Desalinization

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Desalinization, also known as "desalination," is the process of separating sea water or brackish water from their dissolved salts. The average salt content of the ocean water is about 3.4% (normally expressed as 34 parts per thousand). The range of salt content varies from 18 parts per thousand in the North Sea and near the mouths of large rivers to a high of 44 parts per thousand in locked bodies of water such as the Red Sea, where evaporation is very high. The desalination process is accomplished commercially by either distillation or reverse osmosis (RO).

Distillation of sea water is accomplished by boiling water and condensing the vapor. The components of the distillation system consist of a boiler and a condenser with a source of cooling water. Reverse osmosis is accomplished by forcing filtered sea water or brackish water through a reverse osmosis membrane. In a reverse osmosis process, approximately 45% of the pressurized sea water goes through membranes and becomes fresh water. The remaining brine (concentrated salt water) is returned to the sea.

In 1980, the United Nations declared 1981–1990 as the "International Drinking Water Supply and Sanitation Decade." The objective was to provide safe drinking water and sanitation to developing nations. Despite some progress in India, Indonesia, and a few other countries, the percentage of the world population with access to safe drinking water has not changed much since that declaration. In the period between 1990 and 2000, the amount of people with access has only increased by 5%.

The World Health Organization (WHO) estimates that only two in five people in the less developed countries (LDCs) have access to safe drinking water. The WHO also estimates that at least 25 million people of the LDCs die each year because of polluted water and from water-born diseases such as cholera, polio, dysentery, and typhoid. Whether by distillation or by reverse osmosis, desalination of water can transform water that is unusable because of its salinity into valuable fresh water. This could be an important water source in many drought-prone areas.

There are approximately 7,500 desalination plants worldwide. Collectively they produce less than 0.1% of the world's fresh water supply. This supply is equal to about 3.5 billion gal per day (13 million l). The cost and the feasibility of producing desalinated water depends upon the cost of energy, labor, and relative costs of desalinated water to that of imported fresh water. It is estimated that in the United States, commercial desalinated water produced from sea water by reverse osmosis costs about $3 per 1,000 gal (3,785l). This price is four to five times the average price currently paid by urban consumers for drinking water and over 100 times the price paid by farmers for irrigation water. The current energy requirement is approximately three kilowatt hours of electricity per one gallon of fresh water extracted from sea water. Currently, using desalinated water for agriculture is cost prohibitive.

About two-thirds of the desalination water is produced in Saudi Arabia, Kuwait, and North Africa. Several small-scale reverse osmosis plants are now operating in the United States, including California (Santa Barbara, Catalina Island, and soon in Ventura and other coastal communities). Generally, desalination plants are used to supplement the existing fresh water supply in areas adjacent to oceans and seas such as southern California, the Persian Gulf region, and other dry coastal areas. Among the advantages of desalinized water are a dependable water supply regardless of rainfall patterns, elimination of water rights disputes, and the preservation of the fresh water supply, all of which are essential for existing natural ecosystems.

Reverse osmosis involves forcing water under pressure through a filtration membrane that has pores small enough to allow water molecules to pass through but exclude slightly larger dissolved salt molecules. The basic parts of a reverse osmosis system include onshore and offshore components. The onshore components consist of a water pump, an electrical power source, pre-treatment filtration (to remove seaweed and debris), reverse osmosis units connected in series, solid waste disposal equipment, and fresh water pumps. The offshore components consist of an underwater intake pipeline, approximately 1,093 yd (1 km) from shore, and a second pipeline for brine discharge.

Small reverse osmosis units for home use with a few gallons-per-day capacity are available. These units use a disposable reverse osmosis membrane. Their main drawback is that they waste four to five times the volume of water they purify.

Producing potable water from sea water is an energy intensive, costly process. The high cost of producing desalinized water limits its use to domestic consumption. In areas such as the Persian Gulf and Saudi Arabia where energy is plentiful at a low cost, desalinized water is a viable option for drinking water and very limited greenhouse agriculture. The notion of using desalinized water for wider agricultural purposes is neither practical nor economical at today's energy prices and available technology.

Kaufman, D. G., and C. M. Franz. Biosphere 2000: Protecting Our Global Environment. New York: Harper-Collins, 1993.

Nebel, B. J., and R. T. Wright. Environmental Science: The Way the World Works. 4th Edition. Englewood Cliffs, NJ: Prentice Hall, 1993.

Lizarraga, S., and D. Brown. "Fresh Water from Santa Barbara Seas." Reprinted from Desalination and Water Reuse, 1992. Santa Barbara: Department of Water Resources.