May 30, 2008

Providing Pretreatment For Dry-Weather Flows

Monterey Bay is one of the nation’s spectacular marine sanctuaries, but pollution carried by urban runoff is a huge threat to its water quality and marine life. In an effort to protect Monterey Bay, the California central coastal community has made extensive efforts to manage runoff during wet and dry weather.

Runoff treatment during periods of dry weather may not seem necessary, but water still flows off paved areas from activities such as landscape irrigation, car washing and sidewalk and driveway cleaning. Pollutants are carried along with this water; that is why the city of Pacific Grove, Calif., which borders the bay, constructed a diversion system that directs dry-weather runoff from the storm drain system to the sanitary sewer system during the seven-month dry period.

The Solution

The diversion system was designed to accommodate dry-weather flow from two mixed-use drainage basins, where each system accepts runoff from numerous storm drain interceptor manholes. Previously, the runoff from these two drainage basins was discharged directly into Monterey Bay through two drainage pipes. Now, two pump stations divert the dry-weather runoff into the sanitary sewer system. Two CDS units from CONTECH Stormwater Solutions were installed to pretreat the runoff.

The first drainage area is 7.42 acres, with a 2-year, 1-hour storm treatment requirement for the water quality flow. An inline CDS unit was used with a 2-cu-ft-per-second (cfs) design treatment capacity and a 6.13-cfs peak capacity. The unit was retrofitted with the existing 24-in. RCP drainage pipe.

The second drainage area is approximately 220 acres, and the design of the CDS unit was based on a hydraulic analysis of the existing drainage system. An offline CDS unit was designed with a 6-cfs treatment capacity and a 150-cfs peak capacity. This offline unit was retrofitted on the existing 48-in. RCP storm drain.

The Result

Using the CDS system in the dry-weather-flows diversion system provides the city with advanced pretreatment of polluted runoff. The system will effectively remove trash, debris and sediment from the runoff before it is pumped to the sanitary sewer system. The CDS units protect the pumping structure from abrasion, debris and coarser sediment and reduce the solids load to the sanitary sewer facility, which cannot be achieved using a typical screening device.

Compared to the traditional, physical screening device for pretreatment, the high-efficiency CDS technology has a non-blocking screen and no power requirements or moving parts.

During wet-weather conditions, the CDS device will also function as the storm water treatment device to remove trash, debris, sediment and particulate pollutants (phosphorous, metal, etc.) that are carried in storm water runoff.