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City of Redmond

Redmond, Washington

Stormwater Treatment


City of Redmond


City of Redmond



Technical Description:

CDS Hydrodynamic Separator

To treat stormwater before discharging it to the Sammamish River, the City of Redmond, WA, in 1992 installed a coalescing plate oil–water separator in its 184-ac (74.5-ha) catchment area downtown. The system, which consists of three sequential, pipe connected vaults with an oil-water separator in the middle vault, is fairly standard for stormwater treatment, according to Bob Franklin, Redmond’s engineering manager. “We didn’t have enough space for any other type of facility” he explained, “so a below-ground type of system was essentially our only treatment option”

The Challenge

Unfortunately, separator maintenance needs soon overwhelmed Redmond’s public works staff. “The amount of debris and leaves entering the system was more of a problem than anyone anticipated” Franklin said, noting that every major rainstorm washed trash into the system, collapsing the separator’s static screens and clogging its coalescing plates. As a result, oil and other pollutants bypassed the blocked system and entered the river, putting Redmond at risk for non-compliance with state water quality standards.

The Solution

To deal with the trash, city staff purchased a CDS (continuous deflective separation) system. Redmond liked the technology because it has no moving parts, requires little maintenance, and removes virtually all debris and suspended solids from stormwater runoff.

CDS designed a special system to fit in the oil–water separator’s first vault, which measures about 8-ft. by 18-ft. (2.4 m by 5.5 m). The patented system relies on water hydraulics, gravity, and a screen configuration to remove debris from runoff. As stormwater enters the system’s diversion chamber, a weir directs the flow into a separation chamber, where a vortex forms. Floatables and suspended solids are pushed onto a cylinder screen, which deflects them to the center of the separation chamber. The solids then settle into a sump, where they remain until removal by city staff. Stormwater, meanwhile, passes through the screen, out of the separation chamber, and into the coalescing-plate chamber. Because the solids have been removed, the coalescing-plate separator now can function properly, removing nearly all the oil and chemical pollutants from the water.

The Result

“We’ve had no problems since we installed the CDS unit” Franklin said. “Maintenance of the CDS system is minimal. About once a quarter, a [vacuum] truck removes the debris from the unit’s sump. Otherwise, it’s maintenance free. And it was a very affordable retrofit”

Redmond’s public works staff now is looking at installing treatment units in other city stormwater systems, Franklin noted, and “we’re definitely considering CDS units for those pipes as well”


Technical Description:

CDS Hydrodynamic Separator

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