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

Desalination has come a long way in the 2,400 years or so since people boiled salt water and collected the steam in sponges. Yet, the most widely used method is still based on the same principle: distillation. Essentially, distillation artificially mimics what occurs in nature: Heated water evaporates to become water vapor, leaving salts and impurities behind, and then condenses as it cools to fall as freshwater (aka rain). Distillation plants refine and speed up this process by applying artificial heating and cooling and by evaporating water under lower air and vapor pressure, which significantly reduces its boiling point. This method requires a great deal of energy, however, so distillation plants are often located alongside power plants, where waste heat is available to bring the water up to a volatile temperature [source: Water-technology.net].

Another method, reverse osmosis (RO) desalination, uses pressure to force water through filters, straining out other substances at the molecular level. Developed in the 1960s, the process became feasible on a commercial scale in the 1970s, ultimately replacing distillation as the method used in most new desalination facilities, in part because it requires less energy [source: NRC-WSTB]. Besides removing salt, both methods remove virtually every mineral and most biological or organic chemical compounds, producing water that is safe to drink, far exceeding federal and state drinking water standards [source: Maloni].

So how widespread is desalination? Specific figures are elusive, as new plants are constantly being added and little data exists concerning plants that have shut down. It's also tricky to separate counts of distillation versus RO plants. However, a good ballpark figure is 8,000 RO seawater desalination plants globally producing a total of about 10 billion gallons (37,854,117 cubic meters) of drinking water each day, with older distillation plants still outnumbering RO [source: Maloni].

The largest users of desalination globally in terms of volume capacity are (in descending order) Saudi Arabia, United Arab Emirates, United States, Spain, Kuwait and Japan [source: NRC-WSTB]. Desalination provides 70 percent of drinking water in Saudi Arabia [source: NRC-WSTB].

Cruise ships, submarines and ships of war have been using desalination for decades. One impressive example, the aircraft carrier U.S.S. Carl Vinson, can make some 400,000 gallons (1,514 cubic meters) of its own freshwater every day, half of which is excess water that at press time is being used to aid disaster relief in Haiti [source: Padgett].

As much as desalination has increased over the years, it is still just a drop in the bucket. In this next section, we'll look at what's holding us back from a full-on sea change in freshwater supply.

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: 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: 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.