Contractors Northwest Inc., of Coeur d'Alene, has started work on a $10.5 million construction contract for the city of Coeur d'Alene to build a five-structure addition at the city's wastewater treatment plant.

The total cost of the project, including engineering, is about $13 million, says Sid Fredrickson, superintendent of the city's wastewater utility department.

The work is part of about $72 million in upgrades to the facility planned over several years, he says.

The current work includes constructing a digester that will have a capacity of about 250,000 gallons. Along with the digester, a 2,600-square-foot digester-control building will be constructed, for boilers and equipment to be used with the digester.

Contractors Northwest also will build a 650-square-foot biogas-control building, where methane gas emitted from the plant's digesters will be routed to boilers that will burn the gas to heat six structures at the facility.

The contractor also will build a 7,300-square-foot, heated maintenance and storage building for the department's liquid sewage collection equipment, and a 9,000-square-foot two-story building for its administrative offices and laboratory.

Currently, the department leases space offsite in Coeur d'Alene for its administrative offices, Fredrickson says. In addition to office space, the new administrative building will have a conference room, a training room, and a library, he says.

The new structures will be built of cast-in-place concrete and concrete masonry blocks and will have metal roofs, Fredrickson says.

The Missoula, Mont., and Boise offices of HDR Inc. designed and engineered the project, Fredrickson says. He says work on the project began in March and will continue through 2011. In about 2014, the department will begin its next project, implementing one of three pilot phosphorus-removal systems it currently is testing. Ultimately, the city hopes to increase the plant's capacity to about 12 million gallons of wastewater a day. With the upgrades currently under construction, the plant's capacity will be about 6 million gallons a day, he says.

The improvements will be done under the department's facilities plan, which was amended to account for upgrades deemed necessary to meet the terms of a proposed federal discharge permit.