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October 1, 2008

Spokane unveils wastewater plant digesters

Mike Prager
Staff writer - Spokesman Review
October 1, 2008

The view from the catwalk on top of Spokane’s two new egg-shaped sewer digesters offers a panoramic view of the Spokane River.

More than 100 feet below lies a honeycomb of pipes, pumps and valves – the guts of a $45 million investment in the Spokane area’s wastewater treatment system.

The steel-walled digesters replace treatment capacity lost in the 2004 catastrophic collapse that claimed the life of sewer maintenance worker Mike Cmos.

They are 89 feet in diameter, hold 2.8 million gallons each and have a gray-green exterior to blend with the environment. The digesters use bacteria to process sludge into fertilizer.

“It’s pretty impressive the size of these things,” said Tim Pelton, one of two superintendents at the plant.
Today, city officials invited a small group of people involved in the project on a tour as city workers prepare to put the tanks into operation later this year.

The project was completed in August, several months behind schedule because of a boilermakers strike, changes in design and severe winter conditions that slowed work.

In the concrete galleries below ground hangs a maze of pipes hooked to 75-horsepower pumps that can circulate sludge at 8,000 gallons a minute.

“It’s one of the most complicated concrete projects we’ve ever done,” said Hollis Barnett, project manager for Garco Construction of Spokane, the prime contractor.

Garco teamed up with Chicago Bridge and Iron, which had acquired Morse Tank, of Everett, several years ago.

Garco brought its own experience on tank and fueling facilities to the project, said Garco owner Tim Welch.

“It was a great job for us,” he said.

Garco bought a huge crane in Italy for the work and assembled it with a 146-foot tower and 181-foot mast. That crane is now being used for a garage project at Northern Quest Casino.

The digester site is sandwiched on the embankment between the river and the older portions of the plant, which made work difficult for crews.

During construction in 2006, an accident claimed the life of 26-year-old North Idaho resident and Garco employee, Tizoc Gayton, who was killed while removing sheet piling in a temporary retaining wall.

The egg-shaped tanks were created with conical sections of fabricated steel that were shipped to Spokane from Iowa and pieced together in 10 horizontal rings on each tank. Foam insulation covers the steel, and the insulation is clad in an aluminum shell.

Pumps are underground to reduce noise. The safer egg shape design allows for better mixing, less maintenance and lower energy use.

The tanks include “robust foam control” to minimize accumulation of foam on top of the sludge, a problem that contributed to the failure of the pancake-shaped tank in 2004. An earthen containment dike surrounds the tanks to prevent sewage spills in case of another accident. There are redundant systems for safety.

The new tanks are part of a wider effort to modernize the city sewer system at a total cost of $500 million upgrade through 2018. A major part of the spending is going to reduce pollution discharges into the river. A portion of the funding comes from a $13 monthly surcharge on sewer bills.

Mike Prager can be reached at (509) 459-5454 or by e-mail at mikep@spokesman.com.

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