The cows at the Audet family's Blue Spruce Farm make nearly 9,000 gallons of milk a day — and about 35,000 gallons of manure.

It's long been the milk that pays, but now the Audets have figured out how to make the manure pay as well. They're using it — actually, the methane that comes from it — to generate electricity.

With the help of their power company, Central Vermont Public Service Corp., the Audets have devised a way to extract the methane from the manure and pipe it to a generator.

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They make enough electricity to power 300 to 400 average Vermont homes. It's renewable energy, and they're not the only ones interested in it.

Four other Vermont farms now have similar projects in the planning or early construction stages, power company officials said.

The Audets "deserve to be congratulated. They're the pioneers among Vermont farmers," said Dave Dunn, a senior energy consultant with CVPS who worked with them on the cow power project.

Elsewhere in the country, farmers are using similar technology to make energy, said Corey Brickl, project manager with Wisconsin-based GHD Inc., which built a device that the Audets use to harvest the methane.

One in Washington uses tomato waste from a salsa factory and waste from a fish stick plant as fuel, Brickl said.

For the Audets, the electricity has created an important new income stream at a time when low wholesale milk prices have squeezed their margin. The utility pays 95 percent of the going New England wholesale power price for electricity from the Audets' generator.

In addition, the utility charges customers willing to pay it a 4-cents-per-kilowatt-hour premium for renewable energy and then turns the money over to the Audets.

So far, more than 3,000 CVPS customers have signed up to pay the premium to support the renewable energy effort.

The bottom line is more than $120,000 a year from electricity sales. When they add in other energy savings enabled by the project, the Audets expect their $1.2 million investment in project equipment to pay for itself in about seven years.

The program has piqued interest.

Marie Audet, who describes herself as a wife, bookkeeper and milker, has become a tour guide, showing people from the United States and a handful of other countries around the farm's cow power operation.

Managing the hundreds of milking Holsteins — as well as young stock — is a high-tech operation.

In their stalls, cows munch contentedly on a mix of hay and silage while they make an occasional contribution of fuel for the Audets' power plant.

An "alley scraper," which looks like a big squeegee on wheels, comes by to push their manure down the row and through grates to a conveyor belt below.

From there, the manure goes to an anaerobic — meaning oxygen-free — digester, a 100-foot-by-70-foot structure similar to a covered swimming pool built by Brickl's company.

The manure spends 20 or 21 days in the digester, being pushed slowly from one end to other as more is added.

Three products result: a liquid that contains enough nutrients that it can be used as fertilizer for the farm's feed crops; a dry, odor-free, fluffy brown substance that is used as bedding for the cows and some of which goes to a local firm that bags and sells it as fertilizer on the home-and-garden market; and methane.

The methane is piped into an adjacent shed that contains a big Caterpillar engine that powers the 200-kilowatt generator.

Audet said the farm was saving the $1,200 a week it formerly spent on sawdust bedding for the cows, as well as some of the cost of heating the milking barn. A study by agricultural scientists from the University of Vermont found that the bedding produced from the manure was better than the sawdust.

"Wood harbors a lot of bacteria," she said.

With the success of the 200-kW unit, the Audets are expanding by adding a new, 75-kilowatt hour generator. And Audet said she's even grown to like giving the tours.

"It's bringing a lot of people to the farm who are normally very removed from food producers," she said.