Desalination plants and technology

Seawater and other salt-containing waters are converted into potable water by distillation, reverse osmosis, and other processes experimentally, and increasingly practically, in regions where water resources are limited or expensive.

Background

For many years, large ships at sea have used distillation processes to convert seawater into usable water for passengers and crews because it is more economical than carrying enormous quantities of for drinking, cooking, and cleaning. In desert regions and some areas that have limited suitable fresh water available, distillation and, more recently, membrane processes have been introduced for the conversion of brackish water, industrial effluents, wastewater, and seawater. Large-scale pilot processes have been rare. One notable example is a plant that was built in San Diego in the 1950s and later shipped to the U.S. naval base at Guantánamo Bay in Cuba. It can produce 13 million liters of distilled water per day.

Because brackish water and various wastewaters contain between 500 and 5,000 parts per million of dissolved solids, and seawater and geothermally produced brines contain up to 50,000 or more, a number of different processing methods have been developed. In addition, the end use of the water may dictate the superiority of one method above the others. For many agricultural purposes, water containing a few thousand parts per million can be used, whereas U.S. drinking water standards are set at a maximum of 500 (in actuality, many U.S. cities’ water supplies exceed this standard).

Distillation Methods

Distillation methods were first described by Aristotle, but they had their first practical use aboard English naval vessels in the 1600s. Since then they have become much more complex, but they still involve a high-cost, energy-intensive boiling process, and subsequently a cooling process for liquefaction of the steam generated. The original processes required submerged tubes, which became encrusted with chemical deposits. Multistage flash process plants are currently used in which the latent heat of evaporation of the water is captured and reused, and the scaling is diminished by adding chemicals or removing the ions causing the deposits. Newer variations of these processes are being investigated. Some attempts have been made to couple power generation plants with distillation units, which may provide more desirable economy of operation.

Various versions of the multistage flash process are used in many parts of the Middle East and in more than three-quarters of the currently operating systems. Other designs for distillation plants have been proposed, and some have been built. Most of these have used horizontal tube processes with a design that permits multiple stages with vacuum distillation and a gradual reduction of saline content by incorporating steam with the brine. Large installations are currently incorporating this design. Smaller plants have employed a vapor compression procedure for industrial plants and resort hotels, but these are gradually being replaced by reverse osmosis facilities.

Solar distillation procedures would appear to offer great future alternatives in the very regions where water is in short supply. If solar energy could be more cheaply and efficiently obtained, and the land area needed made available, the saline water conversion problem would be solved relatively easily.

Membrane Methods

Although reverse osmosis has been most heavily promoted, there is actually a large group of related procedures that utilize membrane separations to purify water. In ordinary osmosis, such as occurs through cell walls, a semipermeable membrane (one through which only the solvent can flow) allows water to flow from a less concentrated solution into a more concentrated one (thus exhibiting an “osmotic pressure”). In reverse osmosis, pressure is exerted on the more concentrated solution, overcoming the osmotic pressure and reversing the flow. After the (saline water) has been concentrated in this manner, the process is repeated with fresh brine.

Among the membranes that have been utilized, most are polyamides and polyimides, which closely resemble protein structures. Reverse osmosis has been most effective with brackish waters, which do not have the high osmotic pressure of seawater to overcome. However, improved membrane systems have permitted construction of larger seawater charged reverse osmosis plants in the 13-million-liters-a-day range. A procedure known as electrodialysis permits an electric field to assist in directing flow through membranes, which are permeable to either cations or anions; some success in using this method with brackish water has been achieved. Pressurization cycles with ion exchange resins or membranes have been successful with low energy requirements, but experiments have failed to find the high-strength materials required to survive the high pressures needed.

Ion Exchange Methods

Utilizing ion exchange resins in a normal flow-by mode is very reasonable for purifying slightly brackish water. In fact, it is used to soften water in many communities with hard-water supplies. Resins that replace metallic ions with positive hydrogen ions, and nonmetal ions with negative hydroxide ions, can readily accomplish that limited task, but they are not adequate for seawater conversion. The necessity of regenerating the exhausted resins with acid or base make designing a continuous process more difficult.

Freezing and Solvent Extraction Methods

When a solution freezes under equilibrium conditions, the solid formed is pure solvent. Therefore, when an iceberg forms, it contains very pure water. It has been proposed that icebergs could be towed to water-short regions. However, mechanical problems, such as providing appropriate freezing chambers and removing brine from the ice surface, have prevented these methods from being seriously explored. Solvent extraction procedures have been tried experimentally, but solvent use and removal are costly.

Bibliography

Khan, Arshad Hassan. Desalination Processes and Multistage Flash Distillation Practice. New York: Elsevier, 1986.

Lauer, William C., ed. Desalination of Seawater and Brackish Water. Denver, Colo.: American Water Works Association, 2006.

Lesiv, Anna-Sofia. "The Growing Importance of Desalination." Contrary, 8 Sept. 2023, www.contrary.com/foundations-and-frontiers/desalination. Accessed 21 Dec. 2024.

National Research Council of the National Academies. Desalination: A National Perspective. Washington, D.C.: National Academies Press, 2008.

Simon, Paul. Tapped Out: The Coming World Crisis in Water and What We Can Do About It. New York: Welcome Rain, 1998.

Spiegler, K. S., and A. D. K. Laird, eds. Principles of Desalination. 2d ed. New York: Academic Press, 1980.

National Academies Press

Desalination: A National Perspective.