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The Water Treatment Process

illustration of the water treatment process

The Santa Clara Valley Water District provides Silicon Valley with safe, clean, reliable water.

Much of our water comes to us from the Sacramento River. Transforming that silty blend into clean drinking water is quite a process.

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 three drinking water treatment plants: Rinconada, Santa Teresa or Penitencia.

Removing solids
At our Rinconada Water Treatment Plant, we remove the suspended solids in large tanks called clarifiers, the largest feature at the plant. Each tank is approximately 20 feet deep and 117 feet square

We add special chemicals--such as aluminum sulfate--to the water that enters the clarifier. These chemicals, called coagulants, cause the solid particles to clump together. This process is called flocculation.

Eventually, the clumps form a "sludge blanket." The solid clumps are far heavier than the water, so the blanket sinks to the bottom. As it does, the blanket works like a finely-meshed net to catch other smaller particles.

The water at the top of the tank, now free of solids, overflows to the gutter-like spokes you see radiating from the center of the tanks in the picture above.

Rakes slowly rotate along the bottom of the clarifier. They scrape the settled sludge at the bottom of the tank into the center where it is removed periodically through pipes that run under the clarifiers.

Ozone disinfection
Starting in the spring of 2006, new ozone systems will be in place at Penitencia and Santa Teresa water treatment plants. Ozone is a powerful disinfectant, minimizes harmful disinfection byproducts and removes unpleasant odors and tastes.

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 finished the flocculation process is piped into the ozone contactor basins. Ozone is bubbled up through the water. Water typically will spend 15 minutes in this system, traveling up and down a series of columns to maximize the contact with the ozone gas.

The newly ozone treated 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 has six filters each capable of filtering 16.7 million gallons of water per day. Each filtration tank is 13 feet deep and 77 feet long, about half the length of an Olympic-sized swimming pool

At the bottom of each filter are layers of coarse sand (6 inches), fine sand (18 inches), and anthracite coal (1 foot). As the water seeps down the layers of sand and coal, tiny particles as small as one micron are left behind.

Each of our plants will use granular activated carbon, or GAC, in the future instead of anthracite coal. GAC, the same type of material used in many home filtration systems, removes tiny particles and also chemical compounds that affect the water’s taste and odor.

Just like any home water filtration system, these filters get dirty and must be maintained. To keep them functional, they have to be washed periodically. The process, called "backwashing," involves several steps. First, the filter is taken off line and the water is drained down to the filter bed. Then, the air wash cycle is started which pushes air up through the filter bed causing the filter bed to appear to boil. This breaks up the compacted filter bed and forces the accumulated particles into suspension. The air wash cycle lasts for about three minutes.

After the air wash cycle stops, the backwash cycle starts with water flowing up through the filter bed. Most of the accumulated particles are flushed out. This cycle continues until the backwash water looks clean. The filter 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 (starting in the spring 2006), 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. The other two plants may have this capability 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 wiped out water-borne diseases such as cholera and typhoid in the United States. The science of water treatment has progressed so far that detection and control of contaminants in water have reduced health hazards to nearly zero.

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