Water Treatment Process

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The water treatment process is a multi-step process in which water is taken from its raw (untreated) form and through a series of processes it is treated to a level that makes it safe for consumption. The process below shows the path that water takes from our reservoirs to your tap.

1: Source of Water:

source water

City of San Luis Obispo water comes from Salinas Reservoir (Santa Margarita Lake), Whale Rock Reservoir, Nacimiento Reservoir, and ground water.

2: Ozone for Disinfection


Disinfection is the cleansing process in the treatment of water through selective destruction of pathogenic organisms (diseases).

The City of San Luis Obispo’s water treatment plant uses ozone as the primary disinfectant. Ozone is produced by three 250-pound-per-day ozone generators. It is then fed into contact basins where it dissolves into the water to achieve disinfection. Ozone treatment is preferred because it minimizes disinfection byproduct formation resulting from traditional disinfection by chlorination. A thermal catalytic destruct system removes excess ozone gas and limits the concentration of ozone discharged to the environment.

3: Rapid Mix (Coagulation)


During rapid mix, aluminum sulfate (alum) and polymer are added and mixed with the water to start the coagulation process (the formation of clumps from the fine, suspended particles in the water). The alum and polymers neutralize the electrical charges on the particles allowing them to coagulate into larger particles.

4: Ballasted Flocculation (Actiflo®)

ballasted flocculation

Ballasted flocculation combines the traditional flocculation and sedimentation processes to include the addition of a weighting  or ballast material. Light suspended matter in the previously coagulated water is flocculated to form larger particles. This floc then bonds to the ballast material or micro-sand so that it may settle out very rapidly. The settled material is then collected and processed to separate the undesirable solids for disposal and return the micro-sand back into the treatment cycle.

5: Filtration


Filtration removes particulate impurities and residual floc from the water being treated. Fine particles are filtered through multi-media filters consisting of layers of anthracite coal and sand. When the filters become clogged with these particles, they are backwashed with filtered water. The backwashed water goes to a small package plant where it is partially-treated before returning to the head of the treatment process.

6: Chlorine, Fluoride Addition & pH Control

chlorine fluoride ph

Before the treatment process ends, the required dose of fluoride is added to the filtered water (to reduce tooth decay in children). Sodium Hydroxide is added, when necessary, for corrosion control, and sufficient chlorine is added to maintain a residual disinfection capacity as the water moves through the distribution system. Operators test the final treated water every 2 hours to ensure that safe, high-quality  water is being produced.

7: Treated Water Storage


Water flows to treated water storage clearwells, and then on to other City reservoirs. City reservoirs are geographically located to provide water at the proper pressure to customers and for fire suppression.