With concerns about global warming and reducing our dependence on fossil fuels, we hear a lot about the search for renewable sources of energy.

What you may not know is that there’s a potential source of renewable energy right under our noses in Wisconsin…manure.

With the help of an anaerobic digester, farmers can generate heat and electricity using animal waste.

Gale Gordon, of Deer Ridge Dairy, part of Gordondale Farms in Nelsonville, owns what’s called a “mixed-plug flow” anaerobic digester.

It’s a kind of digester that’s been patented by a company called GHD, Inc. in Chilton.

This digester takes the manure and then uses anaerobic bacteria to break it down in a large chamber.

After about 22 days in the digester, out come a solid product, a liquid product, and what’s called biogas.

The biogas is sent to an engine which is hooked up to an electric generator; the electricity is sold to a utility company.

At Gordon’s farm, with more than eight hundred cows, he’s able to generate enough electricity to power 130 homes.

After the solid product comes out of the GHD digester, more than 99% of the harmful bacteria from the manure (like E-coli) have been killed; and the solid product is used as bedding for Gordon’s cows.

The liquid product is rich in nutrients, and is used as a crop fertilizer.

But the benefits don’t stop there…

Heat from the engine that burns the biogas is used to heat the dairy’s water and the floor of the milking parlor.

The solid and liquid product come out free of offensive odor and the biogas is burned, so the air around the farm is more pleasant to breathe.

The digester has also helped Gordon to save money on pest control.

“We’ve not had to do any fly control because the fly eggs are killed in the digester,” says Gordon.

WHAT SETS GORDON'S DIGESTER APART

Not all anaerobic digesters work exactly like the one made by GHD, Inc.

Other digesters may use different processes to digest the manure, or different types of anaerobic bacteria to break down the manure.

There are also some key differences in design that set the GHD mixed-plug flow digester apart from others :

One is that all the manure that goes into the digester stays in it for a consistent amount of time (22 days), which means that more than 99% of the harmful bacteria (like E-coli) are killed in all of the manure that goes into the machine.

Another is that solids don't settle and build up at the bottom of the digester, because the manure is mixed while it's in the digestion tank.

Through their design some other common types of anaerobic digester might see inconsistencies in the amount of harmful bacteria that come out in the final product; or they might have problems with solids building up at the bottom of the digestion tank.

In some countries, different kinds of anaerobic digesters are used simply to generate gas for cooking.

However, many digesters in the U.S. output the same types of products that the one at Deer Ridge Dairy does.

MANURE AS A RENEWABLE ENERGY SOURCE

Even though this is a good source of renewable energy, it’s not realistic to think that manure processed in digesters could ever completely replace fossil fuels as an energy source.

“But it could definitely take a chunk out of the pie,” says Melissa Van Ornum, of GHD, Incorporated, "especially when you can add in other waste streams to it."

And that is something that farmers can do with certain types of digesters.

Food processing plants and restaurants sometimes generate wastes (like fats, greases, or sugars) that wouldn't be able to be fed into a digester by themselves.

However, when a farmer is already using a digester for manure, such food waste streams can be added to the mix, and the digester can do its work and generate more of the same products it normally would.

"I would say it's kind of like adding dessert for the [anaerobic] bacteria," says Van Ornum.

That's good news in an energy situation where every little bit helps.