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Holding tank nears completion

Thu., Aug. 9, 2007

A large underground holding tank designed to minimize the dumping of raw sewage and storm water into the Spokane River is getting its finishing touches at High Bridge Park just west of downtown.

It is the second in a series of tanks to be installed through Spokane’s wastewater department as part of a cleanup program for the Spokane River – at a cost of $250 million or more.

The 200,000-gallon tank is designed to fill when combined sewage and storm water floods into sewer lines from heavy rain or a fast snowmelt. After the storm has subsided, the tank slowly will send the excess water to the city sewage plant for treatment over the next 24 hours.

Lars Hendron, engineer for the wastewater department, said the tank is like a siding on a railroad. Excess water spills into the tank when it reaches a predetermined level, just as a rail siding stores extra rail cars.

Three overflow pipes coming from western areas of the city have been connected to the tank, which is part of state- and federally mandated cleanup of the Spokane River.

The total amount of combined sewage and storm water dumped into the river last year was estimated at 116 million gallons, significantly more than was recorded in each of the three previous years. The high volume was due mainly to a series of spring thunderstorms.

“It is probably the most visible type of pollution of the river,” said Dale Arnold, wastewater director for the city.

Hendron described the problem “as a holdover from the pioneer days.” Many sewers in Spokane are a century old.

Prior to construction of the city’s sewage-treatment plant in 1958, sewage was dumped directly into the Spokane River. Sewers also carried storm water. In some parts of the city, primarily on the North Side, storm sewers were separated from sanitary sewers in the mid-1980s to reduce the combined wastewater releases.

But combined sewers still serve the south and east portions of the city, as well as some locations on the North Side.

Combined sewers are vulnerable to overflows during storms. The excess wastewater and storm water are intentionally diverted to the river during high flows to protect the wastewater plant and its lines from being flooded.

The anti-pollution goal is to reduce combined sewer releases into the Spokane River to only one spill every two years, on average.

CTE Engineers Inc., with an office in Spokane, was hired for design; Clearwater Construction Co. of Airway Heights installed the tank. Total cost of the project at High Bridge Park was $2.3 million.

Funding is coming through sewer fees and a rate stabilization fund collected on monthly utility bills.

At least two dozen more tanks are to be installed with up to 5 million gallons capacity in the largest of them. Locations under consideration include the former Playfair Race Course, Pettet Drive overlooking the river, Northwest Boulevard and I Street, York Avenue and Nettleton Street and Downriver Golf Course.

The tanks will be installed below ground, and the locations are expected to include public improvements, including possible park facilities.

The tank project at High Bridge Park included parking lot improvements and installation of water and power to the site. Those utilities can be used if a white-water park is built in the river near the combined sewer overflow tank.

The tank was designed with no pumps or wiring. It will operate with hydraulics and uses the flow of water to flush itself clean, Hendron said. The design also prevents floating waste from reaching the river.

City officials wanted to build smaller tanks first. The first tank was completed in 2003 at Felts Field.

Engineers are going to study the smaller tanks’ operation to make sure that the designs will work for larger tanks. Included in the design is an estimate of the volume of water that can be expected in a typical downpour, Arnold said.

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