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Frequent Questions


What is AgSTAR?

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AgSTAR is an outreach program that provides an array of information and tools designed 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. 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.

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Does AgSTAR provide cost share, grants, or any other kind of financial support to develop projects?

The AgSTAR Program does not provide project funding. AgSTAR recommends reviewing funding information provided by DSIRE (Database of State Incentives for Renewables & Efficiency), which provides an unofficial overview of financial incentives and other policies.

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How can I get involved in AgSTAR?

AgSTAR informational products are available to everyone on the website. If you wish to receive email notifications as well, you can sign up for the AgSTAR list serv on the Contact Us page.

Biogas industry members (such as commodity organizations, consultants, developers, energy service providers, financiers, manufacturers/distributors, publishers, and universities) are encouraged to provide company information in the Industry Directory. If you are currently listed and want to update your information, please contact us.

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How do you get energy from manure?

Photograph of a methane recovery operation

Photo: RCM International LLC

When livestock manure that is handled as a liquid or slurry decomposes anaerobically (without the presence of oxygen), it produces biogas. In waste management systems that are designed for treatment, such as digesters and anaerobic lagoons, biogas consists of about 60 to 70% methane and 30 to 40% carbon dioxide.

When these gases are collected and transmitted to a combustion device, such as an electric generator, boiler, or absorption cooler, energy is produced.

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What are the unit components of an anaerobic digester waste management system?

Anaerobic digestion systems have four components: a manure collection system, a digester, a gas-handling system, and a gas use device. Effluent storage is also needed to contain the treated effluent until it is land-applied. Solids separators, which remove the digested solids for uses such as bedding materials or soil amendments, are common in dairy applications.

Anaerobic digestion systems separate the treatment and storage functions. This design provides several financial benefits to producers, including:

This multiple-cell process also improves environmental performance. Visit the AgSTAR's AD 101 webpage for more descriptions and photos of anaerobic digestion systems.

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What are the benefits of anaerobic digester systems?

Anaerobic digester systems generate renewable energy and reduce greenhouse gas emissions. Other benefits of these systems include a potential supply of manure fiber that can be used for bedding for cows, as compost, or for a variety of other uses; and high quality fertilizer. They also reduce odors from manure during storage and spreading and help to enhance a farm's public image. 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 fertilization of the fields. Finally, digester systems can provide additional farm revenue from the sale of energy, carbon credits, or other value added products (e.g. compost), and help create jobs.

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What environmental benefits do digesters provide?

Photograph of a storage pond

Photo: Jerry Bingold

Anaerobic digestion technologies provide air and water quality benefits including pathogen destruction, odor control, organic stability, greenhouse gas (methane) and hydrogen sulfide emissions 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.

Waste Management Options Odor Control Greenhouse Gas Emission Water Quality Protection
Anaerobic Digesters E L G
Combined Treatment & Storage Lagoons P-G H F-G
Storage Ponds & Tanks P-F M-H P-F

Key: P=poor, F=fair, G=good, E=excellent, L= low, M=moderate, H=high

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Do all projects have to be on farms?

Photograph of a methane recovery operation

Photo: RCM International LLC

There are two ways to develop a project:

  1. On-farm or farm-scale systems where the system is owned and operated by the farm owner or a third party. Most projects in the United States are on-farm systems.
  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.

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How do I know if anaerobic digestion makes sense for my farm?

Photograph of cows feeding

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.

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What kinds of materials are available through AgSTAR to help me evaluate and develop a reliable system?

AgSTAR Handbook - A concise project development manual, the Handbook contains the information needed to assess a farm's potential to install a commercial biogas recovery system.

Industry Provider List - 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.

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.

Funding On-Farm Biogas Recovery Systems: A Guide to Federal and State Resources - This web page provides information about programs and strategies, such as low-interest loans, grants, and tax incentives, that can help parties interested in implementing anaerobic digestion technology overcome financial barriers to project development.

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How are digester projects usually funded?

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. See the Federal Incentives fact sheet (PDF) (3 pp, 1.3 MB, About PDF) for current Federal funding opportunities.

Since 2000, one of the primary funding sources for anaerobic digester projects is through USDA Rural Development grant and loan programs. In May 2008, Congress passed the Food, Conservation, and Energy Act of 2008 (Farm Bill). This builds upon the first-ever Energy Title of the 2002 Farm Bill, providing new programs and a stronger federal commitment to farm-based energy. Mandatory funding for the Rural Energy for America Program (REAP) - formerly the Section 9006 program - more than doubled from $115 million to $255 million. REAP will continue to include grant and loan funds for construction of proven anaerobic digester systems, but now also includes funds for technical assistance and feasibility studies. For more information, see the USDA Rural Development website.

States and local organizations may provide additional sources of funding.

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Do digesters work for very small farms, such as those having less than 100 cows?

Although the majority of digester systems installed on farms in the U.S. have been on larger farms, viable options for a smaller facility could exist. The primary considerations include frequent manure collection at one point as a liquid, slurry or semi-solid. These systems can be worth the investment if the operator is motivated by odor issues and/or where utility rates are expensive (greater than $0.12/kwh) when coupled with a streamlined utility contract process. Also, thermal applications for the biogas may be an excellent option in place of electricity generation.

The Minnesota Project Exit EPA Disclaimer, which provides information on digester opportunities for mid-sized farms, is a good resource for small farms interested in anaerobic digestion.

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Why have fewer digesters been developed on swine farms than dairy farms?

For more information concerning AD use on swine farms, visit the Anaerobic Digestion on Swine Operations Web page.

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Can anaerobic digesters be used for non-traditional animal manure, such as that from rabbits or horses?

Anaerobic digesters work for manure that is collected as a liquid or slurry and does not contain a large amount of sand or dirt. To generate biogas, the manure must be collected and handled wet because the biogas is generated from anaerobic biochemical processes (those occurring in the absence of air). Anaerobic digestion systems can be used on operations that do not meet these parameters if the operations employ pre-treatment practices to ensure that the slurry is of appropriate consistency for the digester.

However, the key consideration is the biodegradability of the waste. Depending on the animal and its feed characteristics, anaerobic digestion may be a viable option. These systems can be worth the investment if the number of animals is high, the operator is motivated by odor issues, and/or where utility contracts are especially favorable.

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Does the biogas have to be used on-site or can it be piped or transported for sale?

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.

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Why are there so few digesters in operation?

AgSTAR estimates that anaerobic digestion is feasible at over 8,000 farms. There are a number of barriers that contribute to the low number of digesters actually in operation.

First of all, digester systems are technically feasible only on certain farms. The number of animals and the type of manure management system play a big role in determining technical feasibility. Anaerobic digesters are most adaptable to farms that collect large amounts of manure as a liquid or slurry on a daily basis. The systems can be used on other operations, but may require major changes to current waste management processes.

Secondly, projects which 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 the local cost of electricity, the price the local utility will pay, as well as other charges that may be incurred, such as a “standby charge”. Interconnection policies and net-metering options can vary by state and also have an impact on a project's economic feasibility.

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

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Why is there a difference between the amount of methane captured in a digester and the amount of methane reductions that can be claimed for carbon credits?

Photograph of a methane recovery operation

Photo: RCM International LLC

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.

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How can one farm having a much higher animal population than another end up with fewer emission reductions than the other if both install similar digestion systems?

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. 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 to handle manure.

There can be other factors that influence emission reductions, but different baseline cases account for most of the disparity.

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What universities in the U.S. have research programs specifically geared toward anaerobic digestion?

Many universities involved in anaerobic digester research are listed in AgSTAR's suggested links. Some are also highlighted on AgSTAR's Research Web page, which provides examples of pilot studies, evaluations, and technical assistance related to anaerobic digester being conducted at universities around the country.

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I see an error in AgSTAR's online database. Who should I contact about this?

Photograph of cows in pasture

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.

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How can I have our company added to AgSTAR's Industry Directory? What should I do if our listing needs to be updated?

There is an online form for those that would like to be added to the Industry 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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How do animal wastes create methane emissions?

Photograph of pigs feeding

Methane emissions occur whenever animal waste is managed in anaerobic conditions. Liquid manure management systems, such as ponds, anaerobic lagoons, and holding tanks create oxygen free environments that promote methane production. Manure deposited on fields and pastures, or otherwise handled in a dry form, produces insignificant amounts of methane. Currently, livestock waste contributes about 8 percent of human-related methane emissions in the U.S. Given the trend toward larger farms, liquid manure management is expected to increase.

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What factors need to be considered to codigest manure with other organic wastes?

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.

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What are the permitting practices for anaerobic digester systems with co-digestion of other substrates?

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 the Permitting Practices page for an overview of the federal regulations that affect these systems as well as state-specific information.

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