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