San Diego

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Our Water Treatment Process

Photo of Water Treatment Facility

The City's Public Utilities Department provides high-quality drinking water byutilizing proven technology, updated facilities, and state-certified operators. Water is treated at the City's three treatment plants using several processes, with each process providing additional water quality improvements. Using several treatment processes provides multiple barriers for added safety.

Our treatment plants employ a combination of time-tested Conventionalwater treatment processes and innovative disinfection strategies to produce high-quality drinking water. Conventional water treatment consists of coagulation, flocculation, sedimentation, and sand/multi-media filtration; this cost-effective, proven method of treatment is used throughout the modern world. Our advanced disinfection technologies have been implemented to reduce disinfection byproducts and produce better tasting water.


Watershed protection: San Diego receives water from local rain collectedin City reservoirs and from imported water. Imported water comprises 85 percent of our water and travels hundreds of miles before reaching our water treatment plants. Protecting the watersheds prevents contamination of our water supply and is the most cost effective process in water treatment. Extensive measures are being taken to prevent contamination of our local and imported water. So when you see "No Swimming" or "No Dumping" near water supplies we hope you understand this is for the protection of your drinking water. For information on the City's watersheds, the 2010 Watershed Sanitary Survey is available.

Coagulation: This is the chemical process of rapidly mixing coagulantsto the water coming into the water treatment plant (source water). Many of the particles in the source water have negative charges causing them to repel each other, much like two magnets when the negative ends are put together. Coagulation changes the negative charges to neutral.

Flocculation: Coagulated water is slowly mixed causing the neutralparticles to collide. When the collisions occur the particles clump together forming floc. As the floc is formed, particles in the water are trapped within the floc. The floc now looks like snowflakes suspended in the water.

Sedimentation: The floc particles are heavier than water. Mixing is stoppedand the water is allowed to slowly flow through the sedimentation basins. The floc settles to the bottom and is removed. The clear water is collected from the top of the sedimentation basins and sent to the filters.

Filtration: Water is passed through deep filtration beds to produce water that iscrystal clear. Extremely small particles are removed during this process. San Diego's water treatment plants produce water with turbidities (cloudiness) significantly better than drinking water standards.

Disinfection, Primary: Drinking water is further treated to remove orinactivate viruses, bacteria, and other pathogenic organisms. Disinfection is accomplished in a variety of methods. The Alvarado and Miramar Water Treatment Plants use ozone as the secondary disinfectant. The Otay Water Treatment Plant uses chlorine dioxide as the primary disinfectant. These are advanced disinfection processes and have the advantage of providing higher quality water with better taste.

Disinfection, Secondary: Chloramines are created by adding chlorine andammonia to the water as the last step in the treatment process. Chloramines help prevent microbial contamination from occurring in the water distribution system.

Corrosion Control: The corrosivity of the water is controlled by adjusting the pH.

Graphic of Water Filtration Process, Reservoir, Coagulation, Flocculation/Sedimentation, Filtration, Treated Water Storage

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