Water treatment process

To understand why the Rinconada plant must undergo improvements to its reliability, you need to understand the water treatment process. Each day, the facility treats as much as 80 million gallons of water per day. That’s a lot of water coming out of your tap.

At its simplest level, drinking water treatment seeks to remove solids, also known as turbidity, and pathogens. Additional treatment may be applied to deal with specific problems, such as hardness and chemical contamination. The description of drinking water treatment which follows addresses the turbidity and pathogen issues and includes five steps:



Solids are removed by sedimentation, followed by filtration. Small particles are not removed efficiently by sedimentation because they settle too slowly. They may also pass through filters. They would be easier to remove if they clumped together (coagulated) to form larger particles, but they don’t because they have a negative charge and repel each other, like two north poles of a magnet.

In coagulation, the water district adds a chemical such as alum, which produces positive charges to neutralize the negative charges on the particles. Then the particles can stick together, forming larger particles which are more easily removed. The coagulation process involves the addition of the chemical and then a rapid mixing to dissolve the chemical and distribute it evenly throughout the water.


Now that the particles have a neutral charge and can stick together, the water flows into a tank with paddles that provide slow mixing and bring the small particles together to form larger particles called flocs. Mixing is done quite slowly and gently in the floculation step. If the mixing is too fast, the flocs will break apart into small particles that are difficult to remove by sedimentation or filtration.


Next, the water flows to a tank called a sedimentation basin, where gravity causes the flocs to settle to the bottom. Large particles settle more rapidly than small particles. It would take a very long time for all of the particles to settle out and that would mean the need for a very large sedimentation basin. So the clarified water, with most of the particles removed, moves on to the filtration step where the finer particles are removed.


The filtration apparatus contains sand (which does the filtering), gravel (which keeps the sand from getting out) and an underdrain (where the filtered water exits). After the filter is operated for a while, the sand becomes clogged with particles and must be backwashed. Flow through the filter is reversed and the sand and particles are suspended. The particles are lighter than the sand, so they rise up and are flushed from the system. When backwashing is complete, the sand settles down onto the gravel, flow is reversed and the process begins again.


With particles removed, the remaining step is disinfection to ensure no pathogens remain in the water. Bacteria and viruses are now destroyed by addition of a disinfectant and chlorine. Enough chlorine is added so that some remains to go out in the water distribution system, protecting the public once the water leaves the plant.


9 Comments Add yours

  1. Hello,
    At what point of the process will the Fluoride be added?

  2. This question was asked in May, is it a hard one to answer?

    1. Jacqueline, the Fluoridation facility is scheduled for construction in Phase 5 of the project. Start up of Phase 5 is being eyed for December 2020.

  3. Thank you for your response…but my question is at what point of the water treatment process is the fluoridation chemical added?

    1. Hi, Jacqueline, thanks for the clarification. Fluoride gets added at the end of the treatment process, before going into the chlorine contact basin.

  4. Jacqueline Theriault says:

    Are you planning to use Ozone as an alternative to chlorine? Or in conjuntion? Will it be chlorine or the current chloramine that you will use?

    1. Hi, Jacqueline. The plant will be using ozone. I spoke to the treatment plant operator and he said that the facility will use free chlorine through the chlorine contact basin and chloramines at the clearwell, reservoir and the west pipeline as it currently operates.

  5. Vivek Rao says:

    I have a group of six 4th grade Argonaut elementary school students that would like to tour your facility to understand the water treatment process. It is part of project on hydrodynamics that they are working on. How do I arrange this?

    1. Hi, Vivek and thanks for your interest in the water treatment plant! Unfortunately, because Rinconada is in a heavy stage of construction, we are not offering tours of the facility at this time. However, we do have options for your class to learn much about how we treat water for drinking! Our award-winning education program regularly visits classrooms around Santa Clara County and conducts age appropriate presentations. And we do have a water treatment plant in south San Jose that conducts tours on a case-by-case basis. If you’re interested in learning more, please contact us at tmercado@valleywater.org

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