Waste Not: Large and Small Minnesota Dairy Farms Use Manure Digesters to Produce Methane Gas

By Thompson, Stephen | Rural Cooperatives, May-June 2009 | Go to article overview

Waste Not: Large and Small Minnesota Dairy Farms Use Manure Digesters to Produce Methane Gas


Thompson, Stephen, Rural Cooperatives


In the heart of Minnesota's dairy country, two dairy co-op members are looking to a future in which farm waste will be used to supplement America's supply of electric power. Both are using technology that reduces pollution, greenhouse gas emissions and, hopefully, farm operation costs.

Methane digesters have been around for more than 100 years. Using the action of bacteria, they ferment manure and other organic waste to generate a mixture of C[O.sub.2] and methane, called "biogas," that can be burned to power electric generators. Municipal sewage-treatment plants around the country have used digesters for decades to help dispose of sewage sludge. They are also used to create power from farm wastes on a large scale in western Europe.

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But in the United States, use of the technology on farms has yet to become significant. Dairy farmers Dennis Haubenschild and Jerry Jennissen hope to change that by running biogas generators that contribute significant amounts of power to the grid, and make money for their owners in the bargain.

True believer

Haubenschild is a true believer. "I know what happens when you're non-sustainable," he says. In 1953 when his father purchased the land Dennis farms today in Princeton, Minn., it had been completely exhausted by years of crops grown without replenishing the sandy soil. "The humus was just gone," says Haubenschild. "It took years to get it productive again."

His interest in sustainable farming has led him to try a number of unconventional measures on his 868-cow dairy farm, including solar heating a cow barn, water conservation and recycling. But the centerpiece of his conservation effort is an electric generator run by methane produced from his cows' manure.

[ILLUSTRATION OMITTED]

"A day's worth of manure from 100 cows gives you the energy of one barrel of oil," he says.

The methane is produced by a plug-flow digester, a relatively low-tech method that uses a large, plastic-lined pit, covered by another plastic membrane. The manure and bedding are scraped from the floors of the cow barns and milking parlor twice a day and pumped into one end of the digester. As it flows the length of the digester, it ferments, releasing methane gas, which is scrubbed of harmful hydrogen sulfide and dried before being piped to an engine-driven generator.

Some of the waste heat from the methane-powered engine is used to keep the digester at optimum temperature, some warms the cow barns and milking parlor in cold weather, saving hundreds of gallons of fuel per month.

At the other end of the digester, after about 25 days, the fermented manure is run through a mechanical separator that squeezes out most of the moisture. The resulting fluid is piped into a lagoon; the solids, which are free of most pathogens due to the heat of the fermentation, are used as bedding for the cows.

It's an elegantly simple, but effective, process. The fluid digestate is rich in plant nutrients, and--as a result of the digestion--is nearly odorless.

Standing next to the waste lagoon, the odor reduction benefit is instantly, and pleasantly, noticeable. Just a few feet away, one wouldn't even know the lagoon is there, whereas the stench from conventional dairy manure lagoons often attracts the wrath of neighbors, municipalities and environmental authorities.

Environmentally friendly fluid

The digested fluid is environmentally benign; if it should get into local waterways due to runoff or floods, it shouldn't kill fish and other aquatic life. And its 7.8 pH level means it can be spread on cropland without risk of burning crops.

Those are huge advantages at a time during which livestock operations and dairy producers are coming under increasing scrutiny by environmental groups and regulators over the smelly, toxic contents of their manure lagoons.

"We've doubled our output of alfalfa since we started using the digestate for fertilizer," says Haubenschild. …

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