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Water is essential to practically all industrial operations, but it cannot always be used just as it is drawn from stream, lake, or well. The impurities in raw water can seriously affect both equipment and product if appropriate precautions are not taken. Cooling is by far the largest industrial use of water, and for this purpose its quality can generally be quite poor. Corrosion, erosion, and slime growths comprise the major problems where untreated water is used for cooling. Water quality becomes an important consideration where water comes in contact with products; sometimes only traces of certain substances will affect products adversely. Absolutely pure water is a laboratory curiosity. As a result of its almost universal solvent action, all natural water contains various foreign substances. The most common impurities in water are dissolved gases, such as carbon dioxide and oxygen, and soluble mineral matter, including such metal ions as calcium, magnesium, iron, and sodium in chemical balance with such anions as sulfate, bicarbonate, carbonate, hydroxyl, chloride, and others. These substances are dissolved as water flows over or percolates through the ground. Water usually carries variable amounts of organic material, depending upon its source. Typical examples are drainage from peat bogs and cypress swamps, runoff from farms, and decomposition products from aquatic plants and animals killed by changes in weather or environment. Most organic matter persists over relatively short distances in streams because of the purifying capacity of stream organisms that utilize organic matter as food. Some organic substances, for example, humates and tannates, are very resistant to biochemical destruction, and color from them may persist almost indefinitely. In a strict sense these various impurities might be called “natural pollution,” but the term pollution is generally reserved for contamination resulting from the activities of mankind.