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 fresh water 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 projects have been rare. One notable example is a plant that was built in San Diego in the 1950s and later shipped to the US Naval Base at Guantánamo Bay in Cuba. It can produce 13 million liters (3.43 million gallons) 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 US drinking water standards are set at a maximum of 500; in actuality, many US 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 to be 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 brine (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 ion 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

Ion exchange methods use synthetic resins in a normal flow-through mode to remove dissolved ions from water and are well suited for treating slightly brackish or hard water. These systems are widely used in municipal and industrial settings for water softening, where resins replace calcium and magnesium ions with sodium or hydrogen ions. Resins that exchange metallic ions for hydrogen ions and nonmetal ions for hydroxide ions can effectively reduce total dissolved solids at low concentrations.

Despite their effectiveness for limited applications, ion exchange processes are not practical for large-scale seawater desalination because of the high ionic strength of seawater and the rapid exhaustion of resins. The need to regenerate spent resins with acids or bases complicates continuous operation and produces secondary waste streams that require careful handling. As a result, ion exchange is most often employed in desalination plants as a pretreatment or post-treatment technology, such as for hardness control, boron removal, or polishing water following reverse osmosis.

Freezing and Solvent Extraction Methods

Freezing and solvent extraction have long been investigated as alternative desalination techniques, but neither has been widely adopted for large-scale use in the twenty-first century. When saline water freezes under equilibrium conditions, the ice crystals that form consist almost entirely of pure water, leaving salts concentrated in the remaining liquid. This principle explains why naturally formed ice, such as icebergs, contains very low salinity. During the twentieth century, proposals were advanced to tow icebergs to water-scarce regions as a source of freshwater, but logistical, economic, and environmental challenges—particularly those related to harvesting, transport, and controlled melting—have prevented practical implementation.

Artificial freezing desalination has also been tested experimentally in the twenty-first century, but technical difficulties—including energy demands, ice handling, and efficient separation of brine from ice crystals—have limited its feasibility. Solvent extraction methods, which rely on organic solvents that selectively dissolve water while excluding salts, have likewise remained largely experimental. High costs, solvent recovery requirements, and concerns over contamination and environmental impact have restricted their development. As a result, both freezing and solvent extraction have been largely superseded by membrane-based and thermal desalination technologies, which offer greater efficiency and scalability.

Bibliography

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