SUPPLY AND DESALINATION

Of the surface of the earth, 71% (3.60 x 10 km ) is covered by oceans their average depth is 6 km and their volume is 8.54 x 10 km . Unfortunately, this huge quantity of water is not suitable for very many human uses. Water with over 1000 ppm (parts per million by weight, or mg/L) salt is usually considered unfit for human consumption, and water with over 500 ppm is considered undesirable, but ia some parts of the world, people and land animals are forced to survive with much higher concentrations of salts, sometimes of over 2500 ppm. 

Freshwater with less than 500 ppm (or 0.05%) dissolved soHds is generally considered to be potable. Rain is the source of freshwater, and its precipitation of 1.3 x 10 m /d over the earth s surface averages about 1.05 m (depth) per year. Extremes range from almost 2ero ia North Chile s desert borderiag the Pacific Coast to 25.4 m ia some tropical forests and on some high slopes where the high, cold mountains condense floods from the clouds. 

Even rain is not pure water. Reports from the U.S. Geological Survey show that it contains 2.3—4.6 ppm of soflds, or a yearly precipitation of 2.5—5 t/km. Recently (ca 1997), work conducted ia the United States and Europe has underscored the rather dangerous results of iacreased use of fossil fuels, where the SO and NO emissions that end up ia the rain lower its pH from 5.6 (slightly acidic) for uncontaminated rain, to pH4 for acid rains. Such acid rain has serious effects on surface waters (1). About 40 x 10 t of SO and 25 x 10 t of NO were emitted ia the United States ia 1980. There are, however, encouragiag trends the 1970 Clean Air Act has led to a gradual reduction ia these emissions, bringing the SO emissions down from the previous levels cited by 10% by 1990, and the NO emissions down by 6%, with a consequent slight decrease ia rain acidity. A part of the Clean Air Act is also iatended to cap SO emissions from major poiat sources at 13.5 x 10 t (2). Between 1994 and 1995, total SO emissions ia the U.S. decreased remarkably by 13% and total NO emissions by 8%. 

The oceans hold about 97% of the earth s water. More than 2% of the total water and over 75% of the freshwater of the world is locked up as ice ia the polar caps. Of the remaining 1% of total water that is both Hquid and fresh, some is groundwater at depths of 300 m and therefore impractical to obtain, and only the very small difference, possibly 0.06% of the total water of this planet, is available for human use as it cycles from sea to atmosphere to land to sea. Only recently have humans been able to regulate that cycle to their advantage, and even now (ca 1997), only infinitesimally, ia some few isolated places. 

Kirk-Othmer Encyclopedia of Chemical Technology (4th Edition) 

In more modem times. New York City s water system, initiated ca 135 yr ago, stands comparison as a tme engineering marvel. Farsighted action in the late nineteenth century also gave the city extensive upstate watershed rights. This system, with a storage capacity of 2.07 x 10 , safely furnishes 

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Evaporation. In most chemical industry evaporation systems, the objective is product recovery, although occasionally the objective is concentration of an organic waste from an aqueous solution, to facihtate incineration. Similar equipment is used extensively for desalination of salt or brackish water (see also Water, supply and desalination). 

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