Water, Supply and Desalination

Abstract

Of the surface of the earth, 71% ( 3.60 × 10 8 km 2 ) is covered by oceans; their average depth is 6 km and their volume is 8.54 × 10 8 km 3 . 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 in 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 solids is generally considered to be potable. Rain is the source of freshwater, and its precipitation of >1.3 × 10 12 m 3 /d over the earth's surface averages about 1.05 m (depth) per year. Extremes range from almost zero in North Chile's desert bordering the Pacific Coast to > 25.4 m in 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 solids, or a yearly precipitation of 2.5-5 t/km 2 . Recently (ca 1997), work conducted in the United States and Europe has underscored the rather dangerous results of increased use of fossil fuels, where the SO x and NO x emissions that end up in 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 × 10 6 t of SO x and 25 × 10 6 t of NO x were emitted in the United States in 1980. There are, however, encouraging trends: the 1970 Clean Air Act has led to a gradual reduction in these emissions, bringing the SO x emissions down from the previous levels cited by 10% by 1990, and the NO x emissions down by 6%, with a consequent slight decrease in rain acidity. A part of the Clean Air Act is also intended to cap SO x emissions from major point sources at 13.5 × 10 6 t (2). Between 1994 and 1995, total SO x emissions in the U.S. decreased remarkably by 13%; and total NO x emissions by 8%.About 60% of the land area of the earth is arid or semiarid and is not generally considered habitable. Mountainous areas and the polar areas covered by ice must be subtracted from the remainder to arrive at a land area that can be counted as available for human habitation and agriculture. Of this small fraction remaining, certain preferred land areas support the expanding world population more generously; hence, increasingly large amounts of the better lands are used for living, ie, for urban areas, industries, roads, airfields, etc. Areas for living and industry usually also require much more water per unit than do farm lands. Thus, as population and industry increase and the resulting demands for water multiply, agricultural areas with adequate water tend to become smaller.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 in the polar caps. Of t...