The catwalk atop Spokane’s two new egg-shaped sewer digesters next to the Spokane River offers a panoramic view of a honeycomb of pipes, pumps and valves – the guts of a $45 million investment in the area’s wastewater treatment system.
The steel-walled digesters, which use bacteria to process sludge into fertilizer, replace treatment capacity lost in the 2004 collapse that killed sewer maintenance worker Mike Cmos.
The gray-green structures are 89 feet in diameter and hold 2.8 million gallons each.
“It’s pretty impressive, the size of these things,” said Tim Pelton, one of two superintendents at the wastewater treatment plant on Aubrey L. White Parkway.
On Tuesday, city officials invited a small group of people involved in the project to view the digesters as city workers prepare to put them into operation later this year.
The project was completed in August, several months behind schedule because of a boilermakers strike, design changes and severe winter conditions.
In concrete galleries below ground hang 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 with Chicago Bridge and Iron, based in Chicago, which acquired Morse Tank, of Everett, several years ago.
The digester site is sandwiched between the river and older portions of the plant, which made work difficult for crews.
In 2006 a construction accident killed North Idaho resident and Garco employee Tizoc Gayton, 26.
The new tanks were created with sections of fabricated steel shipped from Iowa and pieced together. Foam insulation covers the steel, and an aluminum shell surrounds the insulation.
Pumps are underground to reduce noise. The egg-shape design is safer, officials said, and allows for better mixing, less maintenance and lower energy use.
The tanks include “robust foam control” to minimize accumulation of foam on top 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.
The new tanks are part of an effort to modernize the city sewer system at a total cost of $500 million through 2018. A major part of the spending will reduce pollution discharges into the river. Part of the funding comes from a $13 monthly surcharge on sewer bills.