Desalination: A Change in Proportions
When people were able for the first time to see photographs of the earth taken from space, it was brought strongly to general attention that this planet’s surface presents more water than land. At that time, thoughtful people were reminded of how water, and specifically the planet’s oceans, have affected all of life. In regard to humans, oceans separate us and join us, they nurture and destroy us, they even seem to reflect our moods.
From the standpoint of human need and use, it is significant that 94 percent of the earth’s water is salty, and only 6 percent is fresh. This situation suggests the famous image of the “desert island,” which is surrounded by a glistening sea but which cannot meet the needs of the shipwreck victim who dies of thirst on its shore. Such a lack of water is not only the stuff of tales: the Virgin Islands, Cyprus, Haiti, and Japan are among the islands or island groups that are searching for solutions to their limited supplies of fresh water. On larger landmasses, Saudi Arabia and the state of Florida are among areas looking for ways to increase their access to fresh water; these areas sometimes have large quantities of brackish water that they are currently unable to use (Brackish water is underground water that has a mineral content that severely limits its use by most plants an animals. The United States contains large quantities of brackish water.)
The reader will have noticed that this introduction has attempted to make it clear that from the human point of view, it would be very advantageous to be able to turn salty and brackish water into fresh water, changing the proportion between two absolutely basic characteristics of our planet. That same reader will now be awaiting a solution to the problem that has been raised. The remainder of this essay will discuss a solution that appears to be increasingly practicable: the desalination of water.
One of the appealing characteristics of water desalination is that it is a human version of a natural process. When the sun’s heat evaporates water from the surface of oceans and then returns it to the earth’s surface as rain, a natural form of desalination is taking place. Early forms of desalination followed the concept of the solar process. Sea water was placed in shallow basins covered by tilted glass plates; fresh water evaporated from the sea water in the basins, condensed on the glass plates, and was collected as it ran down into separate containers. This process is feasible or small amounts of water produced at the site of immediate need. It is especially attractive in areas that have very limited fuel and a great deal of sunlight, or where low initial costs are essential. (Perhaps the most dramatic use of this low-level technology is on lifeboats, where production of potable water by a solar device is literally a matter of life and death.) Currently, however, much of the world’s desalination is being done in oi-rich countries where water is scarce, fuel to run mechanical desalination processes is abundant, and funding for the building of complex desalination plants is available.
It is important to be aware that the current (if slow) growth of interest in desalination is based on the increasingly restrictive relationship between available fresh water and word population, or regional populations. As population increases, techniques such as water desalination and the treatment of wastewater are being view as more economically feasible. However, even the most enthusiastic proponents of desalination view the process as appropriate only for the production of pure drinking water, not for the production of the huge amounts of water required for agricultural purposes.
A number of methods are currently used to desalinate water. These include heat-based methods, in which fresh water is condensed from steaming salt water; electric-charge methods, in which salt or other dissolved minerals are attracted to positive or negative electrodes; and osmosis-based methods, in which salts and other impurities are separated from water through the use of a selectively permeable membrane.
A method in increasing use is called reverse osmosis. It is appropriate for seawater and brackish water. In reverse osmosis, water under high pressure is exposed to a membrane that allows water but not salt or other minerals to pass through it. The desalinated water is collected or use, and the water that did not pass through the membrane (which is very highly mineralized) is disposed of. Care must be taken that salt and other minerals in the remaining water do not reach a concentration that produces deposits in the machinery. In reverse osmosis and in some other methods of desalination, it is possible to produce electricity by turning the high pressure that has been applied to the water into rotating energy. The production of electric energy from the desalination process can have a significant impact on the cost of operating a large plant.
In all of the desalination processes, a quantity of “by-product water” is produced that has a high mineral content. The desalination process must therefore include disposal of the mineralized water in an environmentally responsible way. In the case of water withdrawn from the oceans, it is usually possible to return the extremely salty water in a controlled way, using the vastness of the oceans to dilute the returning water. In the case of brackish water, which is usually withdrawn from underground sources, simply returning the remaining, highly mineralized water to the aquifer is not feasible. It may be necessary to dispose of the mineralized water by piping it to a seacoast or other appropriate disposal site, or by evaporating the remaining water, which then requires the disposal of the minerals in dry form.
During the past fifty years, the desalination of water has gone from something of an oddity to being an increasingly viable method of obtaining fresh water. The change has been due to the decrease in the price of desalinated water as the desalination processes have become more effective and efficient, and the increasing cost of fresh water as the global population has increased.
This essay is being written in 2003, declared by the United Nations to be the International Year of Freshwater. It is also being written in a large urban area that receives much of its water from a distant and overused river. In Overview of the National Water Program, the United States Environmental Protection Agency states, “Another risk concerns water availability. Water conservation is important only to those who have grown up in places or during times of severe drought; it is not part of our culture…ours is not yet an environmentally literate society.” In too many ways, that statement describes where and how I live.
Many human societies have shown the ability to adapt and change. It appears that water desalination (which is not mentioned in the Overview produced by the Environmental Protection Agency) is a growing part of human adaptation to the changing relationship between human population and the supplies of fresh and brackish/salty water. Although the total supply of the planet’s water cannot be increased, desalination is a practical way in which the supply of fresh water within the total quantity of water can be and is being increased.
Notes on sources:
Most of the information in this essay is from two sources:
The ABC’s of Desalting by O. K. Buros, published by the International Desalination Association
“Quenching a Nation’s Thirst for Water,” (no author given), Saudi Arabia, spring 2003, published by the Information Office of the Royal Embassy of Saudi Arabia in Washington, D.C.
I sent requests for information on desalination to a number of agencies, governments, and individuals; most did not respond. It appears that this situation is a commentary either on my letter-writing skills or the degree to which desalination is not yet viewed seriously by organizations to which it may become increasingly important in the future.
Erik Felker
United Nations Association of the U.S.A.
San Fernando Valley (California) Chapter
2003