Impermeable cover on a manure storage that allows collection of biogas, keeps rainwater out of the manure and helps minimize odor and ammonia loss, but is it cost feasible? Iowa State University/Daniel Andersen
Impermeable cover on a manure storage that allows collection of biogas, keeps rainwater out of the manure and helps minimize odor and ammonia loss, but is it cost feasible?

Value of manure to energy

Livestock producers need to thoroughly examine the cost-benefit ratio before jumping into a system to turn manure into energy.

Livestock manure has gone from being looked upon as true waste, to becoming a valuable asset as a fertilizer for crop farmers, providing cost savings on producers needing to buy synthetic fertilizers to nourish their crops. Manure applied to farm fields has also proven to improve overall soil health.

But are there ways to get even more value out of the “end” product from your pigs?

Daniel Andersen, Iowa State University assistant professor in Agricultural and Biosystems Engineering specializing in manure management, looks at the latest trend of turning manure into energy in the latest installment of “The Manure Scoop” blog.

“In liquid and slurry manure systems this has typically meant anaerobic digestion for production of biogas, which is rich in methane,” Andersen writes in his blog. In some states, he goes on to write, these systems are rare, largely due to the economics of these anaerobic digestion systems.

He examines one specific type of digester system, the covered lagoon, to see how it may impact manure economics. An integral part of the covered lagoon is the cover itself, which is an “impermeable cover, a plastic film placed on top of the manure storage that is impermeable to both gases and liquid. The idea is that this cover will keep rainwater out of the manure, hold in gases so odor and ammonia emissions are minimized, and capture methane made by the natural breakdown of organic matter in the manure.”

By reducing the amount of rainwater entering the lagoon, that lessens the amount of excess water that needs to be hauled away, “holding onto nitrogen and increasing the fertilizer value of manure, and the value of odor control. By capturing the biogas, it can be used for the generation of electricity, heat or additional processing (via pressure-swing adsorption to separate the methane and compression into pipeline quality gas in this analysis).”

Producers will, of course, have to factor in the added expense of the cover itself, as well as the “cost of cleaning the gas to pipeline quality, and the change in manure application costs, as more land will be required as the cover maintained more nitrogen value.”

On the plus-side, Andersen writes that adding a cover to outdoor manure storage would allow the design of a slightly smaller storage, since producers would not have to allow for a larger design to take in the 25-year, 24-hour rain storm (about 5 to 5.5 inches throughout Iowa). He says this would be a minimal change on the construction costs of the actual storage. “However, putting a cover on a storage does offer the potential for retaining nitrogen in the manure,” he writes, as he explains that 7.8 pounds NH3 is retained per pig per year in typical deep pit storage. “Switching to an impermeable cover would save about 5 pounds of NH3 per pig per year, which on a 4,800-head swine farm amounts to about $7,000 of nitrogen value every year. This would increase our manure application costs slightly as more nitrogen means more acres would be needed and manure would need to be moved a bit further, increasing application manure application costs by about $2,700 every year.”

Andersen concludes that after considering all extra expense while looking at the benefits, such a system “hovers right around the breakeven point on an annual basis.”

Click here to read Andersen’s full Manure Scoop blog.

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