Sources of Drinking Water and Possible Contaminants
Water treatment plants may receive water from surface sources, such as lakes, streams, and rivers, or draw from underground reservoirs. As water flows, whether underground or above ground, it can pick up contaminants that would be unsafe for consumption. Fertilizer from agricultural fields, runoff from livestock, mine drainage, and wastewater from industrial activity are some of the many avenues for pollutants or harmful bacteria to enter the water system. Because there are so many undesirable metals, microorganisms and chemicals, water treatment plants must use sophisticated testing and purification processes to ensure that water is safe to distribute.
Filtration
One of the first steps in treating water for drinking is to filter out unwanted material. Coagulants are added to the water to make particulate matter clump into larger chunks which settle to the bottom. Water flowing through a treatment plant moves very slowly in order to allow these materials to settle and to keep circulation steady. In addition to the coagulants, water usually runs through several types of filters of varying sizes. Sand filters, which can reduce biological content by 90-95 percent, are often used. Activated charcoal is another highly efficient filter.
Purification and Testing
Once water is filtered, disinfectant or other chemicals may be added to meet drinking water standards. If the water is acidic, a plant may add lime or soda ash to raise the pH slightly. Chlorine or chlorine compounds are often added to kill any viruses or bacteria that made it through the filters. Ozone is also very effective, as is ultraviolet radiation. In many areas, treatment plants add fluoride to drinking water to prevent tooth decay. Because the EPA strictly regulates drinking water, all water is tested at many points during the treatment process and before it is released from the treatment plant.
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