Water Purification

a series of technological processes designed to raise to the established standards the quality of water entering pipes from a water source.

The earliest information on water purification is contained in the Sanskrit medical book Susrutasamhita, which was written in India about 4,000 years ago. It says:“It is good to keep water in copper vessels, leave it in the sunlight, and filter it through charcoal.” The ancient Greek physician and naturalist Hippocrates recommended the use of boiled water to avoid disease. The first water-treatment plant with so-called slow filters was built in 1829 in London. In Russia, the first water-treatment plant was built in 1888 in St. Petersburg; the first plant to disinfect water was built in 1910 in Nizhny Novgorod.

The water from surface sources (rivers and lakes) is usually not potable because of its turbidity, chromaticity, and high bacterial content. Therefore, before water is brought into household water pipes it is clarified (purged of suspended and colloidal particles), decolorized, and disinfected (freed of pathogenic microorganisms). Water is clarified and decolorized in the treatment plant by coagulating suspended and colloidal pollutants with aluminum sulfate or ferric chloride. Most of the coagulated pollutants are trapped in sedimentation tanks while the water is further clarified in sand or two-layer filters. Water containing a suspension of less than 150 mg per liter l can be clarified on contact clarifiers by adding a coagulant immediately before the water enters the layers of the filtering charge. Liquid chlorine, chlorinated lime, or ozone is introduced into the original or filtered water to disinfect it. Well-clarified water and water from underground aquifers can be disinfected by bactericidal ultraviolet light with a wavelength of 2,000-3,000 angstroms. Mercury-quartz or argon-mercury lamps are used as sources of ultraviolet radiation.

If water in a water supply source is harder (that is, its total content of calcium and magnesium salts is higher) than is permitted by the standards, it must be softened before it is admitted into the water supply system. The reagent method involves the precipitation of the salts with lime to remove the carbonate hardness and soda to remove the noncarbonate hardness. The total hardness of water can be reduced by this process to 0.5-0.7 milliequivalents (meq) per l. Water can be softened further by using the cation-exchanger method, which reduces water hardness to 0.03 meq per l. If water contains more than 0.3 mg per l of iron, this is removed. Underground water is usually freed of iron by aeration—air is enriched with oxygen to oxidize bivalent iron salts to trivalent salts, which are precipitated as ferric hydroxide. Surface water is freed of iron by coagulation. To remove dissolved salts, water is freshened or desalted by means of ion-exchange resins. It is generally degassed by aeration. (Hydrogen sulfide, methane, radon, carbon dioxide, and other dissolved gases are removed.) Excess fluorine (if more than 1.5 mg per l is present) is removed by filtering the water through activated aluminum oxide. If radioactive substances are present the water is deactivated. Deodorization—that is, removal of substances responsible for tastes and odors—is achieved by using charcoal, which absorbs them, or by oxidation with ozone, chlorine dioxide, or potassium permanganate. Water purification is the largest-capacity industry in the country’s economy. Drinking water purification plants alone in the USSR treated more than 10 billion cubic m of water in 1968.