The Old Mission Creek Summer Urban Runoff Facility (SURF) project was designed to improve water quality entering the creek from the Westside Storm Drain, providing safer conditions for children that play in Old Mission Creek, and residents and visitors that swim in the ocean at East Beach.
Constructed in 2006, the SURF diverts urban runoff in the Westside Storm Drain during low flow conditions (dry weather) to a treatment facility that uses ultraviolet (UV) light to eliminate bacteria and viruses.
Following treatment, the water is then discharged back to the storm drain, and flows into Old Mission Creek at Bohnett Park. The project is closed during the rainy season, when storm water is allowed to flow directly into the creek.
The project is located underground and within the boundary of the City-owned Bohnett Park. A diversion structure was built into the existing 84” diameter storm drain. Dry-weather flow up to 120 gallons per minute are diverted from the drain to an oil/water separator, and then a pump station passes the water to a vault housing the treatment facility.
Water moves through solid-media filters, in order to remove particulates, and then passes under ultraviolet (UV) light bulbs. The residence time of the water under the bulbs and the intensity of the UV light have been designed to eliminate bacteria and inactivate viruses in the water. The treated water is discharged back to the storm drain and then flows into Old Mission Creek in Bohnett Park. A valve allows the facility to be closed during the rainy season.
The project also includes a solid removal device (CDS unit) for removing trash and sediment from higher flows (up to 54 cfs). Although not necessary for the operation of the SURF during low flows, the CDS helps reduce the discharge of trash and sediment in Old Mission Creek during average rainfall events.
Similar projects elsewhere have resulted in non-detectable levels of indicator bacteria after treatment with ultraviolet light, as long as the water had been filtered of fine particles before treatment. Indicator bacteria can return to high levels downstream of a UV treatment facility. However, it is important to note that this likely represents nonpathogenic indicator bacteria that are capable of growing in the natural environment, illustrating the problem with the use of indicator bacteria to assess the risk to human health.