Surface Water
Treatment Process

Surface water treatment involves delivering the water from the source (a lake, river, or creek) to the water treatment plant (WTP) for cleaning and disinfection after which it is pumped to the consumer for commercial and private use.

Twelve steps to surface water treatment.

1. Water treatment starts at the water intake. For surface water, the intake takes the form of a large pipe that conducts the water to the WTP. These pipes are usually very large for big cities (usually larger than 10 feet in diameter). Many employ screens to help prevent large debris from entering the conduit.
   

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2. At the WTP another screen removes larger debris that made it through the intake conduit
   

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3. Chlorine is added in amounts which usually maintain free chlorine residuals around 1 mg/L. The chlorine disinfects the water and can aid in the reduction of bad tastes and odors. Fluoride is also added at this point to help prevent dental decay.
   

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4. Low-lift pumps raise the water so it can flow by gravity through the plant.
   

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5. The water goes through the mixing chamber where aluminum sulfate (or other coagulant) is added to remove small particulate matter. The aluminum sulfate attracts suspended impurities in the water (activated carbon is also added to remove offensive tastes and odors).
   

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6. The water is then gently stirred by giant paddle wheels called flocculators and/or forced to flow through a set of baffles to allow for good mixing of the coagulant. This causes the suspended impurities to form large, flaky particles.
   

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7. The particles settle out during a four hour trip through settling basins which are cleaned periodically to remove the particles accumulated at the bottom of the basin.
   

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8. The water is filtered through filter beds filled with anthracite, sand, and various types of gravel or other aggregate. The filters remove the remaining particles of impurities. A special backwashing cleaning process keeps the filters clean and free of accumulated impurities.
   

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9. When the water leaves the filter beds, more chlorine is added to bring the free chlorine residual up to a suitable level in order to maintain safe disinfectant levels in the water. This post-chlorination is an extra safety factor used to ensure good water quality.
   

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10. Phosphoric acid is sometimes added for corrosion control.
    

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11. The water is sent to either the storage reservoirs or the high-lift pumps depending on demand.
    

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12. The high-lift pumps send the water into transmission mains to millions of users. This is when the water travels to our homes and businesses.
    

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