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Desalinization

Desalinization

Because only 1 percent of the Earth's water is fresh, it is useful to utilize the oceans as a means of supplementing the fresh-water supply. To be potable (drinkable), however, salt and other chemicals must first be removed from the sea water. This process of salt removal, known as desalinization (also called desalination), has been practiced since ancient times. Today, a number of technologies are used.

Heat Distillation

Over 60 percent of the world's desalinated water is produced using heat to distill fresh water from sea water. The distillation process mimics the natural hydrologic cycle in that sea water is heated, producing water vapor, which is in turn condensed to form fresh water.

In a desalinization plant, sea water is heated to its boiling point to allow maximum vaporization . For this to be done economically, the boiling point is lowered by reducing the atmospheric pressure. The reduction of the boiling point is important for two reasons: it allows multiple boiling that results in lower energy requirements, and it controls the buildup of carbonate and sulfate scale production on the apparatus. The distillation process is used successfully in many locations around the world.

Ion Extraction

Another method of desalinization is ion extraction, in which the ionized salts found in sea water are extracted through chemical or electrical means.

Ion Exchange.

The chemical method is called ion exchange. In this method, granules of commercially prepared resin remove the positive ions from the sea water and replace them with ions that are loosely bound in the molecular structure of the resin. Other beds of resin are able to exchange negative ions. However this process is too expensive to be used to desalinate large quantities of sea water.

Electrodialysis.

In contrast to the chemical method, the electrical mechanism of ion removal, commercially introduced in the early 1960s, is much cheaper. It is called electrodialysis, since electric current pulls the ions through membranes that are permeable to only the positive or negative ions. Alternating positive or negative membranes, which number in the hundreds, are bound by a frame and form narrow compartments to trap the ions. When a direct electric current is applied, the positively charged ions tend to migrate through the membranes permeable to positive ions and the negatively charged ions tend to migrate through the membranes permeable to negative ions. By this process, ions move between the compartments and become more concentrated.

The distance the liquid flows in the compartments, the intensity of the current flow, the permeability of the membranes used, and the distance that the membranes are apart govern the efficiency of electrodialysis. The cost depends on the concentration of salts in the sea water, since the electrical power used varies directly with the number of ions to be removed and their electrochemical characteristics. Usually, the electrical power required to separate the ions from the water would be cheaper than the resin and chemicals used in ion exchange. Even then, it is usually so high that only brackish water, less salty than sea water, can be desalinated economically by electrodialysis for large-scale use.

In both ion extraction procedures, sediments and other impurities in the water can greatly reduce the success rate. Careful pre-treatment of the water to remove undesirable materials is usually necessary.

Freezing Desalinization

Extensive work was done in the 1950s and 1960s to develop freezing desalinization. During the process of freezing, dissolved salts are naturally excluded from the lattice structure of ice crystals. Cooling the water to form ice under controlled conditions can desalinate sea water. Before the entire mass of water has been frozen, the ice is removed and rinsed to remove any salts adhering to the ice surface. It is then melted to produce fresh water.

Theoretically, freezing has some advantages over distillation, including a lower energy requirement, little scaling or precipitation , and minimal potential for corrosion. The disadvantage is that it involves handling ice and water mixtures that are difficult to move and process.

Solar Humidification

The use of direct solar energy for desalinating sea water has been investigated and used for some time. During World War II (from 1939 to 1945), small solar stills were developed for use on life rafts. These devices imitate the natural hydrologic cycle in that the Sun's rays heat the sea water so that the production of water vapor (humidification) increases. The water vapor is then condensed on a cool surface, and the condensate collected as fresh water. An example of this type of process is the solar greenhouse in Porto Santo, Portugal, in which the sea water is heated in basins, resulting in the condensation of water vapor on the sloping glass roof that covers the basins.

Although the thermal energy may be free, the stills are expensive to construct, additional energy is needed to pump the water to and from the facility, vapor can leak from the stills, and careful operation and maintenance is needed to prevent scale formation. Generally, these types of solar humidification units have been used for desalinating sea water on a small scale, where solar energy is abundant but electricity is not.

Reverse Osmosis

Reverse osmosis (RO) is relatively new, with successful commercialization occurring in the early 1970s. In RO, sea water is forced through a membrane. As a portion of the water passes through the membrane, the remaining "feed water" increases in salt concentration. Some of the feed water is discharged without passing through the membrane to prevent precipitation of supersaturated salts and increased pressure at the membrane surface. Pretreatment is important in RO because the membranes are fine; suspended solids must be removed and the water pre-treated so that salt precipitation or microorganism growth does not occur on the membranes. Usually the pretreatment consists of fine filtration and the addition of acid or other chemicals to inhibit precipitation. During the 1990s, the development of membranes that can operate efficiently with lower pressures and energy recovery devices has greatly reduced operating costs.

Iceberg Towing

Using icebergs as a source of fresh water is not a new idea. Captain James Cook used icebergs to replenish fresh water supplies aboard his ship The Resolution in 1773. However, it was not until the 1950s that serious consideration was given to towing icebergs from Antarctica to arid regions of the world.

Today, satellites could be used to find suitably sized icebergs that optimize the trade-off between handling costs and ice volume. Once located, a bow could be cut in the iceberg and a Kevlar sheet wrapped around it to prevent melting. Powerful tugboats could tow the iceberg along favorable ocean currents to its destination. A trip to Southern California is estimated to take one year and result in a 20-percent loss due to melting.

At present, the cost of transporting icebergs is prohibitive, and many technological barriers remain. Although towed icebergs would avoid major shipping lanes, towing large icebergs for long distances is not yet possible, and most icebergs are too thick to be towed into shallow seas and ports. Instead, other options for drinking water (like desalinization) are more practical.

see also Cook, Captain James; Drinking-Water Treatment; Ice at Sea; Sea Water, Freezing of; Sea Water, Physics and Chemistry of.

Alison Cridland Schutt

Bibliography

Levine, S. N. Selected Papers on Desalination and Ocean Technology. Gloucester, U.K.:Peter Smith Publisher, 1990.

Speigler, K. S., and Y. M. El-Sayed. A Desalination Primer. L'Aquila, Italy: BalabanPublishers, 1994.

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Desalinization

DESALINIZATION

A solution to hydrate the arid Middle East.

Desalinization, also referred to as desalination, is the removal of salts and dissolved solids from brackish water or seawater. In the past three decades, due to increasing water demand and increasing resource scarcity, desalinization has become a critical, relatively drought-proof, resource of potable, irrigation, and industrial water in arid regions of the globe. In the Arab countries, which have 5 percent of the world population but only 0.9 percent of the water resources, water shortages are a constant challenge, and desalinization is playing an increasing role. Once so prohibitively expensive and technologically troublesome as to be totally impractical, it is now often the solution of choice in conditions of unreliable water resources. Capital investments and costs of production are going down as innovations improve the technologies involved.

Of the more than 12,500 desalinization plants in operation or in construction worldwide, 60 percent are located in the Middle East and North Africa (MENA). The Shuʿayba Plant Phase II in Jeddah, Saudi Arabia, for a time the world's largest plant, supplies the daily water needs of 1.5 million people. Saudi Arabia, whose desalinization output exceeded one billion cubic meters in 2002 to provide 70 percent of its water needs, is the largest desalinated water producer in the world, contributing to 30 percent of global output. Desalinized seawater currently constitutes Saudi Arabia's main source for potable water. This water is transported in a network of 1,550-mile pipelines, 21 pumping stations, 131 depots, and 10 stations for mixing the desalinated water with underground water. Around 1972, the MENA region "ran out of water" as the consumption surpassed the rate of resource renewal. Since then, MENA has relied heavily on desalinization, and is poised to remain for the foreseeable future the largest desalinization market in the world.

Desalinization systems can be membrane-based, such as Reverse Osmosis (RO) and Electro-Dialysis Reversal (EDR), or thermal, such as multi-stage flash (MSF) and multiple-effect distillation (MED). Boiling, leading to desalinization through evaporation, a process of thermal distillation, was known and practiced from ancient times. However, now most desalinization plants use membrane-based reverse osmosis, a process that allows the separation of 99 percent of dissolved salts and impurities from water, by means of pressure exerted on a semi-permeable membrane. Between 15 and 50 percent

bahrain's installed desalination capacity (july 2000)
facility date of commissioning capacity (mgd) technology
source: ministry of electricity and water, bahrain.
table by ggs information services, the gale group.
sitra power and water plant (i, ii, iii) 1974, 1984, 1985 27.5 msf
ras abu jarjur ro plant 1984 12.5 ro
addur ro plant 1992 3 ro
hidd power and water plant 2000 30 msf
total installed capacity 73

of seawater intake into a plant is purified; the rest becomes brine, or high-salt water, in need of dilution, then dumping. Environmental regulations dealing with the impact of desalinization on the environment vary from country to country. In addition to brine, other effluents include discharged process chemicals used for defouling, and toxic metals, as well as small amounts of solid waste (spent pretreatment filters, filtered solid particles, etc.).

Energy requirements for desalinization are high, mostly to power machinery and to heat feed-water. RO and distillation plants are often located with energy generation plants to improve efficiency. In such cases, additional environmental impacts have to be taken into account on a case-by-case basis. Fortunately, many of the water-starved countries such as Saudi Arabia, Kuwait, the United Arab Emirates, and Libya are major oil and gas producers and have significant reserves. They have invested heavily in desalinization from early on. The first plant in Saudi Arabia was inaugurated in 1954. The Saline Water Corporation in Saudi Arabia is the largest investor in, and operator of, desalinization plants in the world.

Cost reduction is the single most important factor necessary to increase the implementation of desalination. Capital investment unit costs range from $1,000 to $2,000 per cubic meter of capacity, and can be amortized over 20 to 30 years. Unit production costs per cubic meter range from $0.50 for large plants to over $1.50 for small plants, or about one-half the cost of desalinated water in the recent past. It is not economical to operate a plant part time. Economies of scale play an important role in investment decision-making but are more easily achieved in distillation than in RO process. The Middle East Desalination Research Center (MEDRC), established in 1996 in Muscat, Oman, has been conducting basic and applied research to reduce the cost of water desalinization.

Bahrain, Jordan, and Kuwait were the only water-scarce countries in the region in 1950s. In 2003, twelve countries have water scarcity, and by 2023, six more countries will suffer the same vulnerability, including Israel and Palestine. If the participants in the PalestineIsrael conflict do not exploit water as a unilateral security issue and insist upon retaining control over water resources in the Occupied Territories, the desalinization option may contribute to a solution.


Bibliography


Rogers, Peter, and Lydon, Peter. Water in the Arab World: Perspectives and Prognoses. Cambridge, MA: Harvard University Press, 1994.

john f. kolars
updated by karim hamdy

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