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| Water Education |
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Anyone who has ever seen a great, or even not so good movie about life on the ocean knows that humans just can't drink seawater. After all, it's full of salt that typically dehydrates the human body. But what if we could make seawater safer to drink? It turns out that we can and the process is called desalination. Desalination is a process that removes dissolved minerals (including but not limited to salt) from seawater, brackish water, or treated wastewater. A number of technologies have been developed for desalination, including reverse osmosis (RO), distillation, electrodialysis, and vacuum freezing. So why is seawater such an attractive water resource? Here's a few reasons why...Seawater provides an unlimited, reliable water supply for coastal populations worldwide; brackish water is a plentiful, relatively drought-proof water resource for inland populations and reduces dependency on imported water. And, of all the Earth's water, 97 percent is salt water, only 1 percent is fresh water available for humans to drink, and 2 percent is frozen. Of the more than 7,500 desalination plants in operation worldwide, 60% are located in the Middle East. The world's largest plant in Saudi Arabia produces 128 MGD of desalted water. In contrast, 12% of the world's capacity is produced in the Americas, with most of the plants located in the Caribbean and Florida. To date, only a limited number of desalination plants have been built along the California coast, primarily because the cost of desalination is generally higher than the costs of other water supply alternatives available in California (e.g., water transfers and groundwater pumping). However, as drought conditions occur and concern over water availability increases, desalination projects are being proposed at numerous locations in the state. Desalination costs are decreasing as technology improves and more plants are built. Today there are more than 15,000 desalination plants in 120 countries. The desalination market is forecast to grow more than $70 billion in the next 20 years. About half of the world's desalted water is produced with heat to distill fresh water from seawater. The distillation process mimics the natural water cycle in that salt water is heated, producing water vapor that is in turn condensed to form fresh water. One such treatment process is called Multi-Stage Flash Distillation. Another desalination treatment process with the most expanded use is membrane-based reverse osmosis,. In this process, pressure is applied to the water, which allows water to flow through a membrane, leaving the ions, salts, and other dissolved solids and nonvolatile organics behind. Some examples of U.S. cities currently using, or planning to use desalination include: Tampa Bay Water has constructed a 25-million-gallon-per-day desalination facility in Apollo Beach in Hillsborough County, FL. The facility began operating intermittently in March 2003 and is expected to be fully functional by 2006; El Paso Water Utilities and Fort Bliss officials are collaborating to build the country's largest inland desalination plant. The plant will draw brackish water from an underground aquifer that provides about 40 percent of El Paso's municipal water supply. Construction of the 27.5-million-gallon-per-day facility began in September 2005 and should be completed in 2007;The Groundwater Recovery Enhancement and Treatment (GREAT) program in Oxnard, CA will blend desalted groundwater with high-quality water the city buys from a neighboring water district. Oxnard broke ground on the project in May 2004, and it should be completed in 2006 or 2007. There are a number of ways to forestall this, however. One of those ways - desalination - is already being used across the country to stretch water supplies, clean up polluted water and provide protection for aquifers. The technique is ancient, dating back to the 4th century B.C. when, according to the National Water Supply Improvement Association, Greek sailors used simple evaporation to desalinate seawater. The technology, however, is far more modern. Desalination - separating saline water into fresh water and water containing the concentrated salts - is accomplished in two main ways: through distillation or use of membranes. Nearly 60 percent of the world's desalted water is produced via the first method by heating salty water to produce water vapor that is then condensed to form fresh water. The second process uses membranes to separate the salts from the water. In reverse osmosis (RO) facilities, water is forced through bundles of membranes under pressure, leaving behind impurities. In electrodialysis reversal (EDR) plants, an electrical current transfers ions through membranes, resulting in desalted water and concentrates. Worldwide, desalting plants have the capacity to produce 3.5 billion gallons of water a day, nearly enough to provide 15 gallons a day for every American. Some nations, such as Saudi Arabia and Malta, desalt ocean water to produce fresh water for public and industrial consumption. In the United States, most desalting plants treat brackish water, a process that costs one-third to one-fourth as much as the treat involved in desalting ocean water. The product water is used for direct supply, reserves or groundwater recharge. According to the American Desalting Association (ADA), other uses include irrigation, wastewater treatment and water purification. Hospitals, resorts, manufacturing plants, oil rigs and pleasure boats also employ desalination technology. During the Persian Gulf War, the Army had mobile desalination units that could produce 3,000 gallons per hour of potable water from brackish pools. In a 1988 report, the Congressional Office of Technology Assessment suggested desalination could find application in treating contaminated groundwater, be it runoff from mines, agriculture, landfills or storage tanks. Of desalting in general, the report noted, "Desalination should be included as a viable option in any evaluation of water-supply alternatives."
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