Garco Construction Inc., of Spokane, has submitted an apparent low bid of $12.3 million for a contract to construct the first round of projects planned in the $100-million second phase of improvements at the city of Spokane's sewage-treatment plant, says Lars Hendron, the plant's principal engineer.

The work will include constructing an odor-control structure, installing a skimmer to remove grease, and installing a clog-resistant piping system, Hendron says.

The Spokane City Council is expected to award the contract this month, and the work is scheduled to start this summer and to take about 18 months to complete, he says.

Other bidders included Hoffman Contractors Inc., of Spokane, which bid $12.5 million, and Stellar J Corp., of Woodland, Wash., which bid $13.7 million. McClure & Sons Inc., of Mill Creek, Wash., withdrew its $10.5 million bid, citing an error in its calculations, Hendron says.

The odor-control structure will cover four settling basins called primary clarifiers at the plant at 4401 N. Aubrey L. White Parkway and will include high-capacity ductwork capable of carrying 25,000 cubic feet of foul air per minute through a new activated-carbon filtering system, Hendron says. The structure is part of an ongoing plan to reduce odors that often drift from the plant into surrounding neighborhoods, he says.

The work also will involve constructing new basins called wet wells that will be equipped with automated skimmers to remove grease from the surface of wastewater and divert it to the system's digesters to be processed with solids.

Another part of the project, a new large-diameter piping system, will allow flexibility in routing flows out of the plant's secondary clarifiers, Hendron says.

Smaller parts of the project will include parking improvements and a pilot program that will test a chemically enhanced phosphorus removal process during primary sewage treatment, he says.

The Spokane office of Denver-based CH2M Hill Inc. is the city's design, engineering, and construction manager for the project, which is just the first of several planned in the second phase of upgrades.

The first phase of upgrades included recently completed improvements to the plant's headworks and construction of two 130-foot-tall, 2.8 million-gallon sewage sludge digesters. Two additional digesters are planned in the second phase.