Disease-producing organisms, such as bacteria, protozoa, and viruses, are sometimes found in water. A positive test for one particular kind of bacteria that is present in the fecal wastes of humans and many animals and birds—E. coli—indicates possible problems with others. Coliform bacteria, including E. coli, outnumber all other disease-producing organisms in water.
To avoid the growth of coliform bacteria, communities choose water sources without much plant or animal life, such as groundwater rather than surface water, and try to keep human activity away from watersheds (the areas that drain into the water supply) to protect against contamination. Fertilizers and nutrient minerals from farms and lawns can encourage bacterial growth. Water stored in the dark and at low temperatures is less likely to promote bacteria. When microorganisms do get into the water supply, they are destroyed at water treatment facilities.
Sometimes microorganisms do not pose a health danger, but multiply and clog pipes and filters. They can affect the water's appearance, odor, and taste. Surface water reservoirs may contain algae. Cooling towers can also have high bacterial counts.
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