aerial view of the twin oaks plant

Treatment Plant Provides Clean Water

Located next to the Water Authority’s aqueduct north of the city of San Marcos, the high-capacity 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. The plant’s location next to the existing pipelines made pumping and new pipeline construction unnecessary, providing significant cost and time savings.

Water Treatment

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

Submerged Membrane Treatment Process

Water from rivers and reservoirs can contain a variety of organisms and inorganic material that must be removed before the water is safe for drinking and other uses. Larger particles such as sand, vegetation and other materials must be screened out first. Smaller particles are removed as water 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. Contaminants that do pass through the membranes are eliminated in a disinfection process in which a long-lasting disinfectant keeps water safe and healthy as it travels through pipelines to reach homes and businesses. In addition, as part of the treatment process, fluoride is added to the treated water.

The Water Authority selected the submerged membrane method because it 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 is able to meet increasingly rigorous state and federal water quality regulations.

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. Nearly all water entering the plant leaves as high-quality drinking water.

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 & Carryover Storage Project greatly improves the region’s water reliability.