Monterey Bay is one of the nation’s spectacular marine sanctuaries. Pollution carried by urban runoff to the Bay is a huge threat to the water quality and the marine life. In an effort to protect Monterey Bay, the California central costal community has made extensive efforts to manage runoff at the bay.
One might not typically think of runoff treatment being needed during periods of dry weather. However, water still flows off paved areas during this time from activities such as landscape irrigation, car washing, and the cleaning of sidewalks and driveways, carrying pollutants with it. That’s why the City of Pacific Grove, Cal., which borders the Bay, constructed a diversion system that would direct dry weather runoff from the storm drain system into the sanitary sewer system throughout their seven-month dry period.
The diversion system was designed to accommodate dry weather flow from two mixed-use drainage basins, where each system accepted runoff from numerous storm drain interceptor manholes distributed throughout.
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 Engineered Solutions were installed to pretreat the runoff prior to diversion.
The first drainage area is 7.42 acres with a treatment requirement for the water quality flow, which a 2-year, 1-hour storm. An inline CDS unit was used with a design treatment capacity of 2.0-cfs and peak capacity of 6.13 cfs. The unit was retrofitted on the existing 24” 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 treatment capacity of 6.0 cfs and peak capacity of 150-cfs. This offline unit was retrofitted on the existing 48” RCP storm drain.
Using the CDS system in the dry weather flows diversion system provides the City with advanced pre-treatment of polluted runoff. The system will effectively remove trash, debris as well as sediments from the runoff before it is pumped to the sanitary sewer system. The CDS units not only protect the pumping structure from abrasion due to debris and coarser sediment, but also reduce the solids load to the sanitary sewer facility, which cannot be achieved using a typical wastewater screening device.
Compared to the traditional physical screening device for pre-treatment in the wastewater facility, the CDS technology provides high efficiency, a non-blocking screen, and has no power requirements or moving parts.
During wet weather conditions, the CDS device will also function as the stormwater treatment device to remove trash, debris, sediments and particulate pollutants (phosphorous, metals, etc.) that are carried in stormwater runoff.