PUBLIC WORKS/Decantation plant saves money, trips to incinerator

In March, Bothell, Wash., opened a decantation plant that separates solid debris from water collected by city maintenance crews while cleaning storm drains and sewer manholes. The $43,000 facility will reduce the city’s expense of transporting the materials to an incineration company for disposal.

Before the plant opened, Bothell trucked all of the material — water and solids — to a privately owned incinerator located approximately 20 miles from the city. The company charged Bothell a flat, per-ton rate for handling the material. Now, with the decantation plant, the city can take just the solids to the incinerator. Meanwhile, the plant discharges the decanted water into King County, Wash.’s sewer system.

“The tonnage that we’re taking [to the incinerator] is much less, and, consequently, our trips there are fewer, too,” says Dave Zabell, director of Bothell’s Public Works Department. “Add it all up, and it’s a major operating savings.” He estimates that the city will save $20,000 annually in part by reducing the tonnage transported to the incinerator by 40 percent and decreasing the number of trips to the facility.

At the plant, water and solids are pumped into a 520-square-foot roofed facility that allows large debris to settle and the water to drain into an underground vault. In the vault — and in an adjacent 15,000-gallon settlement tank — finer solid materials are separated from the water. Eventually, a valve releases the debris-free water into the county sewer system. Meanwhile, the solids are collected and placed in an 800-square-foot drying facility before being shipped to the incinerator.

Instead of bidding out the project, the public works staff designed and built most of the plant. Town and Country, a locally based contractor, designed and constructed the facility’s roof. Bothell officials estimate that designing the facility in-house saved the city $15,000.

Some smaller parts of the plant were built about five years ago, but Bothell did not have the money to substantially invest in the project at the time, Zabell says. However, construction of the facility, much of which is located underground, began in earnest last summer. The Northshore Utility District, a water and sewer utility that serves more than 60,000 residents of King County, paid for the facility’s roof in exchange for use of the plant.