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Desalination

Desalination

Approximately 97% of Earth's water is either sea water or brackish (salt water contained in inland bodies), both of which are undrinkable by humans. Desalination is the process of removing salt from seawater. Natural desalination occurs as a part of the hydrologic cycle as seawater evaporates. Manipulated desalinizationdesalting, or saline water reclamationis an energy expensive alternative to natural desalination.

Sea water contains 35,000 parts per million (ppm) (3.5% by weight) of dissolved solids, mostly sodium chloride, calcium and magnesium salts. Brackish water typically contains 5,000-10,000 ppm dissolved solids. To be consumable, or potable, water must contain less than 500 ppm dissolved solids. The method used to reach this level depends on the local water supply, the water needs of the community, and economics. Growing populations in arid or desert lands, contaminated groundwater , and sailors at sea all created the need for desalting techniques.

In the fourth century b.c., Aristotle related tales of Greek sailors desalting water using evaporation techniques. Sand filters were also used. Another technique used a wool wick to siphon the water. The salts were trapped in the wool. During the first century a.d., the Romans employed clay filters to trap salt. Distillation was widely used from the fourth century onsalt water was boiled and the steam collected in sponges. The first scientific paper on desalting was published by Arab chemists in the eighth century. By the 1500s, methods included filtering water through sand, distillation, and the use of white wax bowls to absorb the salt. The techniques have become more sophisticated, but distillation and filtering are still the primary methods of desalination for most of the world. The first desalination patent was granted in 1869, and in that same year, the first land-based steam distillation plant was established in Britain, to replenish the fresh water supplies of the ships at anchor in the harbor. A constant problem in such a process is scaling. When the water is heated over 160°F (71°C), the dissolved solids in water will precipitate as a crusty residue known as scale. The scale interferes with the transfer of heat in desalting machinery, greatly reducing the effectiveness. Today, the majority of desalting plants use a procedure known as multistage flash distillation to avoid scale. Lowering the pressure on the sea water allows it to boil at temperatures below 160°F (71°C), avoiding scaling. Some of the water evaporates, or flashes, during this low pressure boiling. The remaining water is now at a lower temperature , having lost some energy during the flashing. It is passed to the next stage at a lower temperature and pressure, where it flashes again. The condensate of the previous stage is piped through the water at the following stage to heat the water. The process is repeated many times. The water vapor is filtered to remove any remaining brine, then condensed and stored. Over 80% of land-based desalting plants are multistage flash distillation facilities.

A host of other desalinization processes have been developed. An increasingly popular process, reverse osmosis, essentially filters water at the molecular level, by forcing it through a membrane. The pressures required for brackish water range from 250 to 400 pounds per square inch (psi), while those for seawater are between 800 to 1,200 psi. The pressure required depends on the type of membrane used. Membranes have been steadily improving with the introduction of polymers. Membranes were formerly made of cellulose acetate, but today they are made from polyamide plastics. The polyamide membranes are more durable than those of cellulose acetate and require about half the pressure. Solar distillation is used in the subtropical regions of the world. Seawater is placed in a black tray and covered by a sloping sheet of glass or plastic. Sunlight passes through the cover. Water evaporates and then condenses on the cover. It runs down the cover and is collected. The salts are left behind in the trays.

Modern desalination technology allows use of desalinated water to supplement regular drinking water. The state of Florida, for example, is using dozens of reverse-osmotic plants to treat undrinkable brackish water and then mixing the treated water with the regular water supply. The intent is to extend the local water supply. Another approach is to make traditional methods, like distillation, more economically feasible.

See also Hydrologic cycle; Saltwater encroachment

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water, desalination of

desalination of water, process of removing soluble salts from water to render it suitable for drinking, irrigation, or industrial uses. The principal methods used for desalination include distillation (or evaporation), electrodialysis, freezing, ion exchange, and reverse osmosis.

In distillation saltwater is heated in one container to make the water evaporate, leaving the salt behind. The desalinated vapor is then condensed to form water in a separate container. Although long known, distillation has found limited application in water supply because of the fuel costs involved in converting saltwater to vapor. Representative of the early attempts in this direction were the solar distillation methods employed (c.49 BC) by the legions of Julius Caesar for using water from the Mediterranean. Modern technological advances led to the development of more efficient distillation units using solar energy; however, since these units have small capacities, their utility is restricted.

Distillation plants having high capacities and using combustible fuels employ various devices to conserve heat. In the most common system a vacuum is applied to reduce the boiling point of the water, or a spray or thin film of water is exposed to high heat, causing flash evaporation; the water is flashed repeatedly, yielding fresh distilled water. This multistage flash distillation method is used in more than 2,000 desalination plants, including one in Saudi Arabia that produces 250 million gallons of freshwater per day.

Another method of desalination is by electrodialysis. When salt dissolves in water, it splits up into charged particles called ions. Placed in a container with a negative electrode at one end and a positive electrode at the other, the ions are filtered by the membranes as they are attracted toward the electrodes; they become trapped between semipermeable membranes, leaving outside the membranes a supply of desalinated water that can be tapped. The first large installation using this process began operating in South Africa in 1958, but its electrical demands make it impractical except where such energy is abundant.

By far the most promising approach is the reverse osmosis process, in which pressure is applied to saltwater to force it through a special membrane. Only pure water passes, leaving concentrated seawater behind. Where multistage flash distillation costs about $4 per 1,000 gallons, reverse osmosis costs about half that amount. This process is used by a plant in the Tampa Bay area, Florida, that produces 25 million gallons of drinking water a day. Another type uses an empty hollow sphere of semipermeable material that is lowered into the sea. The water flowing into the sphere is fresh, since the salt is excluded by the membrane that covers the entire sphere and is its guard.

One final approach is under development in Hawaii, where different layers of seawater display a large temperature differential. Here an Ocean Thermal Energy Conversion plant is being built which will use steam produced by the flash method to produce energy, then condense the steam into freshwater. Three such plants could produce a hundred megawatts of power, as well as supply 30% of Hawaii's water needs.

For emergency use, i.e., in lifeboats, various systems are available in addition to solar or fuel-heated distillation devices. One device made of flexible plastic is worn around the waist of the user to employ body heat for evaporation.

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desalination

desalination Extraction of pure water (for drinking, industrial and chemical uses, or for irrigation) from water containing dissolved salts, usually sea water. The commonest and oldest method is distillation. Another method is to freeze the salt solution; salt is excluded from the ice crystals which can then be melted. In reverse osmosis, pure water only passes through a semipermeable membrane against which salt water is pressurized. Other methods include electrodialysis.

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desalination

desalination A series of processes whereby salt water is rendered potable.

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desalination

desalination A series of processes whereby salt water is rendered potable.

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desalination of water

desalination of water: see water, desalination of.

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Desalination

Desalination

Desalination, also called desalting, is the removal of salt from seawater. It provides essential water for drinking and industry in desert regions or wherever the local water supply is brackish. Desalination plants are active in over one hundred countries around the world. Saudi Arabia produces about one-fourth of the worlds capacity of desalinated water. Israel possesses the largest desalination plant at its reverse osmosis plant in Ashkelon. Opened in 2005, it produces 130 cubic yards (100 million cubic meters) of water each year. Most of this water was produced through distillation. However, other methods, including reverse osmosis and electrodialysis, are becoming increasingly important.

Desalination has been used for many centuries. In the fourth century BC, Aristotle (384322 BC) told of Greek sailors desalting water using evaporation techniques. Sand filters were also used. Another technique used a wool wick to siphon the water. The salts were trapped in the wool. During the first century AD, the Romans employed clay filters to trap salt. Distillation was widely used from the fourth century on salt water was boiled and the steam collected in sponges. The first scientific paper on desalting was published by Arab chemists in the eighth century. By the 1500s, methods included filtering water through sand, distillation, and the use of white wax bowls to absorb the salt. The techniques have become more sophisticated, but distillation and filtering are still the primary methods of desalination for most of the world. The first desalination patent was granted in 1869, and in that same year, the first land-based steam distillation plant was established in England, to replenish the fresh water supplies of the ships at anchor in the harbor.

At its simplest, distillation consists of boiling the seawater to separate it from dissolved salt. The water vapor rises to a cooler region where it condenses as pure liquid water. Heat for distillation usually comes from burning fossil fuels. To reduce costs and pollution, desalination plants are designed to use as little fuel as possible. Many employ flash distillation, in which heated seawater is pumped into a low pressure chamber. The low pressure causes the water to vaporize, or flash, even though it is below its boiling temperature. Therefore, less heat is required. Multistage flashing passes the seawater through a series of chambers at successively lower pressures. For even greater efficiency, desalination plants can be linked with electrical power plants. Heat from the hot gasses that turn the generators is recycled to warm the incoming seawater. Distillation is widely used in the Middle East, where fossil fuel is plentiful but fresh water is scarce.

Reverse osmosis uses high pressure to force pure water out of saltwater. Normal osmosis occurs when pure water and saltwater are separated by a semi-permeable membrane, which permits only water to flow through. Under these conditions, the pure water will move into the saltwater side, but if the saltwater is squeezed under high enough pressure, fresh water moves out of it. Pressures on the order of 60 atmospheres (800 to 1, 200 psi [pounds per square inch]) are required to push pure water out of seawater. Reverse osmosis is widely used to desalinate brackish water, which is less salty than seawater and therefore requires pressures only about half as great.

Like reverse osmosis, electrodialysis is presently best suited for desalinating brackish water. Salts consist of ions, which are atoms that have acquired electrical charge by losing or gaining electrons. Because of their charge, ions are attracted to oppositely charged electrodes immersed in the saltwater. They move toward the electrodes, leaving a region of pure water behind. Special membranes prevent the ions from drifting back into the purified water as it is pumped out.

The desalination of seawater and brackish water is still being researched throughout the world. In the United States, desalinization research is being performed by such federal organizations as the Bureau of Reclamation within the Department of the Interior. In 2005, the Long Beach Seawater Desalination Research and Development Facility opened in California. The facility, which produces about 300, 000 gallons (1.1 million liters) of desalinated water each day, will provide the latest information and data on cost-effective and environmentally sound techniques for the desalination of seawater.

Ongoing research seeks to improve existing desalination methods and develop new ones. The costs of distillation could be greatly reduced if clean, renewable energy were used to heat the water. Solar, geothermal, and oceanic temperature differences are among the energy sources being studied. Reverse osmosis could be used on a larger scale, and with saltier water, through development of semi-permeable membranes able to withstand higher pressures for longer times. All desalination methods leave extremely salty residues. New methods for disposing of these must be developed as the worlds use of desalination grows.

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Desalination

Desalination

Desalination, also called desalting, is the removal of salt from seawater. It provides essential water for drinking and industry in desert regions or wherever the local water supply is brackish . In 1991, about 3.5 billion gallons of desalinated water were produced in about 4,000 desalination plants worldwide. Most of this water was produced through distillation . However, other methods, including reverse osmosis and electrodialysis, are becoming increasingly important.

At its simplest, distillation consists of boiling the seawater to separate it from dissolved salt. The water vapor rises to a cooler region where it condenses as pure liquid water. Heat for distillation usually comes from burning fossil fuels . To reduce costs and pollution , desalination plants are designed to use as little fuel as possible. Many employ flash distillation, in which heated seawater is pumped into a low pressure chamber. The low pressure causes the water to vaporize, or "flash," even though it is below its boiling temperature . Therefore, less heat is required. Multi-stage flashing passes the seawater through a series of chambers at successively lower pressures. For even greater efficiency, desalination plants can be linked with electrical power plants. Heat from the hot gasses that turn the generators is recycled to warm the incoming seawater. Distillation is widely used in the Middle East, where fossil fuel is plentiful but fresh water is scarce.

Reverse osmosis uses high pressure to force pure water out of saltwater . Normal osmosis occurs when pure water and saltwater are separated by a semi-permeable membrane , which permits only water to flow through. Under these conditions, the pure water will move into the saltwater side, but if the saltwater is squeezed under high enough pressure, freshwater moves out of it. Pressures on the order of 60 atmospheres (800-1,200 psi) are required to push pure water out of seawater. Reverse osmosis is widely used to desalinate brackish water, which is less salty than sea-water and therefore requires pressures only about half as great.

Like reverse osmosis, electrodialysis is presently best suited for desalinating brackish water. Salts consist of ions, which are atoms that have acquired electrical charge by losing or gaining electrons. Because of their charge, ions are attracted to oppositely charged electrodes immersed in the saltwater. They move toward the electrodes, leaving a region of pure water behind. Special membranes prevent the ions from drifting back into the purified water as it is pumped out.

Ongoing research seeks to improve existing desalination methods and develop new ones. The costs of distillation could be greatly reduced if clean, renewable energy were used to heat the water. Solar, geothermal, and oceanic temperature differences are among the energy sources being studied. Reverse osmosis could be used on a larger scale, and with saltier water, through development of semi-permeable membranes able to withstand higher pressures for longer times. All desalination methods leave extremely salty residues. New methods for disposing of these must be developed as the world's use of desalination grows.

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Desalination

Desalination

Approximately 97% of Earth's water is either sea water or brackish water (a mixture of salt and fresh water). Humans and other animals cannot drink salt water and to do so can bring on dehydration (the loss of the body's existing water) that can lead to illness and in extreme cases, death.

Desalination is the process of removing salt from seawater to make it drinkable (drinkable water is also called potable water) or to make it useable for irrigation (watering fields and crops).

Natural desalination occurs everyday as a part of the world's hydrologic cycle. As salt water from the oceans evaporates (changes from liquid to gas), the salt is left behind and the water that moves into the atmosphere is fresh water. Thus, the water in clouds that eventually falls as rain is fresh water.

Salt can also be removed from water by a series of processes known as manipulated desalinization, desalting, or saline water reclamation (salt water reclamation). All of these manmade processes are expensive in terms of how much money and energy they each require to produce a gallon of water.

Salt is composed (made up) of sodium and chorine atoms (the smallest particles of each element). Seawater contains the same kind of salt (sodium chloride) used everyday on food and in cooking. In addition, seawater also contains many small particles of the chemicals such as calcium and magnesium that also form chemicals called salts. Some of these salts come from chemicals used by industry, others from natural processes. Between three and four pounds out of every 100 pounds of atoms in saltwater (the hydrogen and oxygen atoms that together form water plus the atoms of all chemicals dissolved in the water) are combined into salts. Public health officials who test water use a different scale and label the salt in water as parts (particles) per million (ppm). Using this scale, seawater contains 35,000 ppm of dissolved salts. Brackish water typically contains less than half the amount of salt that is found in seawater, about 5,000–10,000 ppm of salt. Safe drinking water for humans, and water for most types of crops, must contain only 5,000–10,000 ppm of salt.

Methods to remove salt

There are several ways to remove salt from seawater and the method used is determined by the intended use of the water. Salt can also be removed from groundwater contaminated with saltwater. For example, if the water is to be drinkable then more salt needs to be removed than if the water is to be used for crops. Cost is also an important consideration because the more salt that needs to be removed, the greater the cost.

Stories from ancient Greece tell of how sailors obtained fresh water by first removing salt from seawater by evaporating the seawater, and then condensing (changing from a gas to a liquid) the air carrying the evaporated water. This process, because it uses the heat of the Sun is now called solar distillation. Solar distillation is similar to the natural process of the heat of the Sun evaporating water from the oceans that later condense into fresh water drops in clouds. When the water evaporates, only fresh water moves into the surrounding air because the salts are too heavy and are left behind in the ocean. Only fresh water went into the surrounding air (for example, the air over a bucket of seawater). As the air came into contact with cooler sheets or sails spread over the bucket, drops of fresh water would form and could then be collected in a separate bucket.

Other, but far less efficient ways to obtain fresh water included the use of filtering seawater. One method of filtering included the use of a wool wick (a length of rope made of wool) to absorb (siphon) the water. The salts were trapped in the wool and fresh water dripped out. Water was also poured through sand or clay to remove salts.

By the fourth century (400 a.d.) onward, people obtained fresh water by boiling salt water and using sponges to absorb the fresh water in the air above the pot. The first scientific paper on desalting was published by Arab scientists in the eighth century.

The first desalination efforts for industry started in 1869, as land-based steam distillation plants were established in Britain to prepare fresh water for ships going to sea.

Methods of distillation and filtering are still the most widely used methods of desalination used in most areas of the world.

Other modern techniques use complex machines that change the temperature at which water boils away by lowering the pressure of the atmosphere over a sealed container of water. This methods reduces the formation of crusty white salts, which appear similar to the sticky white powder found at the bottom of a pan from which all water has boiled away. These crusty white salts can clog machinery and make it more difficult to heat water. In industry, the crusty residue is called scaling, and the method of lowering the temperature at which water boils is called multistage distillation (multiple stages of distillation). The goal of multistage distillation is to reduce the boiling point of water to a temperature where it will still boil (evaporate) into a collection flask, but that it will not form a crusty salt residue. The residue forms at about 160°F (71°C) so the goal is to reduce the boiling point of water to less than 160°F. In some desalinization plants, distilled water is also filtered of other pollutants to make it ready to drink.

A process called reverse osmosis can also be used to remove salt from water. Water molecules are forced through a plastic membrane (a barrier) with very small pores (openings) that allow the passage of water, but not of salts.

K. Lee Lerner

For More Information

Books

Farndon, John. Water (Science Experiments). Salt Lake City, UT: Benchmark Books, 2000.

Postel, Sandra, and Brian Richter. Rivers for Life: Managing Water for People and Nature. Washington, DC: Island Press, 2003.

Websites

"Chemistry Tutorial. The Chemistry of Water." Biology Project. University of Arizona.http://www.biology.arizona.edu/biochemistry/tutorials/chemistry/page3.html (accessed on August 26, 2004).

"Water Basics." Water Science for Schools, United States Geological Survey.http://ga.water.usgs.gov/edu/mwater.html (accessed on August 26, 2004).

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