Page - 4 - in Water, Energy, and Environment - A Primer

Multi-stage flash (MSF) distillation occurs in several successive stages, each at a progressively lower pressure. The feed water is first heated at high pressure, and the lower pressures in successive stages result in a sequence of sudden evaporation and condensation. In multi-effect distillation (MED), which uses the same principle of evaporation and distillation at progressively lower pressures, the water vapor of each vessel (‘effect’) serves as the heat source for the next vessel. Another variation is vapor compression distillation (VCD), where mechanical compression is used to generate the heat forevaporation. In addition to RO, other membrane technologies are electrodialysis (ED) and electrodialysis reversal (EDR), both of which predate RO. Both are voltage-driven processes which utilize the fact that salts dissolved in water are electrically charged ions (either positive or negative). For example, in a saline solution containing dissolved sodium chloride, the sodium ion has a positive charge (cation) and the chloride ion a negative charge (anion). Applying a voltage across the ED membrane allows either a cation or an anion to pass through the membrane, leaving dilutedwater behind. InEDR the polarity of the driving voltage is reversed several times an hour. Once the desired water quality is achieved and the desalinated water is removed, the concentrate channels are flushed and clean water production resumed. The RO process is a variation on osmosis, the phenomenon in which water with a low-salt concentration passes naturally through a semi-permeablemembrane into a regionof higher salt concentration.Byapplyingpressure to the solutionwithahigher salt concentration, the water is forced to flow in a reverse direction through the membrane, leaving the salt behind. The required pressures range from about 150 pounds per square inch (psi), roughly equivalent to 1035kPa, for low-salinity brackish water up to 800–1000psi (5515–6895kPa) for high- salinity seawater. Water,Energy, andEnvironment–APrimer4