SUNNYSIDE -- The fluids flow downhill, methane flows up and the whole 40-ton pile of manure turns into fertilizer.

And once the contraption starts, nobody needs to touch it for months.

That's the thinking -- and the hope -- of developers who are testing an inexpensive manure digester they believe will be the first to harness methane and produce compost inside one unit.

"It's the Swiss army knife of digesters," said Russell Davis, president of Organix, a Walla Walla-based company that specializes in converting waste organic matter -- normally cow manure -- into compost.

The pilot digester that Organix constructed earlier this month at a dairy north of Sunnyside aims to do anaerobic and aerobic manure conversion in sequence, but inside one "vessel" -- industry speak in this case for a giant plastic bag.

In June, Organix received an $84,000 grant from the U.S. Department of Agriculture to develop the digester and enlisted the help of Conly Hansen, an engineering professor at Utah State University ,who has helped develop manure digesters across the world.

"This is a unique system," Hansen said, sweating under the afternoon heat at a dairy, whose owner has no plans to purchase the digester and requested that the location not be revealed to avoid attracting curious trespassers.

The test measures how much methane the system can harness from the natural breakdown of manure -- an anaerobic process -- as well as how well solid waste can be converted to compost by adding oxygen -- the aerobic side of the equation.

Organix already works with about 15 dairies and a quarter of the cows in Yakima County, home to 215,000 total cattle, 97,000 of them dairy cows, according to U.S. Department of Agriculture statistics.

Davis expects this digester will help smaller dairies, those with 500 or so cows, costing about $250,000.

Manure digesters aren't new, but they're rare.

The sprawling George DeRuyter & Sons dairy in Sunnyside owns the only one in Central Washington, a $3.8 million anaerobic digester that works without oxygen to harness the methane that decomposing manure inevitably produces. The dairy burns it to generate electricity, which it sells to the power grid.

Even with a digester, once the methane is siphoned off there is still a pile of manure to deal with. It's typically heated to let oxygen mix with the microbes to convert into compost or bedding, which the dairies use on their own fields or sell to other farmers.

The Organix digester is a pretty simple set-up.

Manure is stuffed into elongated agricultural bags and laid on a slight descent. A pump at the bottom end collects the fluid as it trickles through the manure and sends it back up through a perforated pipe running along the top to keep the circulation going.

As the bacteria works, the pile releases methane from the bag through a valve at the top end.

In 40 days or so, Organix will open a plastic tube running the length of the bottom of the pile to blow air into the manure and let oxygen convert it to compost.

"It's a good idea if it works," said Steve George, the Yakima field representative for the Washington State Dairy Federation who said he knew little about the digester.

Compost has turned into big business. About 50 percent of the dairy compost made from cow manures is shipped outside the county, George said.

About 25 percent of the Yakima County dairies contract with Organix to make, sell and haul their compost, Davis said.

Organix's digester could provide a cheaper alternative to the expensive digesters that don't pay for themselves with the area's relatively cheap electricity, said Laurie Crowe, coordinator and livestock nutrient management specialist for the South Yakima Conservation District.

"What Organix is coming up might very well be part of the solution," Crowe said.

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