Desalinization
Desalinization, also known as desalination, is the process of removing salt from seawater to make it suitable for drinking, agriculture, and industrial use. Historically, this technique has roots dating back to ancient civilizations, where methods like evaporation and distillation were first employed. With rising freshwater scarcity, especially in arid regions such as the Middle East, the demand for efficient desalination technologies has surged. Modern techniques include reverse osmosis, which uses high pressure to filter out salt through a permeable membrane, and the freezing method, which separates salt as ice crystals form. While these technologies have advanced, challenges remain, particularly in terms of energy consumption and cost, prompting interest in renewable energy solutions. For instance, solar stills harness sunlight for evaporation, and innovative plants powered by renewable energy sources have emerged, such as those in Australia. As the need for freshwater continues to grow globally, the development and refinement of desalinization methods is becoming increasingly critical.
Desalinization
Desalinization, also called desalination, is the process of removing salt from seawater so that it can be used for drinking or agriculture and industrial purposes. To desalinate it, water must be evaporated, frozen, or filtered. Although people often believe desalinization is a modern technology, evidence has shown that the process dates to ancient times.
Overview
Because humans and most land animals need freshwater with a low salt content—less than 1,000 parts per million (ppm)—scientists and inventors have tried to find simple and efficient ways of desalinating seawater or other brackish sources as freshwater becomes scarce. Seawater can contain salt in concentrations of up to 35,000 ppm. Early thinkers and modern scientists have used various methods of removing salt from water, including evaporation, filtering, and freezing. Like many such endeavors, the development of desalination techniques was not a single event, but rather a good idea that was explored and improved upon over time.
Finding cost-effective technology for low-energy systems that can transform large quantities of water efficiently to meet the needs of the world's population is the major challenge of desalinization. Freshwater is becoming scarce in many parts of the world, including desert areas such as the Middle East. According to the United States Geological Survey, about 70 percent of desalinated water is used in countries such as Bahrain, Kuwait, Qatar, Saudi Arabia, and the United Arab Emirates. Some coastal areas of the United States, such as California and Florida, also need additional freshwater supplies for both drinking and irrigation because the groundwater has a high salt content.
Evaporation
Early efforts to desalinize water generally developed from the need for freshwater aboard ships—especially on long journeys. In the first century C.E., Pliny the Elder reported in his Natural History that sailors suspended wool or sheepskins over the sea during the night so by morning the wool had absorbed water that evaporated from below, leaving the salt behind. They could then squeeze the freshwater from the wool directly into their mouths or into containers for drinking. It was a simple method that did not require equipment or fuel for heating the seawater.
Boiling or distilling water is a common evaporation technique that also dates back to antiquity. When saltwater is heated, it produces steam. The vapor can then be cooled and allowed to revert to liquid form—pure water—and the salt remains in the boiling vessel. This method is still useful, although the cost of fuel to heat large amounts of water can be prohibitive.
Reverse Osmosis
Reverse osmosis is a process used to filter water. During natural osmosis, saltwater moves through a permeable membrane, a thin sheet that allows liquids to filter through it, from an area of greater concentration to an area of lesser concentration. Reverse osmosis does the opposite. For freshwater in a saline solution (higher concentration) to move through the membrane, pressure of 800 to 1,200 pounds per square inch (psi) must be applied. When this is done, the water molecules pass through but leave behind the dissolved salt and any other minerals or contaminants. The result is pure water. Reverse osmosis is generally used on brackish water, which has less salt than seawater and requires only about half the pressure to push the freshwater through the membrane. As technology improves, reverse osmosis may become more feasible as a desalination method.
Freezing
The freezing method has great potential as a future source of desalinating saltwater, but technology has not advanced far enough to make it economically feasible. This technique involves using refrigerant to remove heat from briny water, causing ice crystals to form. The salt separates from the water as it freezes. Before the ice can be melted and the pure water used, the salt must be washed from the crystals. The heat removed from the brine as it is frozen is later used to melt the ice crystals into potable water.
Renewable Energy and Desalinization
One of the simplest devices for desalinization is a solar still. A solar still is an evaporation instrument. It has a tank at the bottom, which is filled with salty or undrinkable water. A plastic lid goes over the tank, and when the device is left in the sun, the water evaporates, condenses, accumulates inside the lid, and runs down the sides. It then collects in a trough on the outer edge of the tank, from which it can be drained for drinking or other uses. Small, collapsible versions are available for soldiers, sailors, and others who might need a clean water supply, but large solar still plants are in the development stage.
In 2011, the World Bank funded a study to address the shortage of potable water in the Middle East and North Africa. Because the operating costs of reverse osmosis desalination plants are so high, and it is likely that both demand for water and fossil fuel prices will increase, the study concluded that the use of renewable energy would be more effective over time. Even though large-capacity solar desalinization plants have yet to be developed, their overall advantages led to the study's conclusion that the need for them is urgent and the long-term benefits are strong.
Scientists in Perth, Australia, developed a desalination plant run on renewable energy after they determined that the area's long-term drought was most likely related to global warming. The city built a reverse osmosis facility in 2006 for use with energy provided by a wind farm. The plant produces enough clean water to meet 17 percent of the city's needs without the use of fossil fuels that could exacerbate the climate problems of southwestern Australia.
Bibliography
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"Conclusions." MENA Regional Water Outlook, Part II: Desalination Using Renewable Energy. World Bank Group. Web. 18 Dec. 2014. http://siteresources.worldbank.org/INTMNAREGTOPWATRES/Resources/Use‗of‗Desalination‗and‗Renewable‗Energy‗to‗Close‗the‗Water.pdf
"Desalination." Berkshire Encyclopedia of Sustainability. Ed. Daniel E. Vasey, et al. Vol. 4: Natural Resources and Sustainability. Great Barrington, MA: Berkshire, 2012. 104–106. Print.
Dvoark, Bruce I. "Drinking Water Treatment: Reverse Osmosis." NebGuide. The Board of Regents of the University of Nebraska on behalf of the University of Nebraska–Lincoln Extension. Web. 18 Dec. 2014. http://ianrpubs.unl.edu/live/g1490/build/g1490.pdf#page=4&zoom=auto,-169,708
"Saline Water: Desalination." United States Geological Survey. Department of the Interior/United States Geological Survey. 17 Mar. 2014. Web. 18 Dec. 2014. http://water.usgs.gov/edu/drinkseawater.html