A $35 million expansion at Everett's wastewater-treatment plant at Smith Island will ensure its ability to handle growth during the next...

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A $35 million expansion at Everett’s wastewater-treatment plant at Smith Island will ensure its ability to handle growth during the next five years, city officials say.

For more than two years, construction to expand the city’s mechanical-treatment system has been under way. The project was completed earlier this month.

Specifically, the project improved the treatment process with new clarifiers, odor-control equipment and disinfection systems.

The result is a 30 percent increase in the amount of waste the mechanical systems can treat daily, said Robert Waddle, the city’s operations superintendent, bringing that portion of the plant to a capacity of 21 million gallons of treated sewage daily from 16 million.

Added to the city’s lagoon system, Everett now has the capacity to treat about 36 million gallons of sewage daily, Waddle said.

“The biggest portion of the project was the primary clarifiers,” Waddle said. “They settle out the heavy parts of the wastewater, which allows us to skim the lighter part, mostly oils, off the top. That makes the system more efficient.”

Everett uses two forms of wastewater treatment. Its lagoon system, which measures about 200 acres and dates back to the 1960s, treats wastewater in about 30 days. Wastewater travels through a series of lagoons and at each step bacteria in the lagoons help break down the wastewater until it’s clean enough to be discharged into the Snohomish River.

The mechanical plant, built in 1991, speeds up that process to just a few days. Tanks are super-concentrated with the bacteria that break down waste, making the process much quicker.

The city will most likely expand the mechanical plant again in about five or six years, Waddle said. At that time, the city hopes to add capacity for about another 10 million gallons of sewage daily.

Christopher Schwarzen: 425-783-0577 or cschwarzen@seattletimes.com