Water Education - Water Quality

Discover Water Treatment Plants Page 2

Other chemicals may be added at the same time, including powdered activated carbon (to absorb taste- and odor-causing chemicals or to remove synthetic chemicals); chemical oxidants such as chlorine, ozone, chlorine dioxide, or potassium permanganate (to initiate disinfection, to oxidize organic contaminants, to control taste and odor, or to oxidize inorganic contaminants such as iron, manganese, and sulfide); and acid or base (to control pH). Coagulated particles are aggregated into large, rapidly settling "floc" particles by flocculation, accomplished by gently stirring the water using paddles, turbines, or impellers. This process typically takes 20 to 30 minutes. The flocculated water is then gently introduced into a sedimentation basin, where the floc particles are given about two to four hours to settle out. After sedimentation, the water is filtered, most commonly through 24-30 in (61-76 cm) of sand or anthracite having an effective diameter of about 0.02 in (0.5 mm).

When the raw water is low in turbidity, coagulated or flocculated water may be taken directly to the filters, bypassing sedimentation; this practice is referred to as direct filtration. Once the water has been filtered, it can be satisfactorily disinfected. Disinfection is the elimination of pathogenic microorganisms from the water. It does not render the water completely sterile but does make it safe to drink from a microbial standpoint. Most water treatment plants in the United States rely primarily on chlorine for disinfection. Some utilities use ozone, chlorine dioxide, chloramines (formed from chlorine and ammonia), or a combination of chemicals added at different points during treatment. There are important advantages and disadvantages associated with each of these chemicals, and the optimum choice for particular water requires careful study and expert advice.

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Chemical disinfectants react not only with microorganisms but also with naturally occurring organic matter present in the water, producing trace amounts of contaminants collectively referred to as disinfection byproducts (DBPs). The most well-known DBPs are trihalomethanes. Although DBPs are not known to be toxic at the concentrations found in drinking water, some are known to be toxic at much higher concentrations. Therefore, prudence dictates that reasonable efforts be made to minimize their presence in drinking water. The most effective strategy for minimizing DBP formation is to avoid adding chemical disinfectants until the water has been filtered and to add only the amount required to achieve adequate disinfection. Some DBPs can be minimized by changing to another disinfectant, but all chemical disinfectants form DBPs. Regardless of which chemical disinfectant is used, great care must be exercised to ensure adequate disinfection, since the health risks associated with pathogenic microorganisms greatly outweigh those associated with DBPs.

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