used to assist livestock producers in an effort to have an energy self-sufficient operation as well as, potentially, to sell electricity to the local utility. There are more than 160 AD systems operating on farms nationally (see Table 1 ). Some factors that may be responsible for the low technology adoption rates are high capital costs, reliability concerns, and payment rates for the electricity generated. Congress may consider encouraging increased adoption of the technology by (1) identifying the primary technology benefit, so as to determine whether AD should be pursued in the framework of greenhouse gas emission reduction or clean energy development; (2) determining if the captured methane will count as a carbon offset; and (3) considering additional financing options for the technology. Moreover, Congress may receive insight on collaborative techniques by monitoring the first public-private partnership agreement between the U.S. Department of Agriculture and the Innovation Center for U.S. Dairy to reduce greenhouse gas emissions from dairy operations by 25% by 2020. One goal of the agreement is to "accelerate and streamline the process for adopting anaerobic digesters by the United States dairy farm operators through various USDA programs." A number of developments have occurred since the agreement was signed in 2009, including USDA providing funds to establish 30 more anaerobic digesters in FY2010. This report provides information on AD systems, technology adoption, and challenges to widespread technology implementation, and explores the issues facing Congress concerning adoption of the technology. Most manure management systems used on livestock operations store the manure in an open facility, allowing the manure to decompose naturally, which releases the potent greenhouse gas methane into the atmosphere. By contrast, an AD system feeds manure into a digester that breaks it down in a closed facility in the absence of oxygen (see Figure 3 ). The digested feedstock is contained for a period of time as anaerobic bacteria decompose the manure to produce several outputs, including biogas, liquid effluent, and dry matter. The captured biogas is flared (see footnote 5 ) or used for energy. The liquid effluent may be applied to the land as a fertilizer. The digested dry matter may be sold as a soil amendment product or used for animal bedding. AD systems have other benefits (e.g., odor reduction) that may curb negative impacts of livestock operations, including environmental pollution. An AD system is designed and constructed to suit the needs of an individual livestock operation and is typically selected based on the total solids (TS) content of the manure and the manure handling system (see Figure 3 ). Other criteria taken into consideration when building an AD system include the feedstock quantity, feedstock quality, feedstock availability, feedstock handling, demand for effluent, use of captured biogas, and transportation logistics (e.g., feedstock may be transported to an AD system if it is not available on-site, or effluent transported to a receiving entity if not used on-site). AD systems in the United States are in use on dairy cow, swine, and poultry operations. More than 80% of the operating AD systems are located on dairy cow operations. Some argue that less complex AD systems could be constructed on dairy cow operations at a lower cost and that the manure could be easily transported to the AD system, thus making them more economically appealing. Others contend that fewer AD systems are installed on swine operations because many swine operations currently store liquid manure in pits beneath the livestock; the producer would need to redesign the swine operation to incorporate an AD system. Installation of an AD system on a swine operation may also be more expensive if a storage facility must be constructed to contain the digested feedstock. Some assert that AD systems are not a favorable addition to a poultry operation partly because the litter is dry and may require more resources for transport to an AD system and additional inputs to digest. Methane (CH 4 ) is one of the primary greenhouse gases associated with the agricultural sector. The odorless, colorless, flammable gas is potent because it is 21 times more effective at trapping heat in the atmosphere than carbon dioxide (CO 2 ) over a 100-year timeframe. In other words, it takes 21 tons of CO 2  to equal the effect of 1 ton of CH 4 . Methane has a relatively short atmospheric lifetime (approximately 12 years) when compared to the atmospheric lifetime for carbon dioxide, which has a half-life of roughly 100 years; thus some argue that efforts to capture methane from anthropogenic sources may provide near-term climate change abatement. Major sources of methane emissions from animal agriculture are enteric fermentation and manure management (see Table 1 ). While enteric fermentation emissions are much larger than manure management emissions, they are also much more difficult (nearly impossible) to control. Emission factors affiliated with methane released from enteric fermentation, and conversion factors affiliated with methane released from a manure management system, are used to estimate overall methane emissions from animal agriculture. Estimation incorporates a multi-step process that takes into account livestock population data, waste characteristics, waste management system data, and other variables. Capturing methane with an AD system is beneficial because it reduces emissions of a harmful greenhouse gas from an agricultural source of methane: managed manure (e.g., manure stored in pit storage, an anaerobic lagoon, or a storage facility). Swine and dairy cattle are the two dominant livestock emitters of methane for managed manure (see Table 2 ). Emissions from managed manure vary on a statewide basis depending on the livestock population and the manure handling systems in place (see Figure 6 ). Captured methane may qualify as a carbon offset because the methane would no longer be released directly into the atmosphere. Carbon offsets are a facet of the climate change cap-and-trade program debate that was underway in the 111 th Congress. Accurate quantification and verification of the methane captured from an AD system requires robust data, observed or inferred, to ensure that actual reductions are occurring as projected. Another AD system benefit is the production of biogas, which can be used as a renewable energy source. Biogas consists of 60%-70% methane, 30%-40% carbon dioxide, and trace amounts of other gases (e.g., hydrogen sulfide, ammonia, hydrogen, nitrogen gas, carbon monoxide). Biogas can be explosive if exposed to air, depending on the concentration of methane in a confined space. The quality (i.e., heat value) and amount of biogas produced varies based on the hydraulic retention time of the AD system, the manure total solids content, and temperature. Biogas can be used to produce heat or generate electricity. The biogas may be burned in a boiler for space heating or water heating on-site. Another option is to use an engine-generator set to create electricity from the biogas. Biogas that is upgraded to pipeline quality may be sold to a natural gas utility. Typically a producer will decide on only one use for the biogas—either generating electricity or producing heat—due to the expense associated with energy generation and boiler equipment. Few choose to sell the biogas to a natural gas utility. If electricity is generated from the captured biogas, it may be sold to a utility in addition to its on-site use. A rule of thumb is that manure from approximately five Holstein milking cows supplies fuel for 1 kilowatt capacity. Sales to a utility require a contractual agreement between the producer and the local utility provider that typically outlines safety, reliability, and performance standards. Additionally, any federal and state environmental requirements must be met (e.g., AD system generators may have to meet Best Available Control Technology requirements). Net metering, a preferred form of billing for many renewable electricity generators, is an option for producers selling the electricity generated to electric utilities in 43 states, the District of Columbia, and Puerto Rico. Biogas may be used as a fuel if it contains at a minimum a 50% concentration of methane (CH 4 ). The biogas must be cleaned, upgraded, and compressed if it is to be used in a mobile engine. Cleaning biogas removes hydrogen sulfide. Removing any moisture and carbon dioxide upgrades the biogas and increases the British thermal unit (Btu) value. With an energy content of 600 Btu/ft 3 for biogas with approximately a 60% methane concentration, biogas is a low-Btu fuel compared to other fuels (see Table 3 ).