The city of Sandpoint, Idaho, is seeking bids for a planned expansion of one of its drinking water treatment plants that would about double the city's capacity to supply potable water, says a project manager for the Spokane office of Denver-based CH2M Hill Inc., which is overseeing the project.

The project manager, Greta Gilman, says the engineering estimate for the project cost is $18 million.

The city operates two surface-water treatment plants. One draws from Lake Pend Oreille and the other draws water from Little Sand Creek. The combined treatment capacity varies from 5 million to 8 million gallons a day, the city says.

The project will expand the Lake Pend Oreille plant's capacity for treating drinking water to 10 million gallons a day and the city's total capacity to 15 million gallons a day, Gilman says,

The Lake Pend Oreille plant is on Sandpoint Avenue west of the city center. It was built in 1981, and the expansion will be its first upgrade, Gilman says.

The Little Sand Creek plant is on Schweitzer Road north of the city. It was built in 1964 and upgraded in 1997.

The Lake Pend Oreille plant expansion will include converting the treatment process at that plant to a pressurized microfiltration membrane system from the current gravity-fed sand filtration system. The new system will force water through tiny pores in the membrane, which will filter out contaminants, Gilman says.

She says the project will involve demolishing a filter basin and erecting an 8,000-square-foot treatment building in its place. The contractor also will install the membrane system and retrofit the rest of the plant to make it compatible with the system.

Contractor bids are due on Oct. 26. Gilman says she expects construction will begin next spring and be completed in late 2012. The water system serves 4,000 households and businesses, the city says.