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Why can't we convert salt water into drinking water?


The Cost of Desalination

There's little doubt that the world needs more drinking water. It's also abundantly clear that the need will keep pace with mounting population growth and the pressures brought about by global climate change. In the United States alone, experts agree that water demand already exceeds supply, projecting that 36 states will confront shortfalls within the next three years. Within 15 years, almost 2 billion people globally will live in areas confronting water scarcity, and, according to most model scenarios, such shortfalls will only worsen under climate change [source: Strassmann, IPCC]. Indeed, the availability and distribution of water is widely discussed as a likely determining factor in future global stability [source: USGS].

So, what is holding us back from diving in headfirst? Until recently, purifying seawater cost roughly five to 10 times as much as drawing freshwater from more traditional sources [source: USGS]. RO filters have come a long way, however, and desalination today costs only half of what it did 10 to 15 years ago. Consequently, transportation, energy and environmental costs have now replaced technology as the primary impediments to large-scale desalination [source: Maloni, NRC-WSTB].

Energy consumption accounts for as much as one-third of the total cost of desalinated water, making even coastal plants expensive to operate [source: Maloni, NRC-WSTB]. Inland states must also grapple with the sizeable expense of transporting seawater inland. They can opt to use local brackish (salty) water sources, instead, but then they face a different problem: how to dispose of the byproduct, a concentrated salt solution that coastal sites have the luxury of pumping back into the ocean (a practice that remains controversial in environmental circles). Zero Liquid Discharge (ZLD) plants are one way out, but they drive up the energy costs of what is already an energy-intensive process [source: NRC-WSTB].

Is desalination cost-effective? The answer probably depends on where you live. Given the high costs of freshwater importation and reclamation, desalinating seawater is an increasingly attractive option for water-stressed areas. The potential for desalination is limited mostly by social, political, environmental and economic considerations, which vary from place to place. Any way you look at it, the rising tide of desalination seems likely to remain a growing part of our water portfolio for years to come.


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