With limited sources of fresh water available to growing populations, finding new sources of drinkable water is an increasingly important issue. Improved methods of water treatment are making desalination plants a more attractive method for solving this problem. There are six basic methods of desalination that are based on two distinct types of desalination technology. These technologies are known as thermal and membrane-based desalination.
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Multi-Effect Distillation -- or MED -- is a type of thermal desalination that takes advantage of the fact that the boiling point of water decreases at low levels of ambient pressure to efficiently evaporate and filter salt water. MED uses a series of evaporation chambers with successively lower pressures to efficiently separate salt and water. Heat from water vapour generated in the initial evaporation chambers is used to heat low-pressure chambers in the system to evaporate water while improving the energy efficiency of the plant.
Multi-stage flash distillation -- or MFD -- purifies salt water using evaporation chambers operating at multiple pressure levels to flash boil water. MFD-based desalination plants heat water while under high pressure and then transfer it to evaporation chambers operating at successively lower pressures. Each time the water is transferred to the next tank, a portion of the water boils rapidly due to the pressure differential between the chambers.
Desalination plants using vapour compression rely on the heat generated by the compression of water vapour to evaporate salt water. This type of system is used in conjunction with multi-effect distillation systems or as a stand-alone system. This type of desalination plant is typically used in small-scale applications.
Reverse osmosis is a desalination method that uses membrane-based technology. It separates salt and water by pushing a pressurised stream of salt water against a semi-permeable filtration membrane. The membrane allows water to pass through it but not salt, effectively splitting the pressurised stream of salt water into a stream of concentrated brine and a stream of potable water.
Nano-filtration is used to remove divalent salt ions from water. This technology is similar in application to reverse osmosis filtration. While reverse osmosis is designed to remove sodium- and chloride-based salts from the water, nano-filtration filters are designed to remove salt consisting of calcium, magnesium and sulphate ions.
Desalination plants use two types of systems based on dialysis, known as electrodialysis and electrodialysis reversal. These systems use oppositely charged electrodes to force salt anions and cations through selective membranes. This process is repeated until the majority of the salt present in the water is strained out. Plants using electrodialysis reversal periodically reverse the polarity of the electrodes in the system. This moves water in the opposite direction through the system to flush out the filtration membranes and prevent them from clogging.
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