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Summer/Fall 2008 Vol. 8 No. 2

Table of Contents

A Salty Situation

©National Geographic

Is Desalination the Answer to Increasing the Nation's Fresh Water Supplies?

America's thirst for water is leaving regions of the country parched. Atlanta is struggling to find resources to support its expanding urban areas, while Florida is working to maintain adequate flows from the same watershed to protect its ecosystems and wetlands. The Colorado River's water is continually disputed among seven western states and Mexico, and California faces perennial water problems. Such regional water management dilemmas are becoming commonplace, and the overall pressure on the nation's limited fresh water resources to satisfy demands for domestic, agricultural, and environmental purposes will only continue to intensify.

For a nation that has two vast oceans as bookends, the solution seems simple enough: Why can't we tap the oceans as a resource and produce fresh water by removing the salt? The practice of separating salt from ocean water has been around for centuries, dating back to when salt, not water, was a precious commodity.

Historically, the high cost and energy required for desalination confined its use to places where energy was inexpensive and freshwater was scarce. Nevertheless, advances in technology have reduced its costs and generated new interest in desalination in the United States, and a recent report from the National Research Council explored desalination's potential for boosting future U.S. water supplies.

"To make desalination a more attractive option for communities facing water shortages, we must better understand and minimize the environmental impacts and develop approaches that lower the financial costs," said Amy K. Zander, chair of the committee that wrote the report and a professor at Clarkson University, Potsdam, N.Y. "A coordinated and sufficiently funded, long-term research agenda between agencies and the private sector could help reduce costs and ensure environmentally sustainable approaches to desalination."

Currently, desalination generates less than 0.01 percent of the water used in the U.S., although plants exist in every state. Most use a method called reverse osmosis, which pushes seawater or brackish groundwater through a membrane to separate out the salts.

Limited studies suggest that desalination may be less environmentally harmful than other ways to supplement water supplies -- such as diverting freshwater from sensitive ecosystems -- but a definitive conclusion cannot be made without further research. For instance, the extent to which fish and other creatures become trapped in saltwater intake systems is not well-understood.

To help further minimize environmental impacts, researchers should also examine the longer-term ecological effects of disposing of the salt concentrate that remains and develop cost-effective, environmentally sustainable disposal options. The report adds that several detailed environmental evaluations of new desalination plants also should be conducted, including ecological monitoring before and after a plant begins operating. Desalination efforts, however, need not be halted until the research is complete, the report suggests.

Research and development are also needed to continue lowering desalination's financial costs and energy use, which could occur by making the membranes used in reverse osmosis more permeable and improving pretreatment processes, the report says. However, the energy used at reverse osmosis facilities probably cannot be reduced more than 15 percent below current levels. Even if costs are lowered, conserving water or transferring it from one use to another will, in most cases, remain a less expensive option than adding water through desalination, the report notes.

"A variety of financial, social, and environmental factors constrain the potential for desalination, not necessarily the technology," said Zander. "There is a common belief that desalination could one day be used to meet all future U.S. water demands, but considering the fundamental energy requirements, desalination is more likely to complement a broad portfolio of water management approaches."   -- Jennifer Walsh & Sara Frueh

Desalination: A National Perspective. Committee on Advancing Desalination Technology, Water Science and Technology Board, Division on Earth and Life Studies (2008, 312 pp.; ISBN 0-309-11923-5; available from the National Academies Press, tel. 1-800-624-6242; $45.00 plus $4.50 shipping for single copies).

The committee was chaired by Amy K. Zander, professor and director of the interdisciplinary engineering and management program at Clarkson University, Potsdam, N.Y. The study was funded by the U.S. Bureau of Reclamation and U.S. Environmental Protection Agency.

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Copyright 2008 by the National Academy of Sciences