• facebook
  • twitter
  • YouTube
  • RSS feed
  • 651-201-6000
  • 800-967-2474
  • 711 TTY

NodeFire Save Document
Home > Renewable Energy > Energy from Waste: Anaerobic Manure Digestion > Anaerobic Manure Digestion FAQs

Frequently Asked Questions:
Anaerobic Manure Digestion for Livestock Operations

What is anaerobic manure digestion (AD)?

A biochemical process by which organic matter is decomposed by bacteria in the absence of oxygen, producing methane and other byproducts. The complete mixture of this gas is called biogas.

What type of digesters are used in livestock applications?

  • Plug flow
  • Complete mix
  • Fixed film
  • Covered lagoon
  • Temperature-phased Anaerobic Digestion
  • Anaerobic Sequence Batch Reactor

What is biogas?

A combustible gas derived from decomposing biological waste. Biogas normally consists of 50 to 60 percent methane. Methane gas is an odorless, colorless, flammable gas with the formula CH4 that is the primary constituent of natural gas. The remaining portion of the gas is made up of primarily carbon dioxide and with trace amounts of hydrogen sulfide and nitrogen gas.

What factors determine how much biogas will be produced?

  • Volatile solid content (amount of organic material in manure)
  • Hydraulic retention time (amount of time manure is in digester)
  • Temperature (thermophillic vs. mesophillic digesters)
  • Management

What are the environmental benefits?

  • Reduced pathogens in manure (ex. fecal coliform bacteria)
  • Reduced odor and reduction of odor producing compounds (H2S)
  • Manure stabilized and nitrogen more readily available to crop
  • Reduced green house gas emissions
  • Production of “green energy”
  • Reduced dependence on fossil fuels

What are the economic benefits?

  • Reduced electrical, natural gas, and heating costs
  • Revenue stream from selling excess energy back to the grid
  • Enhanced fertilizer value of the digested manure

What are the risks?

  • High initial capital costs may cause a financial burden for the operation
  • Complications in operating and maintaining another biomechanical process in the farming operation
  • Some safety issues with working with a flammable gas
  • Availability of a utility that will want to buy back energy from the facility at a reasonable rate
  • Availability of an engineer and/or contractor locally that can build and maintain a manure digester

What types of livestock operations can use this technology?

Dairy, beef, poultry, and swine manure have all been used for AD. For Minnesota, Dairy manure has a high potential for AD because the ratio of liquids:solids is favorable to producing biogas efficiently from AD.

Where is this technology being used on livestock farms in Minnesota?

  • Haubenschild Dairy, Princeton
  • Northern Plains Dairy, St. Peter

How many livestock does a producer need to use this technology?

  • Any size of operation can be used for manure digestion, but economics play a role in the size needed to produce enough biogas to pay for the digester and it’s maintenance
  • In the case of dairy, to be economically sustainable a facility would need at least 400 cows

What types of financial assistance is available for this technology?

  • Minnesota Department of Agriculture Methane Digester Loan Program
  • USDA energy grants and loans
  • Minnesota Department of Commerce has programs to assist energy projects, which includes a $.015/kWh energy credit for 10 years for new operations
  • Excel energy grant program
  • Private grants from businesses
  • Grants from local units of government or municipalities

What is the cost?

  • The Haubenschild Dairy digester cost approximately $350,000 in 1999 for a relatively simple plug flow digester system designed for a capacity of 1,000 cows.
  • For larger livestock operations manure digester costs may escalate in the millions of dollars, depending on several variables including digester design.
  • According to the US EPA AgSTAR Program, the cost for a manure digester is approximately $550 per cow.

Why has the adoption of ADs been slow in Minnesota, as well as other parts of the United States?

  • The relatively high costs of ADs.
  • Grants are hard to come by and many grants have stipulations attached to them that some producers are unwilling to go along with.
  • Lenders are unwilling to provide funds for ADs because they are considered a high risk venture.
  • Poor commodity prices (milk prices and hog prices) in recent years have drained the resources of many livestock producers and they don’t have enough of their own funds to start the process.
  • Relatively low electrical buyback rates has been a significant issue and concerns about connections to the grid and unfamiliarity of the technology on the part of some electrical utilities have also been issues.
  • The process of lining up permits, financial resources, and engineers can take many years and that dissuades producers from undertaking AD.

What are other States doing?

  • Iowa. Iowa has a few dairy and hog farms that have used AD. One hog farm has been using AD for decades. Methane Energy Recovery in Iowa.
  • Wisconsin. Most of their digesters are for dairy farms and the number of farms with digesters is increasing. Wisconsin Biogas Development Group.
  • California: With the largest dairy industry in the U.S., California has a large potential for AD with dairy. They have many energy problems and distributed energy is a strategy with AD. California Energy Commission.

What are other countries doing?

  • The Netherlands have taken the lead in anaerobic digestion in Europe. They have constructed numerous digestion plants throughout the country in recent years. Other European countries (Britain, Germany) have also been developing digestion plants
  • European countries tend to develop larger AD plants that serve many different livestock operators.
  • India has developed some AD plants that are large scale. In Asia, a lot of small scale digesters are used in small villages and farms to produce gas for cooking.

What types of generators are used for electrical generation from biogas produced from ADs?

  • Primary Technology: Internal combustion engines (natural gas engines)
  • Some of the following technologies have seen limited adoption, mostly through research and demonstration projects
    1. Turbines
    2. Microturbines
    3. Sterling engines
    4. Fuel cells

    What are other alternative uses for biogas?

    • Boilers
    • Water heaters
    • Absorption chillers and refrigeration units
    • Replacement for natural gas/LP on-farm
    • Biogas is flared off to reduce green house gas emissions and odor
    MDA Contact

    David Weinand
    David.Weinand@state.mn.us ~ 651-201-6646

    Ag Marketing & Development Division