Can Desalination Help SolvE The Water Crisis

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With the hydrologie cycle changing through climate change and increased water use, it is important to find new sources of water. Desalination includes a group of water-treatment processes that remove salt from water; it is becoming increasingly more important as freshwater supplies dwindle and population grows, yet desalination is exorbitantly expensive and cannot be afforded by many countries. A number of different processes can accomplish desalination of salty water, whether it comes from the oceans or the ground. These are divided broadly into thermal processes, membrane processes, and minor techniques such as freezing, membrane distillation, and solar humidification. All existing desalination technologies require energy input to work and end up separating a clear fraction or stream of water from a stream enriched in concentrated salt that must be disposed of, typically by returning it to the sea.

Thermal distillation processes produce about half of the desalted water in the world. In this process salt water is heated or boiled to produce vapor that is then condensed to collect freshwater. There are many varieties of this technique, including processes that reduce the pressure and boiling temperature of water to cause flash vaporization effectively, using less energy than simply boiling the water. The multistage flash-distillation process is the most widely used around the world. In this technique steam is condensed on banks of tubes that carry chemically treated seawater through a series of vessels known as brine heaters with progressively lower pressures, and this freshwater is gathered for use. Multieffect distillation has been used for industrial purposes for many years. Multieffect distillation uses a series of vessels with reduced ambient pressure for condensation and evaporation, and operates at lower temperatures than multistage flash distillation. Salt water is generally preheated then sprayed on hot evaporator tubes to promote rapid boiling and evaporation. The vapor and steam is then collected and condensed on cold surfaces, where the concentrated brines run off. Vapor compression condensation is often used in combination with other processes or by itself for small-scale operations. Water is boiled, and the steam is ejected and mechanically compressed to collect freshwater.

Membrane processes operate on the principle of membranes being able to separate salts selectively from water. Reverse osmosis, commonly used in the United States, is a pressure-driven process in which water is pressed through a membrane, leaving the salts behind. Electrodialysis uses electrical potential, driven by voltage, to move salts selectively through a membrane, leaving freshwater behind. Electrodi-alysis operates on the principle that most salts are ionic and carry an electrical charge, so they can be driven to migrate toward electrodes with the opposite charge. Membranes are built that allow passage of only certain types of ions, typically either positively (cation) or negatively (anion) charged. Direct-current sources with positive and negative charge are placed on either side of the vessel, with a series of alternate cation and anion selective membranes placed in the vessel. Salty water is pumped through the vessel, the salt ions migrate through the membranes to the pole with the opposite charge, and freshwater is gathered from the other end of the vessel. Reverse osmosis appeared technologically feasible only in the 1970s. The main energy required for this process is for applying the pressure to force the water through the membrane. The salty feed water is preprocessed to remove suspended solids and chemically treated to prevent microbial growth and precipitation. As the water is forced through the membrane, a portion of the salty feed water must be discharged from the process to prevent the precipitation of supersaturated salts. Presently membranes are made of hollow fibers or spiral wound. Improvements in energy recovery and membrane technology has decreased the cost of reverse osmosis, and this trend may continue, particularly with the use of new nanofiltration membranes that can soften water in the filtration process by selectively removing calcium (Ca2+) and magnesium (Mg2+) ions.

several other processes have been less successful in desalination. These include freezing, which naturally excludes salts from the ice crystals. Membrane distillation uses a combination of membrane and distillation processes, which can operate at low temperature differentials but require large fluxes of salt water. Solar humidification was used in World War II for desalination stills in life rafts, but these are not particularly efficient because they require large solar collection areas, have a high capital cost, and are vulnerable to weather-related damage.

See also atmosphere; clouds; flood; glacier, glacial systems; ocean basin; river system.


Botkin, D., and E. Keller. Environmental Science. Hobo-

ken, N.J.: John Wiley & Sons, 2003. Buros, O. K. The ABCs of Desalting. Topsfield, Mass.:

International Desalination Association, 2000. Gordon, N. D., T. A. McMahon, and B. L. Finlayson. Stream Hydrology: An Introduction for Ecologists. New York: John Wiley & Sons. 1992. Intergovernmental Panel on Climate Change home page. Available online. URL: Accessed January 30, 2008. Intergovernmental Panel on Climate Change 2007. Climate Change 2007: The Physical Science Basis. Contributions of Working Group I to the Fourth Assessment

Report of the Intergovernmental Panel on Climate Change, edited by S. Solomon, D. Qin, M. Manning, Z. Chen, M. Marquis, K. B. Averyt, M. Tignor, and H. L. Miller. Cambridge: Cambridge University Press, 2007. Also available online. URL: http://www.ipcc. ch/index.htm. Accessed October 10, 2008.

Leopold, L. B. A View of the River. Cambridge, Mass.: Harvard University Press, 1994.

Ritter, D. F., R. C. Kochel, and J. R. Miller. Process Geomorphology. 3rd ed. Boston: WCB-McGraw Hill, 1995.

Schümm, S. A. The Fluvial System. New York: Wiley-Inter-science, 1977.

U.S. Geological Survey. Water Resources of the United States home page. Available online. URL: http://water. Accessed October 8, 2008. Updated daily.

ice ages Times when the global climate was colder and large masses of ice covered many continents are referred to as ice ages. At several times in Earth's history large portions of the Earth's surface have been covered with huge ice sheets. About 10,000 years ago all of Canada, much of the northern United States, and most of Europe were covered with ice sheets, as was about 30 percent of the world's remaining landmass. These ice sheets lowered sea level by about 320 feet (100 m), exposing the continental shelves and leaving present-day cities including New York, Washington, D.C., and Boston 100 miles (160 km) from the sea. In the last 2.5 billion years several periods of ice ages have been identified, separated by periods of mild climate similar to that of today. Ice ages seem to form through a combination of several different factors, including the following:

• the amount of incoming solar radiation, which changes in response to several astronomical effects

• the amount of heat retained by the atmosphere and ocean, or the balance between the incoming and outgoing heat

• the distribution of landmasses on the planet. Shifting continents influence the patterns of ocean circulation and heat distribution, and a large continent on one of the poles causes ice to build up on that continent, increasing the amount of heat reflected to space, lowering global temperatures in a positive feedback mechanism.

Glaciations have happened frequently in the past 55 million years and could occur again at almost any time. In the late 1700s and early 1800s Europe expe rienced a "little ice age" during which many glaciers advanced from the Alps and destroyed small villages in their path. Ice ages have occurred at several other times in the ancient geologic past, including in the Late Paleozoic (about 350-250 million years ago), Silurian (435 million years ago), and Late Protero-zoic (about 800-600 million years ago). During parts of the Late Proterozoic glaciation, it is possible that the entire Earth surface temperature was below freezing and the planet was covered by ice.

In the Late Proterozoic the Earth experienced one of the most profound ice ages in the history of the planet. Isotopic records and geologic evidence suggests that the entire Earth's surface was frozen, though some scientists dispute the evidence and claim that there would be no way for the Earth to recover from such a frozen state. In any case it is clear that in the Late Proterozoic, during the formation of the supercontinent Gondwana, the Earth experienced one of the most intense glaciations ever, with the lowest average global temperatures in known Earth history.

One of the longest-lasting glacial periods was the Late Paleozoic ice age, which lasted about 100 million years, indicating a long-term underlying cause of global cooling. of the variables that operate on these longtime scales, the distribution and orientation of continents seems to have caused the Late Paleozoic glaciation. The Late Paleozoic saw the amalgamation of the planet's landmasses into the supercontinent of Pangaea. The southern part of Pangaea, known as Gondwana, consisted of present-day Africa, South America, Antarctica, India, and Australia. During the drift of the continents in the Late Paleozoic, Gondwana slowly moved across the South Pole, and huge ice caps formed on these southern continents during their passage over the pole. The global climate was much colder overall, with the subtropical belts becoming very condensed and the polar and subpolar belts expanding to low latitudes.

During all major glaciations a continent was situated over one of the poles. Currently Antarctica is over the South Pole, and this continent has huge ice sheets. When continents rest over a polar region, they accumulate huge amounts of snow that gets converted into several-mile-thick ice sheets that reflect more solar radiation back to space and lower global seawater temperatures and sea levels.

Another arrangement that helps initiate glaciations is continents distributed in a roughly N-S orientation across equatorial regions. Equatorial waters receive more solar heating than polar waters. Continents block and modify the simple east to west circulation of the oceans induced by the spinning of the planet. When continents are present on or near the equator, they divert warm water currents to high latitudes, bringing warm water to higher latitudes. Since warm water evaporates much more effectively than cold water, having warm water move to high latitudes promotes evaporation, cloud formation, and precipitation. In cold, high-latitude regions the precipitation falls as snow, which persists and builds up glacial ice.

The Late Paleozoic glaciation ended when the supercontinent of Pangaea began breaking apart, suggesting a further link between tectonics and climate. The smaller landmasses might not have been able to divert the warm water to the poles anymore, or perhaps enhanced volcanism associated with the breakup caused additional greenhouse gases to build up in the atmosphere, raising global temperatures.

The planet began to enter a new glacial period about 55 million years ago, following a 10-million-year-long period of globally elevated temperatures and expansion of the warm subtropical belts into the subarctic. This Late Paleocene global hothouse saw the oceans and atmosphere holding more heat than at any other time in Earth history, but temperatures at the equator were not particularly elevated. Instead the heat was distributed more evenly around the planet, leading to fewer violent storms (with a small temperature gradient between low and high latitudes) and more moisture overall in the atmosphere. Several factors contributed to the abnormally warm temperatures on the planet during this time, including a distribution of continents that saw the equatorial region free of continents. This allowed the oceans to heat up more efficiently, raising global temperatures. The oceans warmed so much that the deep ocean circulation changed, and the normally cold deep currents became warm. These melted frozen gases (known as methane gas hydrates) that had accumulated on the seafloor, releasing huge amounts of methane into the atmosphere. Methane is a greenhouse gas, and its increased abundance trapped solar radiation in the atmosphere, contributing to global warming. In addition volcanic eruptions released vast outpourings of mafic lavas in the North Atlantic Ocean realm, and the accompanying liberation of large amounts of CO2 would have increased the greenhouse gases in the atmosphere and further warmed the planet. Global warming during the Late Paleocene was so extreme that about 50 percent of all single-celled organisms living in the deep ocean became extinct.

After the Late Paleocene hothouse, the Earth began a long-term cooling that continues today despite the present warming of the past century. This current ice age was marked by the growth of Antarctic glaciers, starting about 36 million years ago, until about 14 million years ago, when the Antarctic ice sheet covered most of the continent with several miles of ice. At this time global temperatures had cooled so much that many of the mountains in the Northern Hemisphere were covered with mountain and piedmont glaciers, similar to those in southern Alaska today. The ice age continued to intensify until 3 million years ago, when extensive ice sheets covered the Northern Hemisphere. North America was covered with an ice sheet that extended from northern Canada to the Rocky Mountains, across the Dakotas, Wisconsin, Pennsylvania, and New York, and on the continental shelf. At the peak of the glaciation (18,000-20,000 years ago) about 27 percent of the lands' surface was covered with ice. Midlatitude storm systems were displaced to the south, and desert basins of the southwest United States, Africa, and the Mediterranean received abundant rainfall and hosted many lakes. sea level was lowered by 425 feet (130 m) to make the ice that covered the continents, so most of the world's continental shelves were exposed and eroded.

The causes of the Late Cenozoic glaciation are not well known but seem related to Antarctica coming to rest over the south Pole and other plate tectonic motions that have continued to separate the once contiguous landmasses of Gondwana, changing global circulation patterns in the process. Two of the important events seem to be the closing of the Mediterranean ocean around 23 million years ago and the formation of the Panama isthmus 3 million years ago. These tectonic movements restricted the east-to-west flow of equatorial waters, causing the warm water to move to higher latitudes where evaporation promotes snowfall. An additional effect related to uplift of some high mountain ranges, including the Tibetan Plateau, has changed the pattern of the air circulation associated with the Indian monsoon.

The closure of the Panama isthmus correlates closely with the advance of Northern Hemisphere ice sheets, suggesting a causal link. This thin strip of land has drastically altered the global ocean circulation such that Pacific and Atlantic Ocean waters no longer communicate effectively, and it diverts warm currents to near-polar latitudes in the North Atlantic, enhancing snowfall and Northern Hemisphere glaciation. Since 3 million years ago the ice sheets in the Northern Hemisphere have alternately advanced and retreated, apparently in response to variations in the Earth's orbit around the Sun and other astronomical effects. These variations change the amount of incoming solar radiation on timescales of thousands to hundreds of thousands of years (Milankovitch Cycles). Together with the other longer-term effects of shifting continents, changing global circulation patterns, and abundance of greenhouse gases in the atmosphere, most variations in global climate can be approximately explained. This knowledge may help predict where the climate is heading and may help model and mitigate the effects of human-induced changes in the atmospheric greenhouse gases. If Earth is heading into another warm phase and the existing ice on the planet melts, sea level will quickly rise by 210 feet (64 m), inundating many of the world's cities and farmlands. Alternately, if the Earth enters a new ice sheet stage, sea levels will be lowered, and the planet's climate zones will be displaced to more equatorial regions.

See also atmosphere; glacier, glacial systems; greenhouse effect.


Erickson, J. Glacial Geology: How Ice Shapes the Land.

New York: Facts On File, 1996. Kusky, T. M. Climate Change: Shifting Deserts, Glaciers, and Climate Belts. New York: Facts On File, 2008.

igneous rock A rock that has crystallized from a melt or partially molten material (known as magma) is classified as igneous. Magma is a molten rock within the Earth; if it makes its way to the surface, it is referred to as lava. Different types of magma form in different tectonic settings, and many processes act on the magma as it crystallizes to produce a wide variety of igneous rocks.

Most magma solidifies below the surface, forming igneous rocks (igneous is Latin for fire). Igneous rocks that form below the surface are called intrusive (or plutonic) rocks, whereas those that crystallize on the surface are called extrusive (or volcanic) rocks. Rocks that crystallize at a very shallow depth are known as hypabyssal rocks. Intrusive igneous rocks crystallize slowly, giving crystals an extended time to grow, thus forming rocks with large mineral grains that are clearly distinguishable to the naked eye. These rocks are called phanerites. In contrast, magma that cools rapidly forms fine-grained rocks. Aphanites are igneous rocks in which the component grains cannot be distinguished readily without a microscope and are formed when magma from a volcano falls or flows across the surface and cools quickly. Some igneous rocks, known as porphyries, have two populations of grain size—a large group of crystals (phenocrysts) mixed with a uniform groundmass (matrix) that fills the space between the large crystals. This indicates two stages of cooling, as when magma has resided for a long time beneath a volcano, growing big crystals. When the volcano erupts, it spews out a mixture of large crystals and liquid magma that then cools quickly.

Once magmas are formed from melting rocks in the Earth, they intrude the crust and can take several forms. A pluton is a general name for a large, cooled, igneous, intrusive body in the Earth. The specific type of pluton is based on its geometry, size, and relations to the older rocks surrounding it, known as country rock. Concordant plutons have boundaries parallel to layering in the country rock, whereas discordant plutons have boundaries that cut across layering in the country rock. Dikes are generally thin with parallel sides exhibiting tabular shapes and cut across preexisting layers, and are therefore said to be discordant intrusions. In contrast, sills are tabular intrusions oriented parallel to layers and said to be concordant intrusives. Volcanic necks are conduits connecting a volcano with its underlying magma chamber (a famous example of a volcanic neck is Devils Tower, Wyoming). Some plutons are so large that they have special names. Batholiths, for example, have a surface area of more than 60 square miles (100 km2).

The mechanisms by which large bodies of magma intrude into the crust are debated by geologists and may be different for different plutons. One mechanism, assimilation, involves the hot magma melting the surrounding rocks as it rises, causing them to become part of the magma. As the magma cools, its composition changes to reflect the added melted country rock. Magmas can rise only a limited distance by assimilation because they quickly cool before they can melt their way significant distances through the crust. If the magma is under high pressure, it may forcefully push into the crust. One variation of this forceful emplacement style is diapirism, where the weight of surrounding rocks pushes down on the melt layer, which then squeezes up through cracks that can expand and extend, forming volcanic vents at the surface. Yet another mechanism is stopping. During pluton emplacement by stopping large

Crota Terrestre

blocks of the surrounding country rock get thermally shattered and drop off the top of the magma chamber and fall into the chamber, eventually melting and becoming part of the magma.

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