Sonoma County supervisors are expected Tuesday to approve a proposed energy plant powered by chicken waste near the county's airport.

The project, put forward by the private partnership OHR BioStar and the Sonoma County Water Agency, would generate methane gas and power a 1.4 megawatt fuel cell.

Electricity from the fuel cell would be sold to the county Water Agency, supplying about a quarter of the agency's power needs. The unused methane — about three-quarters of the total produced — would be sold to PG&E.

The $48 million project is to be financed through bonds sold by the California Municipal Finance Authority. The county would bear no construction, liability or closure costs.

Supervisors, who serve as directors of the water agency, are set to sign off on a 20-year power purchase agreement with OHR BioStar and a site lease for the plant. The hearing is one of two set for 10 a.m.

The project, including three 65-foot-tall methane digester towers, is is to be built on 5.4 acres of Water Agency land near an existing wastewater treatment plant off Skylane Boulevard.

The county's zoning board, in a unanimous May 26 decision, approved a use permit for the facility and signed off environmental studies of the project.

Opposition to the plant has largely faded since it was switched from a previously proposed location south of Sonoma.

At that time, impacts from odor, traffic and poor aesthetics were mentioned by airport-area interests concerned about the plant. Others, including a representative of the area's largest commercial landowner, the Airport Business Center, said they were satisfied by the project's studies and plans.

A list of 97 conditions in its use permit spells out reporting requirements and limits on noise, traffic, odor and other impacts.

"It looked like the environmental documentation was in order," said Larry Wassem, who leads the Airport Business Center.

County officials have praised the project, noting it will use a local waste product — chicken manure and egg shells — and will move the water agency about three quarters of the way toward its goal of getting all its electricity from carbon-free sources by 2015.

Still, the higher cost for the energy produced by OHR BioStar has raised some eyebrows.

In the first year, for the same amount of energy now purchased on the wholesale market, the water agency will pay $330,000 more for power from the plant. Over the 20-year term of the agreement, the net increase amounts to $1.45 million.

But annual labor and operational savings of $80,000 from the project and the plant's yearly $120,000 lease income, plus the benefit of a locked-in power rate are all expected to produce a net gain for the county of $2.55 million over that 20-year period, officials said.

The savings may be less, officials acknowledged, or they may be greater, depending on prices on the wholesale market. Currently, those prices rise about 5.5 percent every year, while the cost for the OHR BioStar power is set to rise only by about 2.57 percent, the consumer price index for the Bay Area.

The two rates are set to be equal by the 16th year of the project, according to county projections; by the 20th year, the new power would be about $140,000 cheaper.

County officials expressed confidence in those projections.

"Over the life of this agreement, the county will save $2.5 million. I do think that is conservative," said Supervisor Mike McGuire, who represents the airport area.

Rates paid by customers would not be affected if the project does not produce savings, a Water Agency official said. Any increase in power costs could be paid for from an account set aside for that purpose, said Dale Roberts, a Water Agency principal engineer.

"It won't result in a rate increase," he said.