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The desalination of ocean water or brackish groundwater is an alternative to obtaining water from fresh surface or groundwater sources, and could be used to replace the need for a water supply dam. Several different technologies exist to remove salt and other impurities from ocean water. The two most commonly used technologies are thermal distillation, which mimics the natural water cycle by using heat to create a vapor that is converted into freshwater, and reverse osmosis, which involves pushing water through a porous membrane that filters out salts and other impurities. Desalination is a process that is coming of age and is already used as a main source of potable water in the Caribbean, Mediterranean and Middle East.
For coastal states, desalination represents an opportunity to draw on oceanic water resources. If the appropriate conditions are present, a desalination plant has the potential to replace an existing or a planned dam.
In order for a desalination plant to be a viable alternative to a water supply dam, the water users must be located fairly close to a coast. Desalination is also a technology that can have adverse environmental impacts of its own, as plants are very energy intensive and must dispose of a highly concentrated saline byproduct into the ocean or estuarine ecosystem. Additionally, desalination plants can be costly to construct and operate, and the facilities require large amounts of land.
Desalination can be a very expensive process due to the high capital cost of desalination facilities and the large amounts of energy required to pump water through membranes to extract the salt or heat the water for distillation. In the case study below, the desalination plant built in Tampa, Florida cost $110 million, of which the Southwest Florida Water Management District paid $85 million. The water produced in this plant is expected to sell for about $2 per 1,000 gallons, far below the desalination industry standard. The cost of regular groundwater sources is about $1.00 per 1,000 gallons. As technology continues to progress, the cost of desalination is expected to decrease, particularly when compared to many of the alternatives.
Desalination case study
Tampa, Florida is home to the largest desalination plant in the United States. It is projected to produce 25 million gallons per day in order to meet 10 percent of the region’s water needs. The saltwater undergoes osmosis and is then treated with lime and chlorine to ensure proper alkalinity. Historically, this region has derived its drinking water supply from groundwater. However, their new water plan calls for production cutbacks at the 11 existing northern Tampa Bay well fields to allow environmentally stressed areas to recover. To accommodate these cutbacks and still produce enough water for the region, Tampa Bay Water is turning to alternative sources for water, like desalination. Unlike other desalination plants in the United States, the Florida plant is not an emergency water source, but an economically sound, major source of a consistent water supply.
For more information on the Florida desalination plant, visit Tampa Bay Water at http://www.tampabaywater.org/MWP/MWP_Projects/Desal/TAMPABAYdesalinationproject_inro.htm.
Where you can go for help
- For more information, contact your state natural resources agency, such as Department of Natural Resources or Department of Environmental Protection.
- International Desalination Association: http://www.idadesal.org/
- Water Treatment Engineering and Research Group, U.S. Bureau of Reclamation: http://www.usbr.gov/pmts/water/desalnet.html.
- Buros, O.K. The ABCs of Desalting. 2nd ed. Topsfield, MA: International Desalination Association, 2000.
- The Surfrider Foundation, Seawater Desalination Plants, http://www.surfrider.og/desal (13 May 2001).
- The U.S. Bureau of Reclamation (BuRec) commissioned a study of low energy alternatives for desalination in 1995. The study found that using VARI-ROÔ technology would result in an energy cost-savings of $2.45 billion per year (compared to existing desalting technology) and a 7 percent reduction in water cost. VARI-ROÔ (VRO) technology involves the use of positive displacement pumping for greater energy recovery instead of the centrifugal pumps used in current reverse osmosis desalination. The study commissioned by BuRec specifically examined how the VRO system could be used to improve desalting plans in San Diego. Studies by the Middle East Desalination Research Center have also used VRO technology.
- Tampa Bay Water, Tampa Bay Seawater Desalination, December 2002, http://www.tampabaywater.org/MWP/MWP_Projects/Desal/TAMPABAYdesalinationproject_inro.htm(15 July 2003).