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How is Water Treated?
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Sedimentation

After flocculation, the water and floc moves slowly through large basins known as sedimentation or settling basins. The water moves very slowly through these basins due to their large size. This allows the floc to settle to the bottom of the basin. The floc that falls to the bottom of the basins is collected into a hopper by large rotating scrapers where it is removed several times daily by the plant operators. Clear water above the floc layer (referred to as treatment residuals) flows out of the sedimentation basin and to the filters. Removal of particles in the sedimentation basin improves the operation of the filters that comprises the next treatment process after sedimentation.

The sedimentation process removes many particles including clay and silt based turbidity, natural organic matter, and other associated impurities. These impurities include microbial contaminants, toxic metals, synthetic organic chemicals, iron, manganese and humic substances. Humic substances come from soil are produced within natural water and sediments by chemical and biological processes such as the decay of vegetation. Removal of humic substances from drinking water is desirable since they form disinfection byproducts when chlorine is added to the water. At high concentrations, disinfection by-products such as trihalomethanes are a public health concern.


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Sedimentation Basin- Click for full image

Filtration

The last step in purifying the water is accomplished by passing water through a bed of sand and gravel. As water filters through the sand, the remaining particles of suspended matter are trapped in the sand bed. In the filtration process, water flows on top of the sand bed and travels through the bed until it is collected at the bottom in underdrains. Filtered water flows from the underdrains into clearwells or filtered water reservoirs. The rate of filtration is regulated using controllers. The filters must be cleaned periodically as material becomes trapped in the filter and reduces the rate of filtration. Cleaning of the filters is also needed to prevent solids from passing through the filters into the filtered water. To keep the filters operating at their best performance, the filter operators take each filter out of service so they can be backwashed. Backwashing cleans the filters by forcing clean water backward through the sand to remove the solids which are deposited in waste drains. Filters are backwashed on a rotating schedule to ensure that plants can continuously operate.
Click for full image Filtration Plant- Click for full image

Fluoridation

Fluoride is added to the filtered water at each of the plants to reduce tooth decay. The plants maintain fluoride levels of approximately one part per million in the treated water.

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