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Manure-digester opportunities weighed

Manure-digester opportunities weighed

Manure handling can be a significant cost on most dairies even when considering the value of nutrients in manure. Seldom do producers consider their costs of handling and applying manure on a per hundredweight of milk sold-basis like they do with many of their other costs.

Annual grain production requires replenishment of soil fertility. Deep soils and high yields are a perfect solution for recovering value from manure. Technologies for better application and more efficient handling are available. Dairy-manure values are such that corn producers can have more than enough fertility brought onto their farms to fully fertilize the following year’s crop.

Varying manure systems may offer environmental benefits and financial impacts across dairy-farm systems and climates. The potential to produce renewable natural gas from manure can offset manure-handling costs, especially with the current market value of manure-derived renewable natural gas as much as $80 per dekatherm. Several companies are promoting digesters in dairy-dense areas.

The Iowa State University-Extension and Outreach Dairy Team and EcoEngineers, a clean-energy consulting firm, recently hosted Dairy Discussions. The program focused on manure-management systems, opportunities in renewable natural gas, and the benefits of co-locating a dairy and biofuel facility.

Shelly Peterson is a program manager with the state energy office, housed at the Iowa Economic Development Authority. The energy team supports emerging energy opportunities in Iowa such as renewable natural gas. Peterson discussed how the Iowa Economic Development Authority can support Iowa dairies adapting digester technology.

She outlined on-farm potential for renewable natural gas from manure, cover crops, food waste and landfills. She also discussed water-quality benefits, odor control and soil-health benefits.

On-farm potential for renewable natural gas centers on swine and dairy facilities in northwest Iowa that can combine for economies of scale. They also are located near natural-gas pipelines. Peterson also cited the Innovation Center for U.S. Dairy’s environmental-stewardship goals to be achieved by 2050.

  • Becoming carbon-neutral or better
  • Optimizing water use while maximizing recycling
  • Improving water quality by enhancing use of manure and nutrients

The Dairy Discussions program also featured Kris Kohl, Iowa State University-Extension and Outreach agricultural engineer. A major problem with dairy manure is that it’s too thick to pump and too thin to scoop, he said. Dairy cows produce between 12 gallons to 20 gallons of manure daily. Consisting of 10 percent solids, manure is expensive to handle, store and land-applying. Its fertilizer value won’t normally cover those costs, he said.

There are also “hidden hazards,” one of which is sand. It’s the gold standard bedding in free-stall barns, but it damages steel and moving parts in the manure system, he said. Another issue centers on farmers’ use of distillers grains. Those grains promote elevated levels of hydrogen sulfide and sulfuric acid, which can be deadly as well as corrosive to manure systems and handling equipment, he said.

He also discussed maintenance on pumps – greaseable bearings, screens, parts and parts availability. Impeller pumps must have non-clog type impellers and large inlets and outlets to reduce blockages. Piston pumps have high pressures and low flow rate and can transfer high solid-content manure. He shared four keys to effective effluent pumping.

  • Know what you are pumping
  • Know how much you need to pump
  • Select the proper pump for the job
  • Install pump and related parts correctly

Brad Pleima, a senior engineer and regulatory consultant at EcoEngineers, discussed opportunities to producing renewable natural gas from dairy digesters. There are currently about 15 dairy renewable natural gas operations in the United States. There are three uses for biogas produced by digesters – power, heat production or renewable natural gas that can be sold into the grid or as transportation fuel. Pleima outlined opportunities for producers.

  • Divert manure from lagoons to a new anaerobic digestion system
  • Generate biogas, upgrade to renewable natural gas and inject into a nearby natural-gas pipeline
  • Sell renewable natural gas volumes into California as compressed natural gas or liquefied natural gas and generate renewable identification numbers and low-carbon fuel standard credits
  • Use revenues to pay for the project and supplement dairy income
  • Works best with more than 2,000 head of milking-cow equivalents
  • Third-party developers and finance can build with no capital involvement from the dairy farm

The value of the gas in dekatherms is currently $2.50 to $3. With the value of renewable identification numbers and low-carbon fuel standard credits, the value is about $78.

For a farm milking 3,000 cows with a lagoon-based manure system, the capital cost would be $10 million to $12 million with an annual operating cost between $750,000 and $1 million. Gross revenues would be $4 million annually. Earnings before interest, taxes, depreciation and amortization would be about $2.5 million annually. That assumes current programs and prices remain in place.

Visit and search for "dairy team webinars" or contact or or for more information. 

Fred Hall is a dairy specialist and Kris Kohl is an agricultural engineer, both with Iowa State University-Extension and Outreach.

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