The temperature of treated water is generally the same as that of raw water. Highly corrosive raw waters may be treated to reduce this property in conjunction with other types of treatment required. Some waters may also contain dissolved minerals, which deposit on the inside of pipelines, resulting in scale formation. The corrosiveness of surface waters and groundwaters varies widely, depending on their pH, hardness, and other characteristics. Treatment of raw water might include coagulation, sedimentation, filtration, softening, and iron removal in addition to disinfection. Surface waters normally require more extensive treatment than do groundwaters. On the other hand, it may be necessary to remove certain objectionable constituents in the water and/or to reduce others to tolerable limits, depending upon the type of contamination, applicable criteria or standards, and/or the desire of the users. To be made acceptable for a public water supply, groundwater may require only disinfection to ensure adequate health protection. The temperatures of groundwaters are also more constant, normally approaching the average annual temperature of the region instead of the constant fluctuations reflected in the temperatures of surface waters. Over time, the quality of groundwaters is usually more constant than that of surface waters. In general, the mineral content of groundwaters reflects the mineral characteristics of the soil in the area. Conversely, the mineral content, including calcium and magnesium ions-the main contributors to "water hardness"-may be higher in groundwater than in the nearby surface waters. The temperature of surface waters fluctuates with climatic variations.Īlthough groundwaters are also subject to contamination as a result of human activities, they are often clear, colorless, and possess lower concentrations of organic matter and microorganisms than does surface water because of the natural filtration effected by the percolation of water through soil, sand, or gravel. Surface waters are subject to contamination by sewage from cities, industrial wastes, agricultural runoff, and waste from animals and birds. They may also have an undesirable color, taste, and odor. Characteristically, such waters contain microorganisms as well as inorganic and organic particulate matter and dissolved solids.
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