Environmental Engineering Reference
In-Depth Information
due to contaminated water—a child under the age of 5 dies every 20 s. The most com-
mon waterborne diseases, affecting millions of people worldwide, are malaria, cholera,
and diarrhea. In addition, it has been estimated that >20 million people are affected by
arsenic poisoning in the Bengal region of the Indian subcontinent (including Bangladesh).
Given these brief statistics, it is not surprising that there is a renewed focus on developing
new technologies for water puriication [1-5], including desalination and demineraliza-
tion of various source waters to enhance freshwater supplies. However, in many regions,
it is not just a matter of implementing an effective water treatment system to generate
high-quality potable water; the water supply itself can also be an issue. To this end, sup-
plementing the water supply by desalination of salt water has been identiied as a key
step toward making progress for developing sustainable sources for freshwater [6-8]. To
create effective desalination and subsequent distribution strategies, it is useful to briely
review how the water is distributed around the planet. The total available water on Earth
is approximately 1.4 × 10 21 liters (or about 332,500,000 mi 3 ). Of this seemingly enormous
water supply, >99% of the water is currently inaccessible to human use, and >97% of the
earth's water exists in oceans, bays, seas, and saline aquifers as large reservoirs of salt
water. In fact, several estimates place approximate supply of usable freshwater at ~0.7%
or approximately 9.8 × 10 18 liters. While this may appear to be a large number, accounting
for population increase, demands on freshwater for agriculture, industry, power plant
cooling; a changing climate; declining freshwater quality from worldwide contamination
via industrial, municipal, and agricultural discharge; and increasing energy needs lead
to a rather bleak picture for future availability of clean water for human use. In a recent
editorial review [6], which reports data from the World Meteorological Organization, the
combination of uneven population and water distribution is causing rapidly increasing
water withdrawal rates as a fraction of total available water (Figure 27.1) and projects that
by 2025 (only 11 years from now), most of the world's population will be facing serious
water stresses and shortages. Although infrastructure repair, improvement, and water
conservation can help relieve water stress, increasing the available freshwater supply by
desalination is also an attractive option. Therefore, the ability to affordably and sustain-
ably desalinate water can resolve many of the impending and projected water crises. In
this chapter, a discussion of existing and emerging water desalination technologies is
1995
2025
Water withdrawal as percentage of
total available
Over 40%
40%-20%
20%-10%
Less than 10%
FIGURE 27.1
(See color insert.) Water withdrawal rate as a percentage of total available water, projected till 2025. (From
Service, R.F., Science , 313, 1088, 2006.)
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