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The Water Treatment Process

illustration of the water treatment process

Valley Water operates three drinking water treatment plants, Rinconada, Santa Teresa or Penitencia, to provide Silicon Valley with safe, clean, reliable water.

Much of our water comes to us from the Sacramento-San Joaquin Delta and our local reservoirs.

The three water treatment plants transform the untreated source water into clean drinking water via a complex treatment process consisting of clarification, filtration, and disinfection steps.

The first thing we do is remove the solid particles in the water -- mostly silt mixed in during the water's long journey from the Sierra-Nevada mountains to one of our water treatment plants.

As the silty water first enters our water treatment plants, we add chemicals like aluminum sulfate and polymers to the water. This process is called coagulation. As the water then travels through the plants, large mixers are used to blend the chemical and silt together to form heavy solid particles in the flocculation process. The particles then gradually sink to the bottom of the sedimentation basin, while the clarified water, free of solids, overflows and proceeds to the next treatment step. Rakes slowly rotate along the bottom of the sedimentation basin. They scrape the settled sludge at the bottom into a collection channel that removes the sludge out of the basin.

Ozone disinfection
Following major upgrades in 2006, ozone was added at Penitencia and Santa Teresa Water Treatment Plants. Ozone is a powerful disinfectant that minimizes harmful disinfection byproducts and removes unpleasant tastes and odors.

The first step in ozone disinfection is to generate ozone gas. Liquid oxygen is transported to the plant and stored for use. It is then vaporized into oxygen gas. When electric currents are applied to a flow of oxygen gas, some oxygen molecules (O2) are split and bond to other oxygen molecules to form ozone molecules (O3).

Next, water that has completed the sedimentation process is conveyed into the ozone contactor basins. Ozone is bubbled up through the water. Typically, the water spends 15 minutes in this system, traveling up and down a series of reaction cells to maximize the contact with the ozone gas.

Ozonated water then moves on through the pipes to the next step, filtration.

Meanwhile, the ozone that was used in the process is converted back into harmless oxygen and released into the atmosphere.

Next, the water is filtered to remove microscopic particles.

Rinconada and Penitencia Water Treatment Plants both use filters with anthracite coal and sand. Santa Teresa Water Treatment Plant uses filters with granular activated carbon and sand.  One of the advantages of granular activated carbon (GAC) is that aside from trapping tiny particles, it also removes chemical compounds that affect the water’s taste and odor.

 As the water seeps down the layers of sand and coal, tiny particles as small as one micron are removed from the water.

These filters are periodically washed to remove the microscopic particles adsorbed onto the filter media during normal operation. This involves taking the filter offline, draining, air scouring, and backwashing the filters. The wash uses air to agitate the filter beds and scour the trapped particles into the water and uses water to flush out the particles. After the filter has been cleaned, it is then refilled with water and put back on-line.

The backwash water flows into the recovery ponds where the solids in the washwater settle out and the water is pumped back to the beginning of the process to be treated again. At the Santa Teresa plant, the waste backwash water enters a mini-treatment plant to pre-clean it before it is pumped back to the starting point again. This will further reduce the possibility of parasites like giardia and cryptosporidium cycling back to the main treatment process.

 At the Rinconada plant, the waste backwash water is conveyed to the Washwater Recovery Facility for settling and decanting, with clarified washwater returned to the raw water inlet pipe. At the Penitencia plant, washwater is decanted in recovery ponds prior to returning to the main treatment process. A more robust facility for clarifying the washwater is currently in design and will be constructed in the future.

Final disinfection
The filtered water goes through one last step, secondary disinfection, to provide continuous disinfection when it is delivered to water users. Our treatment plants use chlorine and chloramine to kill any bacteria or viruses that may be present in the pipes from our plant to your tap. Chloramine is a combined chlorine and ammonia compound used to disinfect potable water.

Chlorine was first successfully used as a disinfectant for water in 1908. Chlorine disinfection has just about eliminated water-borne diseases such as cholera and typhoid in the United States.  Advancements in water treatment science have reached a point where the detection and control of water contaminants have minimized health risks to almost negligible levels.

After the water is treated, it flows through the pipelines all across the Santa Clara Valley. Your water retailer takes it from there and distributes the water to homes and businesses.