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Water Part III: Get the Salt Out

“If we could ever competitively, at a cheap rate, get freshwater from salt water, that would be in the long-range interest of humanity and would dwarf any other scientific accomplishments” – President John F. Kennedy April 1961
I love this quote from President Kennedy.  As an engineer and a “science-guy” I enjoy the notion that the president who led the charge to put a man on the moon also appreciated the significance of something as seemingly mundane as removing salt from water.
If President Kennedy were alive today, he would see that his focus on water supply was spot on.  The world is currently suffering from what can only be described as a serious water crisis.  Worldwide, one in six people lack access to safe drinking water and one in three do not have sufficient water for adequate hygiene and sanitation.  According to approximately 3.6 million people die each year due to lack of clean water.  Shortages of clean water impose a disproportionate burden on women and girls who are usually assigned the laborious and time-consuming task of fetching water.  In many parts of the world, this prevents girls from attending school, impairing their chances for a more prosperous life.
Under the leadership of Dr. Jamie Bartram, UNC has devoted significant resources and focus to this critical global issue with the formation of the Water Institute.  In addition, UNC has recently declared a new two-year, campus-wide academic theme focused on water, Water in Our World.  Last week Dr. Bartram was tapped to join the U.S. Water Partnership, a group recently chartered by U.S. Secretary of State, Hillary Clinton.  Go UNC!
Given the critical importance of adequate supply, all possible efforts to increase access to clean water must be explored. Last week in “Water Part II:  Water, Water Everywhere, Nor Any Drop to Drink” I explained the harm that salt water can cause to land plants and animals.  This week I am addressing the changes of getting the salt out.  It’s simple but expensive.
There are two commercial-scale desalination technologies, distillation and reverse osmosis.  Distillation, which accounts for about 85% of all desalinated water, involves boiling the water into steam, which leaves the salt behind, and then condensing the steam back to water.  Note that this process mirrors how nature makes fresh water through evaporation followed by rain.  Both the addition of heat to boil the water and the refrigeration needed to condense the steam require a tremendous amounts of energy.  In reverse osmosis, salt water is pumped at extremely high pressure through a membrane which filters out the salt.  Reverse osmosis uses 30% less energy than distillation, but has lower throughput which offsets the energy savings.  The energy requirements associated with both of these desalination technologies increase the cost of supplying domestic and agricultural water by a factor of 4 or 5. 
There are approximately 12,000 commercial desalination plants around the world, with 70% of them located in the Middle East. All together these facilities provide only a fraction of 1% of the world’s fresh water supply.  So you can see that we are still far from President Kennedy’s vision.
Given the energy intensive nature of desalination, it will never be able to provide a significant portion of the world’s water requirements.  The other sources of fresh water are rain and subsurface aquifers (which I will be addressing next week in part IV).  In part V, the final installment, I will discuss the necessity and the process for the world’s population to come back in to balance with our annual allotment of rain.  Please be advised that part V is a bit maudlin.
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