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An Alternative Solution to Animal Waste Management: Biogas

February 10, 2015

If you raise livestock, you produce manure! Consequently, proper manure disposal is a concern for most operations. Most farming operations typically use their manures as fertilizers, but there are limits to how much manure you can apply to cropland—too much manure on fields can reduce your yield and may result in a visit from the local feedlot officer. The most common disposal solution is to build a lagoon, to hold the manure for application when appropriate or for sale to interested farmers. Obviously, manure pits can produce odors offensive to nearby residents, which is why many communities have established “Right to Farm” ordinances to protect farmer interests and balanced zoning laws to serve the well-being of area neighbors. Zoning matters, in combination with increased competition in the marketplace, has given many farmers reason to consider anaerobic digestion of animal wastes! Anaerobic digestion is a simple biochemical process by which waste products are converted to energy. Mixing acetogens and methanogens (acid and gas producing bacteria) with manure, plant waste, crop residue, food scraps or other waste products, farmers produce and capture biogas for energy use on-farm or for sale to local utilities.

Anaerobic digestion works in a 2-stage process to decompose organic material (volatile solids) in the absence of oxygen. Biogas is produced as a waste product of digestion. In the first stage, the volatile solids in manure are converted into fatty acids by anaerobic bacteria known as “acid formers,” or acetogens. In the second stage, these acids are further converted into biogas by more specialized bacteria known as “methane formers,” or methagens. Key by-products of anaerobic digestion include digested solids and liquids, which may be used as soil amendments or liquid fertilizers. Methane, the primary component of “biogas,” can be used to fuel a variety of applications.

Biogas Plant – wikipedia.org

If you’re a farmer exploring biogas, fuel supply (manure in our case) is critical for any project. The amount of waste depends on the type being produced. Wastes with high moisture content (greater than 75%), like cow manure, are typically processed using anaerobic digestion technologies, while drier wastes, such as turkey excrement, can be burned directly without anaerobic digestion. Another consideration is how the animals are housed. In a free-stall dairy where animals remain confined throughout the year, manure can be collected daily. If the animals are pastured in the summer, however, waste cannot be efficiently collected and the biogas project would only be operational during periods when the animals are confined. Furthermore, there are certain economies of scale associated with making a biogas project cost effective.

Biogas, produced from anaerobic digestion (or just gasification), can be used to replace electricity, natural gas, propane, or fuel oil used for electricity, heating, or cooling. The most common approach to producing electricity is to burn biogas in a conventional spark ignition engine coupled to a generator.  In addition to generating electricity, biomass can be used to produce space heat. The most profitable bio-gas energy projects generally produce electricity with waste heat recovery, in a process known as cogeneration.

There are different types of digesters: Covered lagoons, complete mix digesters, and plug flow digesters. Covered lagoons are the least expensive type of digester. Lagoons can be covered to collect and flare the gas produced. But since the manure is not completely digested, lagoon digesters do not entirely eliminate odors. Furthermore, this type is not very effective in regions that experience a wide range of temperatures, like the Midwest, because temperature directly affects the production of methane and effective energy recovery requires equipment sized to a regular gas flow.

Complete mix digesters consist of an engineered tank (round or square, which is located below or above ground; burying the tank helps insulate the system in cold weather states). The complete mix digester can use slurry manures with a solids concentration ranging from 3-10% and the volume of the tank needs to equal 15-20 days’ worth of manure and waste water production. The tanks are heated using waste heat recovered from the burner and mechanical mixers keep the manure in suspension within the tank as to prevent the formation of a surface crust.

Complete Mix Digester – epa.gov

And finally, plug flow digesters are rectangular in-ground tanks where manure flows first into a mixing pit, allowing the solids content to be adjusted by adding water. The contents of the mixing pit are added to the tank daily, slowly pushing the older manure down the tank. This digester can only use manures with solids content between 11-13%, so it’s only compatible with dairy scraped manure.

Plug Flow Digester – epa.gov

Building, operating, and optimizing biogas production systems takes work and is not right for everyone. Your regional climate, available feedstock and the ability to use gas at your site will determine if the project is viable. For more information, please visit www.mnproject.org or call us at 651-789-3330.

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