| Methane Digesters |
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| Introduction |
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Anaerobic digestion involves the decomposition of manure, processing by-products, and other materials into effluent and biogas. Microorganisms perform the decomposition process in an anaerobic digester, which can be designed in several ways. Once biogas is harvested from the processed manure, it can be run through an engine to generate electricity, used in place of natural gas, or flared.
Many digesters are successfully operating in the U.S. and Europe. As digesters have grown in popularity in the U.S., so have the options for ownership and financing of digesters. A variety of incentives are available for digester construction, and tax incentives also exist for manure supplied to digesters.
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| Benefits of digesters |
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- Reduced odor. Anaerobic digesters allow a greater portion of odor-causing volatile acids to be converted to biogas, resulting in less odor-causing compounds than would be present in manure in a typical liquid storage system.
- Reduced bacteria and pathogens. Depending on the retention time and operating temperature, a digester can significantly reduce pathogens in manure.
- Greater nutrient management flexibility. In the process of anaerobic digestion, the organic nitrogen in the manure is largely converted to ammonium, which is readily available and taken up by plants. Separated, digested solids contain most of the phosphorus in the manure, and operators may choose to export those solids or continue to apply them to their land, depending on their available acreage for manure application.
- Renewable energy production. Biogas harvested from the digestion process may be run through a gen-set to generate electricity. The system may be set up to offset the operation's electricity use, or the operator may negotiate a power purchase agreement with a utility to sell power at wholesale rates.
- Solid co-products. Separated, digested manure solids may be used for animal bedding, composted on-farm, or exported from the farm for use as compost.
- Reduced greenhouse gas emissions. Methane is a greenhouse gas that remains in the atmosphere for approximately 9 to 15 years. By capturing methane and converting it to heat or electricity, methane digesters greatly reduce greenhouse gas emissions from manure.
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| Successful projects |
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Here are just a few of the digesters operating today in the U.S.
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| Incentives for digesters |
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Oregon’s Business Energy Tax Credit (BETC) provides a 50 percent credit for the capital costs of biofuels and bioenergy projects. You must apply for the credit before beginning your project. If you do not have the tax liability to be able to use the credit yourself, you may pass the credit through to another entity that owes Oregon taxes. In exchange, you receive a cash payment equal to about 33% of your eligible project costs.
Biomass tax credits are available for certain materials converted to biofuel or bioenergy. For manure, the credit is $5.00 per wet ton. Based on USDA estimates that a dairy cow produces 15 tons of manure per year, a 400-cow dairy could supply 6,000 tons of manure through a digester and generate an annual biomass tax credit of $30,000. This is an estimate and would need to be checked against the digester capacity and operational run time. The biomass credit may also be passed through to another entity if the dairy operator does not have the tax liability to use the credits. Operators do not need to apply in advance to use this credit, but need to calculate the amount of the credit using guidelines from the Oregon Department of Energy and forms from the Oregon Department of Revenue. The Oregon Department of Energy Loan Program offers 5 to 20 year loans for up to $20 million. Energy Trust of Oregon's Biopower Program provides financing to help make digesters economically viable. Energy Trust may also offer assistance with feasibility studies and grantwriter time. Two USDA Rural Development programs can help support digester projects. The Value Added Producer Grant (VAPG) program provides cost-share for planning or operational expenses that add value to an agricultural product. VAPG funds can help cost-share a feasibility study for an anaerobic digester. The Rural Energy for America Program (REAP) provides grants and loan guarantees for renewable energy projects, including anaerobic digesters. The USDA-Natural Resources Conservation Service may have cost-share funding available for digesters through the Environmental Quality Incentives Program (EQIP). For more information, contact your local USDA Service Center. Greenhouse gas reduction credits, often called carbon credits or offsets, can be sold for anaerobic digesters because digesters reduce methane emissions associated with manure. The Climate Trust currently has a solicitation for offsets for anaerobic digesters in the Pacific Northwest. Renewable energy certificates, or green tags, represent 1,000 kilowatt-hours of renewable electricity generation. Depending on how your digester is financed, you may own the green tags generated by the digester and sell the green tags on the open market.
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| How to learn more |
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A variety of organizations provide more information about digesters and how they may help meet operators' nutrient management, odor reduction, public relations, and financial goals. The following are just a few resources that can help you evaluate whether a digester is right for your operation.
- Energy Trust of Oregon has partnered with the Dairy Farmers of Oregon and Oregon State University to provide technical assistance to dairy farmers considering methane digesters. For more information, contact Amanda Green at 360-751-4190 or Mike Gamroth at 541-231-0928.
- Pennsylvania State University's Biogas and Anaerobic Digestion Web site has a questionnaire for operators considering a digester, and a variety of publications on the fate of nutrients, bacteria, and odor-causing compounds.
- Iowa State University maintains an Anaerobic Digestion Web site that includes research and case studies.
- The National Center for Appropriate Technology published Factors to Consider in anaerobic digestion of animal wastes.
Check out other successful digesters and talk to operators who are managing or hosting these systems. Find out what works and what doesn't, and estimated costs per cow for the type of design you are planning.
Evaluate digester technologies and technical service providers carefully. Ask for references and evaluate each provider's track record, just as you would with any other builder or contractor.
Depending on your specific operation, you may wish to secure financing and operate a digester yourself, co-own the digester with partners, or work with a third-party developer to operate a digester at your operation.
Digester design, construction, and commissioning involve large capital costs. Cost per cow will vary depending on the type of system and size of operation.
Consider partnering on both inputs and outputs. If your operation is very close to other operations, you may be able to supply materials into a single digester. Other agricultural industries can provide additional feedstocks, such as vegetable or fruit processing waste, rendering and slaughter by-products, or whey, for co-digestion. In some cases, co-digesting manure with other feedstocks can significantly increase biogas production. You may also wish to partner with other operators or companies to market some of the digested products, such as compost.
Digesters, gen-sets, and digestate require extensive management. Be sure to develop a total plan of operation and marketing before you get too far along.
Check with county/city planners, your local power company, ODA, DEQ, and the Oregon Department of Energy about digester permitting requirements well before you begin your project. This will ensure a smooth process and allow for construction without unnecessary delays.
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