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AgSTAR

Is Anaerobic Digestion Right for Your Farm?

Anaerobic digesters can be installed for many reasons, including to treat waste, reduce odors, provide a revenue source and improve public image. While simple in concept, there are many sizes, styles and applications of digesters. Projects require detailed technical and financial planning to be successful.

To find out if your farm may be a good candidate for anaerobic digestion, consider the following questions:

  • 1. What type of manure does your farm produce?

    If your farm produces manure from cattle, hogs and/or poultry operations, anaerobic digesters are technically feasible for you.

  • 2. Is your livestock farm “large”?

    For preliminary screening purposes, livestock farms with these characteristics are potential candidates for anaerobic digestion:

    • Minimum of 500 head of cattle, 2,000 hogs with anaerobic lagoons or liquid slurry manure management systems, or 5,000 hogs with deep pit manure management systems
    • Minimum of 90 percent of the manure is regularly collected

    However, smaller confined facilities could also support successful recovery projects, given certain site-specific and market conditions. In addition, codigestion of other organic wastes generated nearby may make a smaller project feasible.

  • 3. Is your manure management technique compatible with biogas recovery technology?

    Biogas production is best suited for farms that regularly collect liquid, slurry or semi-solid manure with little or no bedding. This requires the farm to collect manure:

    • As a liquid, slurry or semi-solid (see definitions in table below)
    • At a single point
    • Every day or every other day
    • Free of large amounts of bedding or other materials (e.g., rocks, stones, straw, sand)

    Types of Manure

    Manure Type Definition Compatible with Anaerobic Digestion?
    Liquid Manure Has been diluted to a solids content of less than 5%. This manure is typically “flushed” from where it is generated, using fresh or recycled water. The manure and flush water can be pumped to treatment and storage tanks, ponds, lagoons or other suitable structures. Maybe. Can be adapted for biogas production and energy recovery in warm climates. In colder climates, it may be limited to gas flaring for odor control unless other organic materials are codigested.
    Slurry Manure Has been diluted to a solids content of 5-10% and is usually collected by a mechanical “scraper” system. It can be pumped and is often treated or stored in tanks, ponds or lagoons prior to land application. Yes. For biogas recovery and energy production, depending on climate and dilution factors.
    Semi-Solid Manure Is handled as a semi-solid, with a solids content of 10-20%, and is typically scraped. Water is not added to the manure and the manure is typically stored until it is spread on local fields. Yes. Fresh scraped manure (less than one week old) can be used for biogas and energy production in all climates, because it can be heated to promote bacterial growth.
    Solid Manure Has a solids content of greater than 20% and is handled as a solid by a scoop loader. Maybe. Aged solid manure or manure that is left “unmanaged” (i.e., is left in the pasture where it is deposited by the animals) or allowed to dry is not suitable for traditional digesters.  However, regularly collected manure could be used in a digester.

    Farms with different methods of housing animals and managing manure can use anaerobic digestion, although pretreatment or modifications may be needed. The animal housing and manure management techniques that are most compatible with anaerobic digestion include:

    • Cattle: flushed or scraped freestall barns and drylots
    • Hogs: houses with flush, pit recharge or pull-plug pit systems
    • Poultry: houses with flush systems

    Anaerobic digesters can use single or multiple feedstocks. Digesters that codigest manure with other feedstocks (e.g. fats, oils & grease, food wastes, cheese or wine wastes, manure) can increase biogas production. Additional pre-processing equipment and holding tanks may be required for codigesting. Also, codigestion can increase the amount of nutrients in the effluent, so farms considering codigestion should ensure that they will still comply with their nutrient management plans.

  • 4. Is there a use for the energy recovered?

    Photo of a 120 kW engine generator set combusts recovered biogas, generating electricity for on-farm usePhoto of a 120 kW engine generator set combusts recovered biogas, generating electricity for on-farm useThe use or sale of biogas energy can increase the cost effectiveness of a project. The value of the energy produced from the gas may offset the cost of collecting and processing the gas.

    Biogas can:

    • Generate electricity to:
      • Fuel a reciprocating engine or gas turbine.
      • Operate equipment on-farm. For example, dairies operate vacuum pumps, chillers, feed mixers and fans. Hog farms typically operate heat lamps and ventilation equipment.
      • Sell to the local power grid.
    • Be used directly on-farm to:
      • Fuel boilers or heaters, and in most processes requiring heat, steam, or refrigeration. Dairies and hog farms generally require hot wash water for cleaning and other operations.
    • Be processed into higher quality fuels, including:
      • Pipeline quality renewable natural gas
      • Compressed natural gas (CNG) to fuel vehicles
    • Be flared to:
      • Control odor
      • Reduce greenhouse gas emissions
  • 5. Will you be able to manage the system efficiently?

    Biogas systems require the owner/operator to:

    • Pay regular attention to system operations
    • Provide necessary repair and maintenance
    • Have the desire to see the system succeed

Contact us if you answer yes to one or more of these questions and want to learn more.


Additional Resources