The California Department of Transportation (Caltrans) builds and maintains some of the world's busiest highways, including the I-210, a major east—west state highway in the Greater Los Angeles area. The eight-lane freeway bisects numerous densely populated suburban communities so traffic is usually heavy, well in excess of 100,000 vehicles per day.
The highway is drained by a reinforced concrete box culvert measuring 3 x 1.5 meters. In L.A.'s arid climate, rainfall events are infrequent but often intense, and the runoff is usually packed with all kinds of pollutants and trash. Stormwater runoff from I-210 contains the usual debris: cups, cans, plastic and paper. And like most highways, it also has a toxic mix of metals such as beryllium from tires, aluminum from auto chassis parts, iron from engines and copper from brake linings.
Caltrans looked at several options for cleaning the highway's stormwater runoff. Installing screens or baskets at stormwater drains was quickly ruled out. Such devices require cleaning after almost every rain event, and the drains are located within a few feet of high-speed traffic. The high level of maintenance and the safety factor for workers made this option unacceptable.
An end-of-line cleaning process was preferred, but there were other factors affecting the decision. Volumetric-type devices that catch and hold large amounts of water for a long time were considered, but in this densely populated area it would have required Caltrans to displace several homes, and it would have cost millions of dollars. Caltrans was also interested in a system that would minimize maintenance events and costs.
Ultimately, Caltrans selected a CDS hydrodynamic separator from Contech Engineered Solutions. The CDS uses swirl concentration and continuous deflective separation to screen, separate and trap trash, debris, sediment, and hydrocarbons from stormwater runoff. The CDS system is entirely self-operating, relying on water hydraulics, gravity, and a non-blocking screen.
One of the biggest benefits of the CDS technology in this application was that Caltrans could fit a CDS unit into a small patch of land that could be easily and affordably obtained. There are actually five CDS units incorporated into the new I-210 freeway system, one of which is the largest CDS unit ever installed in the U.S. The pit excavated for the largest unit was 40 ft wide, 40 ft long and 38 ft deep. The cylindrical screen assembly on the unit is 15 ft in diam. by 15 ft tall. The unit is designed to filter a "first flush" runoff event of 175 cfs (78,400 gpm).