The city of Spokane plans to upgrade 3.5 miles of aging water transmission main between the Division-Ruby couplet and its Upriver Dam beginning this spring.

Halme Construction Inc., of Davenport, Wash., is the apparent low bidder, with a bid of $3.2 million, for the work. The low bid was about 28 percent below an engineer's estimate of $4.48 million for the project.

Gary Nelson, a senior city engineer, says the new, 30-inch iron water main will replace a 100-year-old, 24-inch steel pipe there.

The water line to be replaced lies between Jackson Avenue at Ruby Street and Upriver Dam, located on the Spokane River near Upriver Drive and Stanley Lane.

Nelson says most of the work will be done on local access streets, and street closures will be required where work is occurring.

The work is expected to take about 80 days to complete, he says.

Separately, Halme Construction also is the apparent low bidder, with a bid of $285,000, for a city of Spokane project to update several combined sewer-overflow structures, called weirs. Halme's bid was about 39 percent lower than an engineer's estimate for that project, which was $474,000.

The weirs divert sewage from old storm drain pipes that historically dumped raw sewage directly into the river before the city began treating sewage in about 1950. With the older system, sewage drops through holes in the bottom of the pipes into sewer lines below them.

The sewer lines carry the sewage to the wastewater treatment plant, says Lars Hendron, the city's principal wastewater management engineer. During heavy rains, however, faster water flow causes the mixed storm water and sewage to continue on to the river.

The older weirs are being replaced because they are prone to blockages that can cause sewage to overflow to the river even during dry weather, Hendron says.

In the 45-day project, the contractor will replace weirs in three locations in the northwest part of Spokane: on Sherwood Avenue near Summit Drive; at the intersection of Pettet Drive and Nora Avenue; and on Ohio Avenue near its intersection with Nettleton Street.

The newer type of weir, called a straight edge weir, is an adjustable-height wall built into a concrete vault, which separates the sewer pipes from the river outflow pipes. When combined water and sewage levels rise, the flow fills the concrete vault on one side of the weir, overtopping the weir and continuing to the river when it reaches a certain predetermined level.

As the water rises, a float-driven gate closes off part of the sewer pipe to regulate flows to the wastewater treatment plant to keep from swamping the plant with too much flow. The new weirs are equipped with an electronic measuring system so overflows into the river can be gauged more accurately , and the 10-foot-square vaults are less prone to blockages, Hendron says.

The city ultimately will update all of its older overflow diversion structures.