About 97 percent of the water on Earth is in the salty oceans. In their thirst for water, people have looked longingly throughout history at this endless supply. Some brackish (slightly salty) water is found inland. Today, more than ever, many people believe that desalting ocean water and brackish water holds the answer to the ever-increasing demand for fresh water in many areas.

The salt in seawater is mostly the same substance as common table salt. A person can safely drink water that contains less than 1/2 pound of salt to 100 pounds of water, or 0.5 kilogram to every 100 kilograms. But seawater has about seven times this amount of salt. A person who drinks only seawater will eventually die. The body's cells will dehydrate (dry out) as they try to get rid of the excess salt from the seawater. Nor can people use seawater in agriculture or industry. It kills most crops, and quickly rusts most machinery.

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Rivers and lakes.

People have found many ways to desalinate (remove the salt from) seawater and brackish water. Desalination offers hope of relieving water shortages near the seacoasts. However, desalination does not hold the answer to all of Earth's water problems. Even if the oceans contained fresh water, people would still have to face such problems as pollution, flood control, and water distribution.

The desalination processes used most commonly today are distillation, reverse osmosis, and electrodialysis. These processes produce fresh water from salt water.

Distillation is the oldest method of turning salt water into fresh water. Most ocean ships use it to obtain drinking water. Seawater can be distilled simply by boiling it in a teapot, and piping the steam into a cool bottle. The steam rises, leaving the salt behind. As the steam cools in the bottle, it condenses into fresh water.

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Every day, the sun evaporates millions of tons of water from the ocean's surface. The water vapor then condenses and falls back to Earth as fresh water. For centuries, people have copied nature and used the sun's heat to distill seawater. Two thousand years ago, Julius Caesar used solar distillation in Egypt to obtain drinking water for his soldiers.

Solar distillation can be done by filling a shallow basin with seawater and covering it with a transparent plastic dome, or a sloping sheet of glass. The salt water turns to vapor under the sun's heat. The vapor rises until it hits the underside of the dome or glass, where it condenses. The fresh water runs down into collecting troughs. This type of distillation produces little water. In one day, such a basin in a sunny climate can produce only about a pint of water per square foot (5 liters per square meter) of the basin's surface area. Solar distillation is not a commonly used method of distillation because it is expensive. The cost stems from the fact that this method requires the use of an enormous land area in order to produce sufficient quantities of water. Solar distillation also is less efficient in operation than other methods of distillation.

Most modern distillation plants use a process called multistage flash distillation. This is a type of the age-old method of boiling and condensation. In flash distillation, preheated seawater flows into a large chamber in which the pressure is low. The low pressure causes some of the water to flash (turn quickly) into steam. The steam is condensed into salt-free water. The seawater passes through several distillation chambers. Each of the chambers has a lower pressure than the previous chamber. Often, the final water is so pure that it is tasteless, and some salt must be tossed back in to give it flavor. The desalting plant at the United States naval base at Guantanamo Bay, Cuba, uses this process. It produces more than 1 million gallons (3.8 million liters) of fresh water a day.

Reverse osmosis is a widely used method for desalting seawater and brackish water. In normal osmosis, a less concentrated liquid flows through a membrane into a more concentrated liquid. Thus, if salt water and fresh water are separated in a chamber by a special semipermeable membrane, the fresh water will flow through the membrane into the salt water. However, if enough pressure is placed on the salt water, this normal flow pattern can be reversed. Fresh water will then be squeezed from the salt water as it passes through the membrane, leaving the salt behind. The reverse osmosis desalting process works in this way.

A desalting plant at Cape Coral, Fla., uses reverse osmosis. This plant can produce about 14 million gallons (53 million liters) of fresh water a day.

Electrodialysis is used chiefly to desalt brackish ground water and water from estuaries (river mouths). Electrodialysis is based on the fact that when salt is dissolved in water, it breaks up into ions (electrically charged particles) of sodium and chloride. Sodium ions carry a positive charge, and chloride ions carry a negative charge.

Electrodialysis uses a large chamber divided into many compartments by stacks of thin plastic membranes. Two types of membranes are used, and they are used in pairs. One type allows only positive ions to pass through it. The other lets only negative ions through. One of the end compartments contains a positive electrode (electrical pole). The other end compartment contains a negative electrode.

When an electric current is sent through the water, the negative ions are drawn through the membranes permeable to negative ions toward the positive electrode. The positive ions are drawn through the membranes permeable to positive ions toward the negative electrode. Thus, the salt in every other compartment is drawn off, leaving fresh water.

A desalting plant on Sanibel Island, Fla., uses the electrodialysis process. This plant produces about 2 million gallons (7.6 million liters) of fresh water daily.

Other desalting processes are also being studied. During the 1970's, several plants experimented with freezing as a method of desalination. When seawater freezes, the ice crystals produced are pure water in solid form. The salt is separated and trapped between the ice crystals. There are several freezing processes that may be used to desalt water. The main problem lies in separating the ice crystals from the salt. This is usually done by washing off the salt with fresh water. The ice is then melted and becomes fresh, liquid water. High costs and engineering problems have prevented the widespread commercial use of freezing as a desalting method.

The future of desalting


The future of desalting.

All methods of desalination are costly, largely because desalting plants use large amounts of energy, and energy is expensive to produce. In addition, plants must pay to dispose of the salt that is removed during the desalination process. It costs from about $4 to about $7 to produce 1,000 gallons (3,800 liters) of fresh water from seawater. The cost depends on such factors as the capacity of the treatment plant and its location. Desalting brackish water costs less than desalting seawater. Engineers and scientists are continuing to work on the development of less expensive methods of desalination.

The thousands of desalting plants around the world together produce more than 31/2 billion gallons (13 billion liters) of fresh water each day. A large facility, such as the plant in Al Jubayl, Saudi Arabia, can produce about 250 million gallons (950 million liters) of fresh water daily. Although desalting plants meet only a small part of the world's daily demand for fresh water, they are essential to millions of people.

About water
Interesting facts about water
Water in our daily lives
Parts of plants
Nature's water cycle
The water supply problem
City water systems
Fresh water from the sea
What water is and how it behaves
Water and the course of history

Many desalting plants are small facilities that serve isolated military posts, oil-drilling crews in deserts, island resorts, and industrial plants. The largest number of plants are in the Middle East, where fresh water sources are scarce. As the cost for desalting water drops, more and more towns and cities may begin using desalted water.

Contributor: Thomas M. Keinath, Ph.D., Dean, College of Engineering, Clemson University.