The city of Spokane has awarded a $4.1 million contract to Halme Construction Inc., of Davenport, Wash., to construct two new sewer overflow basins near the southern bank of the Spokane River along east South Riverton Avenue, in East Spokane.

The roughly 250,000-gallon underground tanks, called combined sewer overflow basins, or CSO basins, are intended to detain combined storm and sewer water and keep it from flowing into the Spokane River during heavy storms or rapid snow melt-off, says city engineer Dan Buller.

Buller says that currently a heavy amount of stormwater flowing into street manhole drains can overload the city's sewer plant, called the Riverside Park Water Reclamation Facility. When that occurs, combined storm runoff and untreated sewage bypasses the facility and flows directly into the river.

The CSO basins are designed to hold large amounts of combined wastewater during such events until the wastewater treatment facility can accommodate the excess water. Then, the untreated wastewater from the basins is gradually pumped into the city's system and carried to the wastewater treatment facility.

"The idea is to eliminate the number of points of discharge during storm events, and to reduce the number of times we discharge," Buller says. "After the new tanks are installed, it will take a huge storm before any discharge."

Buller says the new CSO basins will be installed at two locations along South Riverton between Magnolia and Regal streets. During construction, that section of South Riverton will remain closed, he says.

Associated sewer mains also will be installed in the area between Magnolia and Altamont streets, and between Regal and Cook streets.

The new overflow basins will eradicate the need for two of the city's current combined sewer overflow discharge points into the river, near the intersection of South Riverton and Regal, and at the intersection of South Riverton and Altamont, Buller says.

The project is being funded by the city's wastewater department and Buller says construction will begin in mid-April, wrapping up in October.

The project to install the two new overflow basins is being mandated by the U.S. Environmental Protection Agency and is being enforced by the Washington state Department of Ecology, he says.

Buller says that in the next six years, the city plans to install around 25 more CSO basins at various locations around the city at an estimated cost of around $287 million. A total of three CSO basins have been installed in recent years.