A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

WATER, OVERVIEW

DOI: 10.1615/AtoZ.w.water_overview

Water is used by municipalities and industries in vast amounts as an engineering material as well as one of life's necessities. It serves as a solvent, suspending and transporting medium, fire extinguisher, heat-transfer agent, source of power and as a chemical reagent. Water outranks all other raw materials. A typical industrial city uses about 1/4 to 1 tonne of water per person per day for domestic and industrial purposes. The U.S. uses 6 t/d and Europe 2 t/d for all purposes. In the heat of summer a person will drink about 5 liters per day in various forms, depending upon level of activity. About 70 tonnes of water are used to make 1 tonne of paper, 1200 tonnes to make 1 tonne of aluminum, 250 tonnes per tonne of steel and 230 tonnes to grow a tonne of tomatoes. Often water is not consumed in the process but its physical state and impurity level may be altered.

Domestic consumption of water has been increasing so rapidly in recent years that governments have become alarmed at the growing scarcity. Incentives are used to curb profligacy and penalties are levied for wastage. Fresh water is not limitless and research focuses on economic methods for obtaining and purifying water (see Water Treatment). The U.S. withdraws an estimated 1.54 × 109 tonnes (1993) of fresh water a day from lakes, streams and underground sources (see Figure 1).

This is nearly 10% less than in 1980 despite the population increase.

Trends in Freshwater Use in the U.S. 1960-1990. [Graves (1993)].

Figure 1. Trends in Freshwater Use in the U.S. 1960-1990. [Graves (1993)].

Main Uses

Water is the only substance which can exist in all three phases of matter at "normal" temperatures. Key properties are its: high heat capacity, latent heat of evaporation, polarizability and solvency; having good convective heat-transfer coefficients; and being relatively inert, stable, nontoxic, cheap and abundant (see Water Properties).

  1. Heat Transfer Medium for temperate heating and cooling—food processing, condensers, coolers, cooling towers, quenching.

  2. Steam. In electricity generation; as a heating medium, since it is an easy form of energy to transport around factories (approximately 2000 kJ/kg depending on pressure) and has good temperature control via pressure regulation; and for inert blanketing.

  3. Raw Material. As a solvent and chemical reagent (e.g. sulfuric acid manufacture, beverage industry).

  4. Washing. With its high polarizability, water is a good ionizing solvent.

  5. Miscellaneous. Hydraulic mining, sluicing, fire control, log debarking, nuclear shielding, irrigation, sanitary and other domestic uses.

Water Resources

The oceans hold 97% of the planet's water, a further 2% is frozen. The total amount of rainfall available worldwide is, however, more than adequate for our purposes.

The problem is one of 'supply' as rainfall is unevenly distributed. Table 1 lists rainfall and runoff for the continents. Australia is the world's driest continent with, not only the lowest rainfall and runoff in proportion to its area, but also the lowest ratio of runoff to rainfall.

Table 1. World rainfall and runoff [See Fenton and Gerofi (1985)]

*Runoff—Proportion of rain flowing to the sea; balance evaporates or fills underground aquifers.

Sources of Natural Water

Water is never chemically pure in nature. However, since fresh water is usually better than 99.9% pure it is usual to indicate purity by level of impurities (as mg/L, equivalent to parts per million, or ppm).

Rain

Rain is generally the purest form of natural water, especially if collected towards the end of a shower. Impurities consist of dissolved gases (O2, N2, CO2), salt (NaCl), oxides, and suspended material (dust, pollen, spores).

Surface water

Water from lakes contains larger amounts of dissolved and suspended material than rainwater. If the soil contains gypsum (CaSO4·2H2O) the natural water will contain more calcium; water from swampy areas will contain a variety of organic acids (humic and fulvic) and color (tannin) from decomposing humus, steams can contain suspended clays or sand, depending on flow rate. Acidic gases from industry (SO2, SO3, CO2, NO2, NO) lead to the formation of "acid rain" which can accelerate the weathering of rocks and soils and increase the quantity of dissolved salts in surface water. Curiously the amount of fine suspended dust from the predominately limestone soils of Western Australia and South Australia result in a somewhat alkaline rain across southern regions.

Springs and rivers

Springs and rivers contain higher quantities of suspended matter than lakes and are usually higher in dissolved material. However, some lakes are noted for their exceptionally high salinity where they are the terminus for streams or rivers with no outlet to the sea, e.g., Lake Eyre (Australia), Dead Sea (Asia).

Wells and bores

Wells and bores often have very high levels of dissolved material even to the extent of becoming saturated with soil salts. However, the clarity is good with little suspended matter and low microorganism counts. Wells in shallow Aquifers can have high biological activity due to surface contamination.

Sea water

The concentration of dissolved salts in sea water is very constant throughout the world's oceans at about 3.5%, or 35,000 ppm (see Table 2). Local variations occur where climatic conditions are severe. Thus in the Baltic Sea, the large runoff of fresh water from Scandinavia and Northern Europe reduces salinity, and in the Northern Spencer Gulf of South Australia solar evaporation results in a salinity of 45,000 ppm. The usefulness of a water source depends on the nature and concentration of contaminants, as well as its quantity and temperature. The treatment method depends on both source and end use. (See Desalination; Water preparation.)

Table 2. Composition of sea water, excluding dissolved gases [See Weast (1971)]

REFERENCES

Considine, D. M. Ed. (1974) Encyclopedia of Chemical and Process Technology, McGraw-Hill, NY.

Fenton, G. G. and Gerofi, J. P. (1985) Desalination in Australia: past and future, Solar Desalination Group, Uni. of Sydney.

Graves, W. Ed. (1993) Water, National Geographic Special Edition, NGS, Washington, Nov.

Weast, R. C. Ed. (1971) Handbook of Chemistry and Physics, 51 ed., CRC Press.

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