|Fig. 1: Interior of modern reverse osmosis desalination plant. (Source: Wikimedia Commons)|
Many human activities, such as drinking, agriculture, sanitation, and electricity generation, among others, require significant amounts of water. Fortunately, in many cases, centers of population are located near sources of useable water. However, oceans, which cover more than 70 % of the earth's surface and contain 97 % of the earth's water, have salt water.  Since this salty water is unsuitable for many applications, it must be desalinated (have the salt content reduced or eliminated) before it can be used. Several years ago, more than 13000 desalination plants processed 12 billion gallons of water daily.  However, desalination tends to be energy intensive, causing significant economic and ecologic impact from desalination.
At least three principle methods of desalination exist: thermal, electrical, and pressure. The oldest method, thermal distillation, has been around for thousands of years. In thermal distillation, the water is boiled and then the steam is collected, leaving the salt behind. However, the vaporization phase change requires significant amounts of energy. More modern methods of distillation make use of various techniques such as low-pressure vessels to reduce the boiling temperature of the water and thus reduce the amount of energy required to desalinate.
A second major type of desalination utilizes electric current to separate the water and salt. Typically, electric current will be used to drive ions across a selectively permeable membrane, carrying the dissociated salt ions with it. A key characteristic of this method is that the energy requirement depends on how much salt is initially present in the water. Consequently, it is suitable for water with initial salt concentrations but too energy intensive for sea water. 
A third principle method of desalination is reverse osmosis, in which pressure is used to drive water through a selectively permeable membrane, leaving the salt behind.  Similarly to electrically-driven separation, the amount of energy required for desalination depends on the initial salt content of the water. Again, this renders reverse osmosis unsuitable for sea water purification.
Despite the innovative refinements of desalination, the energy requirements are still tremendous. State-of-the-art desalination still requires 7 to 30 kW-h of energy per 1000 gallons of desalinated water.  The energy required can vary significantly based on the type of desalination used as well as the initial salt content of the water. Thus, to desalinate 12 billion gallons of water daily, the world uses at least 84 million kW-h of energy; the actual number is likely significantly higher as many plants use older technology that requires more energy per 1000 gallons of purified water. Since a gallon of gasoline contains about 33 kW-h, the world uses the equivalent of at least 2.5 million gallons of gasoline daily to desalinate water. 
As the world's population continues to grow, existing water supplies will become increasingly insufficient. As more and more water is required to meet mankind's needs, desalination of sea water will become an increasingly important source of useable water. Any comprehensive plan addressing mankind's energy usage or ecologic impact must account for the effect of desalination; responsible development requires attention to the most energy-efficient methods of purifying water.
© Tom Parise. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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