SAN DIEGO—The San Diego County Water Authority (SDCWA) recently opened the world’s largest submerged-membrane water treatment plant at Twin Oaks Valley in northern San Diego County. CH2M HILL designed and built and now operates the facility for SDCWA. The 100 million-gallon-per-day (mgd) water treatment plant is the first to be built and owned by the SDCWA, which is a wholesale distributor of both treated and raw water to its 24 member agencies.

SDCWA awarded the design-build-operate (DBO), $157 million contract to CH2M HILL in September 2005. The contract includes an annual operation and maintenance fee of $6 million. The DBO approach fast-tracked the project, allowing concurrent design, procurement, construction, and permit approvals to support a tight construction timetable, saving SDCWA both time and money.

According to CH2M HILL, SDCWA selected the firm’s water treatment plant concept that incorporated the submerged-membrane treatment process because it produces high-quality water, is less expensive than conventional processes, and is more environmentally friendly. The membrane option allowed for a smaller footprint, less concrete, fewer chemicals to store, and fewer solids to dispose of than traditional treatment methods. The ZeeWeed submerged-membrane technology and equipment for the project was provided by GE Water & Process Technologies.

In addition to submerged-membrane filtration, other treatment processes in the plant include fine screening, ozonation for additional disinfection, ozonation with peroxide for taste and odor control, biologically active carbon contactors, chlorine addition, and 15-mgd of treated water storage.

CH2M HILL has completed similar projects around the world, including a 72-mgd submerged-membrane water treatment plant in Singapore; the Regional Municipality of Peel in Toronto has commissioned a 95-mgd plant. Both use different pre- and post-treatment systems because of unique temperature and water quality conditions.

CH2M HILL also said that it recently launched in Singapore the world’s first full-scale, seawater desalination plant to use large-diameter seawater reverse osmosis (SWRO) membrane technology. The 2.6-mgd plant uses 16-inch-diameter membrane elements, instead of the current industry standard 8-inch diameter, the firm said.