Page - 3 - in Water, Energy, and Environment - A Primer

How does one remove salt from highly saline or brackish water to produce drinkable (potable) water? Quite a few technologies exist to do this separation, the oldest being sun- heated water that evaporates and is then condensed on a cold surface, leaving the salt behind. (Note: this also describes the first stage of the Earth’s hydrologic cycle, in which sun-heated water evaporates from the oceans andother bodies ofwater into the atmosphere.) References to this process of evaporation and condensation, known as distillation, can be found in historical records going back centuries. Variations are widely used at sea today and, in the past, helped keep many early explorers and traders alive during long ocean trips. Amodern-day example is a United States nuclear-powered aircraft carrier that uses waste heat from its nuclear reactor to desalinate 400,000 gallons of seawater per day. The technologies used to perform this separation can be categorizedbroadly as either thermalormembrane technologies. The most popular today is reverse osmosis (RO) in which a pressurized streamof salinewater is forced throughamembrane which allows the smallwatermolecules to pass through, but not the various salts found in brackish water or sea water. Several stages of such separation can lead to freshwater at or below the 1000ppm level of salt. Other major desalination technologies are listed inTable1.1. Table1.1 Desalination technologies. ThermalTechnology • multi-stage flashdistillation (MSF) • multi-effect distillation (MED) • vapor compressiondistillation (VCD) MembraneTechnology • reverseosmosis (RO) • electrodialysis (ED) • electrodialysis reversal (EDR) Waterand itsglobal context 3