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Frequent Questions

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Visit the anaerobic digestion, tools, and library pages for more information. Contact us to submit a specific technical inquiry.

Answers to frequent questions are provided below, grouped by topic. Click on a question to view the answer.

About AgSTAR


AgSTAR is a voluntary program—coordinated by U.S. EPA, in cooperation with USDA—that supports farmers and industry in the development and adoption of anaerobic digester systems, which are specialized manure management systems that capture biogas. These technologies are most effective at confined livestock facilities that handle manure as liquids and slurries. The captured biogas, which is 60 to 70 percent methane, can be used to generate electricity or replace fossil fuels for other energy needs. Gas recovery systems and digester technologies may provide enhanced environmental (air and water) and financial performance when compared to traditional waste management systems such as manure storages and lagoons.

AgSTAR is an outreach program that provides an array of information and tools to assist livestock producers (typically swine and dairy farms) in the evaluation and implementation of methane recovery systems appropriate for confined livestock facilities that handle manure as liquids and slurries. AgSTAR’s tools and resources include:

  • Outreach materials and project planning tools
  • Financing information
  • Events
  • Operating Digester Projects
  • Technical and regulatory assistance
  • Newsletters and listservs

No. While the AgSTAR Program does not provide project funding, it does offer assistance in exploring financing resources at the federal, state, and local levels or through private entities. AgSTAR recommends reviewing funding information provided by the Database of State Incentives for Renewables & Efficiency (DSIREExit EPA Disclaimer). DSIRE compiles government, utility, and nonprofit financial incentives and other policies for renewable energy. Additionally, AgSTAR offers tools and resources for project Financing.

AgSTAR informational products are available to everyone on the website. The AgSTAR Listserv provides timely news about digester projects and programs across the country and information about AgSTAR’s conferences and other workshops. To sign up for the email distribution list, visit the Contact Us page.

The Industry Directory for On-Farm Biogas Recovery Systems is intended to assist farm owners and others involved in developing anaerobic digestion technologies to identify designers, project developers, energy service providers, equipment manufacturers and distributors, and commodity organizations operating in the agricultural biogas sector. Biogas industry members are encouraged to provide company information in the Industry Directory. If you are currently listed and want to update your information, or want to be added to the Directory, please contact us.

General Information on Anaerobic Digestion (AD)
The AgSTAR website offers educational materials such as Anaerobic Digestion 101, a number of fact sheets, and sample evaluation forms. 

Project Development Resources
A list of Project Development Tools and Resources are available on the AgSTAR website, including additional guidance on farm-based anaerobic digester systems.

Operating Projects
The AgSTAR website provides useful information about successful projects across the country, including an interactive map of anaerobic digester systems operating at commercial livestock farms, farm project profiles, and a comprehensive project database. 

Vendor Directory
A list of vendors and service providers is available to assist with project development. It summarizes each company's experience with anaerobic digestion and gas recovery from livestock manure. Note: The information in the directory is provided by the listed organizations. AgSTAR neither guarantees the accuracy of the information nor endorses any of the listed organizations.

A compilation of tools and resources to support preliminary and advanced project planning, as well as a list of funding resources and an overview of project financing steps are provided on the AgSTAR website.

If you notice any errors or out-of-date information in our database, we encourage you to contact us at agstar@epa.gov with details.

There is an online form for those that would like to be added to the Vendor Directory. Please be as specific as possible with your company description so that readers will understand the services you provide. If you are already listed in the directory and your information needs to be updated, please contact us at agstar@epa.gov.


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Anaerobic Digestion Systems

Anaerobic digestion is a biological process that occurs when organic matter (in liquid or slurry form) is decomposed by bacteria in the absence of oxygen (i.e., anaerobic). As the bacteria “work,” biogas is released, which consists of approximately 60% methane and 40% carbon dioxide. This gas can be recovered, treated and used to generate energy in place of traditional fossil fuels. The effluent remaining after controlled anaerobic decomposition is low in odor and rich in nutrients.

AgSTAR estimates that there are over 200 anaerobic digester systems operating at commercial livestock farms in the United States. The AgSTAR database provides more detailed information about these projects.

AgSTAR estimates that anaerobic digestion is feasible at over 8,000 farms, yet there are less than 300 AD systems in operation. There are a number of barriers that account for the low number of digesters actually in operation.

The number of animals and the type of manure management system play a big role in determining an AD system’s technical feasibility. Anaerobic digesters are most adaptable to farms that collect large amounts of manure as a liquid or slurry on a daily basis. AD systems may require major changes to a farm’s waste management processes.

Second, projects that may be technically feasible for a farm may not be financially feasible. Capital costs can be quite high, and the return on investment depends on a number of factors including the sale of energy to a local utility and the price that utility will pay; the current costs for electricity charged by the utility; potential use and/or sale of the AD system’s co-products, such as fertilizers, compost, or other soil amendments; and state environmental credits for biogas production (e.g., carbon credits).

Other factors that affect the affordability of a project are the availability of state and federal grants and loans, requirements for interconnection with the local utility, and whether or not the state has a renewable portfolio standard.

Photograph of a methane recovery operation

Photo: RCM International LLC

Methane emissions occur whenever animal waste is managed in anaerobic conditions (i.e., free from oxygen). Manure deposited on fields and pastures, or otherwise handled in a dry form, produces negligible amounts of methane. Through the use of liquid manure management systems such as ponds, anaerobic lagoons, and holding tanks, anaerobic digesters provide an ideal, oxygen-free environment for methane production through bacterial decomposition (the resulting biogas is a mixture of about 60 percent methane and 40 percent CO2). A cover and gas collection system is an essential part of the process; otherwise, methane produced by the system would simply escape to the atmosphere.

Biogas is produced through the decomposition of livestock manure, through the process described in “How do animal wastes create methane emissions” above. When this biogas is collected and transmitted to a combustion device, such as an electric generator, boiler, or absorption cooler, energy is produced.

Anaerobic digestion systems have four components:

  • Manure collection system—A system to collect manure and transport it to the digester. Existing liquid/slurry manure management systems can readily be adapted to deliver manure to the anaerobic digester.
  • Anaerobic digester—Commonly in the form of covered lagoons or tanks, digesters are designed to stabilize manure and optimize the production of methane.
  • Biogas-handling system—A system to collect, treat and pipe the biogas (a product of the decomposition of the manure, typically comprising about 60 percent methane and 40 percent carbon dioxide) to a device that can put the gas to use.
  • Biogas use device—Biogas can be used to generate electricity, as a boiler fuel for space or water heating, upgraded to commercial natural gas quality, or for a combination of these and other uses. Flares may also be installed to burn off extra gas and as a back-up mechanism for the primary gas use device.

Effluent storage is also needed to contain the treated effluent until it is applied to land. Solids separators, which remove the digested solids for uses such as bedding materials or soil amendments, are common components in dairy applications.

Anaerobic digestion systems differ from conventional manure storage lagoons because they separate the treatment and storage functions. This design provides several benefits, including:

  • Lower total volume requirements, which reduce excavation costs and the land area required for the waste management system; and
  • Lower cover costs for covered lagoons because of smaller lagoon surface areas.

The separation of treatment and storage areas also improves environmental performance. Visit AgSTAR's Anaerobic Digestion 101 webpage for more descriptions and photos of anaerobic digestion systems.

Photograph of pigs feeding

A general lack of knowledge related to AD system technology and economics has led to some misconceptions and negative perceptions of biogas systems in the swine farm community. For information concerning the use of anaerobic digester systems on swine farms, visit AgSTAR’s Anaerobic Digestion on Swine Operations webpage.

Anaerobic digestion systems can be worth the investment if the number of animals producing manure is high, if the operator is motivated by odor issues, and/or if the utility contracts are especially favorable to the system operator. However, anaerobic digesters utilize manure that is collected as a liquid or slurry and does not contain a large amount of sand or dirt. Anaerobic digestion systems can be used on operations that do not meet these parameters if pre-treatment practices are in place to ensure that the slurry is of appropriate consistency for the digester. A key consideration is the biodegradability of the waste, which can vary depending on the animal and its feed characteristics.

Photograph of a chicken behind a chain link fence

Anaerobic digester projects are often eligible for various sources of funding, including grants, loans, and income from utility contracts. Since 2000, one of the primary funding sources for anaerobic digester projects has been through USDA grant and loan programs. States and local organizations may provide additional sources of funding. AgSTAR recommends reviewing funding information provided by the Database of State Incentives for Renewables & Efficiency (DSIRE). DSIRE compiles government, utility, and nonprofit financial incentives and other policies for renewable energy. Additionally, AgSTAR offers tools and resources for project financing.     


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Benefits and Products of Anaerobic Digestion

Photograph of a storage pond

Photo: Jerry Bingold

Anaerobic digestion offers environmental, agricultural, economic, and social benefits.

  • Environmental—Anaerobic digester systems generate renewable energy and reduce greenhouse gas emissions. Digesters can also improve water quality. More detail on environmental benefits is provided below.
  • Agricultural—The benefits of these systems include a potential supply of manure fiber that can be used as bedding for cows, as compost, or for a variety of other uses such as high quality fertilizer. During anaerobic digestion, some of the organic nitrogen in the manure is converted to ammonium, which is easier for crops to incorporate. This gives farmers more flexibility in the timing of fertilizing their fields.
  • Economic—Digester systems can provide additional farm revenue from the sale of electricity to the local utility, from carbon credits, or from potential sales of system-related bi-products (e.g., composts and fertilizers).
  • Social—AD systems help to reduce odors from manure during storage and spreading, eliminating a common area of complaint from communities surrounding farms. Anaerobic digesters are “good neighbors.”

Compared to traditional manure storage ponds and tanks, anaerobic digesters provide air and water quality benefits including pathogen destruction, odor control, organic stability, greenhouse gas and hydrogen sulfide emission reductions, and some nutrient management benefits. They also offset the environmental impacts of fossil fuel generation and provide rural electrical benefits, such as green power, distributed generation, and voltage support.

Photo of a truck fueled with compressed natural gas.

Photo: OWS/Phase3Renewables

Although biogas is most commonly used on-site, it can be treated and upgraded to pipeline quality gas or compressed natural gas (CNG) and sold as renewable fuel. Biogas can also be converted for use as vehicle fuel. When considering these applications, the additional energy, operational and management requirements must be assessed.

A common mistake is the assumption that the mass of methane captured and combusted is the basis for calculating carbon credits. Carbon credits are based on the amount of methane reduced through the installation of an anaerobic digester; in other words, credits are based on how much methane was originally emitted before the digester was installed. These emissions are referred to as `baseline’ emissions. Digesters generally produce more methane than what is emitted from the 'baseline' scenario.

This question often arises when discussing emission reductions. The discrepancies sometimes seen between methane reduction estimates for various farms are a result of different “baseline” scenarios. The “baseline” is essentially how the operation would be handling the manure if there was no digester in place. Certain baselines result in more methane emissions; storage lagoons, for instance, will emit far more methane than manure stacks. Therefore, a digester project that produces large amounts of biogas but originally managed manure in stacks may not reduce emissions by as much as a project on a smaller farm that originally used a storage lagoon. There can be other factors that influence emission reductions, but different baseline cases account for most of the disparity.

Biogenic waste digesters such as manure digesters or co-digesters processing mixtures of biogenic wastes may be eligible to generate Renewable Identification Numbers (RINs) under the Renewable Fuel Standard (RFS) if the biogas is processed into a transportation fuel and used in the transportation sector. More information about the RFS, how to register an approved biogas pathway to generate RINs, or how to petition for a new biogas pathway to generate RINs can be found on the EPA’s website: http://www.epa.gov/otaq/fuels/renewablefuels/compliancehelp/index.htm .


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Anaerobic Digestion Project Decisions

Photograph of a methane recovery operation

Photo: RCM International LLC

There are two ways to develop an AD project:

  1. Most of the currently operating anaerobic digester projects in the United States are on-farm systems, where the AD system is owned and operated by the farm owner(s).
  2. Regional or centralized digesters that transfer manure from multiple farms to an off-farm digester are often operated by a third party. Typically this approach requires fresh manure to be collected with very little process water from farms located within a few miles of the processing location.
Photograph of cows in pasture

To determine if a biogas recovery system is right for your facility, you will need to consider the following factors: how manure is handled at your facility, the frequency of manure collection, and the options available for using the recovered biogas.

  • Manure handling. Biogas digester systems can accommodate manure handled as a liquid, slurry, or semisolid (with little or no bedding added). The total solids content of the manure—a measure of manure thickness—determines these classifications.
  • Frequency of manure collection. Facilities best suited for biogas digester systems typically have stable year-round manure production and collect at least 50 percent of the manure daily.
  • Gas use. Biogas can be cleaned to various degrees of purity and used in boilers, in combustion engines, in electrical generators, as vehicle fuel, or injected into the natural gas pipeline. A combustion process is required when farms are participating in a carbon reduction program. When choosing among the options for gas use, you will need to take into account how the option affects a system’s financial performance, the labor requirements associated with the option, and the skills needed to maintain and repair any associated equipment.
Photograph of cows feeding

Although the majority of digester systems installed on farms in the United States have been on larger farms, viable options for a smaller facility could exist. Primary considerations include frequent manure collection at one point as a liquid, slurry or semi-solid. These systems can be worth the investment depending on the rates obtained for the sale of electricity back to the local utility, from carbon credits, or from potential sales of system-related byproducts (e.g., composts and fertilizers). Also, thermal applications for the biogas may be an excellent option in place of electricity generation. The Minnesota ProjectExit EPA Disclaimer, which provides information on digester opportunities for mid-sized farms, is a good resource for small farms interested in anaerobic digestion.

Many digester projects are incorporating other feedstocks to increase methane production. However, there are a variety of items that need to be taken into account to determine feasibility, which AgSTAR has summarized on its codigestion webpage.

EPA’s AgSTAR website provides a range of tools and information to help farmers evaluate a proposed project and install an anaerobic digester system. Resources include the Vendor Directory, project profiles, news and events, technical articles and reports, and areas of general information. Further, it is recommended that farmers confer with universities and agricultural organizations in their state for additional guidance and support.


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Additional Resources

While AgSTAR is the only U.S. EPA program focused on agricultural methane emissions, several U.S. EPA programs support greenhouse gas reductions. Visit AgSTAR’s Partner  page for more information. Additionally, USDA is a collaborative partner and offers several resources to support these projects.

Many of the universities involved in anaerobic digester research are provide in AgSTAR’s Schools of Interest list. Some are also highlighted on AgSTAR’s Research page, which provides examples of pilot studies, evaluations, and technical assistance related to anaerobic digesters being conducted at universities around the country.

On-farm anaerobic digestion systems that digest organic wastes and manure may be subject to federal and/or state air, solid waste, and water permitting requirements. Visit AgSTAR’s Permitting Practices for Co-digestion Anaerobic Digester Systems page for an overview of the federal guidelines that affect these systems as well as state-specific information.


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