Twin Oaks Valley Water Treatment Plant

About This Project

The Twin Oaks Valley Water Treatment Plant is one of the largest submerged membrane water treatment plant in the world and the first treatment plant built by the San Diego County Water Authority. Construction began in 2005 and was completed in April 2008.

Located next to the Water Authority’s aqueduct north of the city of San Marcos, the high-capacity treatment plant can produce up to 100 million gallons of treated water per day -- enough to supply up to 220,000 typical four-person households each year.


About Water Treatment

Water from rivers and reservoirs can contain a variety of organisms and inorganic material that must be removed at a water treatment plant before the water is safe for drinking and other uses. Treatment plants vary in the specific processes they use, but they generally follow the same basic steps.

Larger particles such as sand, vegetation and other materials must be screened out first. Smaller particles are removed in a later, separate process. To eliminate organisms that can cause disease or unpleasant odors and taste in water, disinfecting chemicals are added. In many treatment plants, water passes through an additional filtering medium such as sand, gravel, carbon or anthracite, to remove any remaining tiny particles. The last step is to apply an additional, long-lasting disinfectant that will keep water safe and healthy for the days or weeks it may travel through pipelines to reach homes and businesses.

The Water Authority selected the submerged membrane method for treating water at the plant. This method of separating water molecules from contaminants is safe and highly effective in producing high-quality treated water. The membrane treatment, in conjunction with the other processes at the plant, has such a high degree of contaminant removal that the plant will be able to meet increasingly rigorous state and federal water quality regulations.


What is Submerged Membrane Water Treatment?

Water treated at the Twin Oaks Valley Water Treatment Plant is drawn through very fine pores in membrane fibers. The pores are just large enough for water to pass through, but small enough to leave behind contaminants and particles, such as dirt, dust, bacteria, cryptosporidium, giardia, and others. The contaminants that do pass through are eliminated in a disinfection process. In addition, as part of the treatment process, fluoride is added to the treated water.

Please refer to the fact sheet below for more information.


Treatment Plant Benefits

The Twin Oaks Valley Water Treatment Plant is a project with many benefits beyond the high-quality water it produces. Its strategic location, creative design, and use of membrane technology make it an efficient, money-saving facility.

  • High Volume – Each day 100-million gallons of water can be treated at the facility.
  • Scarcely a Drop Wasted – Nearly all water entering the plant leaves as high-quality drinking water.
  • Energy and Money Savings – The plant’s location next to the Water Authority’s existing pipelines made pumping and new pipeline construction unnecessary.
  • High Quality – The facility not only meets current regulations, but is prepared to meet more rigorous water quality regulations anticipated in the future.
  • Emergency Water Availability – The plant can supply treated water to the region’s emergency water system if catastrophe strikes (no water boiling required). For more information about the Emergency Storage Project system, click here.

Connected to Emergency Water System

In the event of a prolonged drought or earthquake, the delivery of imported water from Northern California and the Colorado River could be interrupted. The Twin Oaks Valley Water Treatment Plant’s strategic location enables the Water Authority to provide an emergency supply of treated water, eliminating the need for consumers to boil water. Integrating the treatment plant with the Emergency Storage Project will keep high-quality drinking water flowing in the event of an earthquake or other interruption in imported water deliveries.

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3566 N. Twin Oaks Valley Road
San Marcos. CA 92069

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We’re Committed to Our Neighbors

The Water Authority values community input and strives to provide clear, open communication early in the planning process on its construction projects. This commitment to the Twin Oaks Valley community prompted the formation of the Twin Oaks Valley Working Group. Working group members included Twin Oaks Valley residents, a business owner, and representatives from the Twin Oaks Valley Sponsor Group and the Twin Oaks Valley Property Owners’ Association.

The working group identified 50 suggestions to improve traffic, noise, lighting, chemical safety, landscaping, and visual impacts. Thirty-five of the working group’s ideas were incorporated into the design, construction and operation of the water treatment plant. These contributions to the design process helped the Water Authority build a better facility.

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For more information, please call the 24-hour toll-free project information line at (877) 682-9283, ext. 7010 or email Your call will be returned within one business day.

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