DEFINITION OF DESALINATION

A group of water treatment processes that

remove salt from water. It is becoming increasingly more

important as freshwater supplies dwindle and population

grows on the planet, yet desalination is exorbitantly expensive

and cannot be afforded by many countries. Only 6 percent

of the water on the planet is freshwater, and of this 27

percent is locked up in glaciers and another 72 percent is

groundwater. The remaining 1 percent of the freshwater on

Earth is becoming rapidly polluted and unusable for human

consumption.

There are a number of different processes that can

accomplish desalination of salty water, whether it comes from

the oceans or the ground. These are divided broadly into

thermal processes, membrane processes, and minor techniques

such as freezing, membrane distillation, and solar

humidification. All existing desalination technologies require

energy input to work, and they end up separating a clear

fraction or stream of water from a stream enriched in concentrated

salt that must be disposed of, typically by returning it

to the sea.

Thermal distillation processes produce about half of the

desalted water in the world. In this process, saltwater is heated

or boiled to produce vapor that is then condensed to collect

freshwater. There are many varieties of this technique,

including processes that reduce the pressure and boiling temperature

of water to effectively cause flash vaporization,

using less energy than simply boiling the water. The multistage

flash distillation process is the most widely used around

the world. In this technique, steam is condensed on banks of

tubes that carry chemically treated seawater through a series

of vessels known as brine heaters with progressively lower

pressures, and this freshwater is gathered for use. A technique

known as multi-effect distillation has been used for industrial

purposes for many years. Multi-effect distillation uses a series

of vessels with reduced ambient pressure for condensation

and evaporation, and it operates at lower temperatures than

multistage flash distillation. Saltwater is generally preheated

and then sprayed on hot evaporator tubes to promote rapid

boiling and evaporation. The vapor and steam are then collected

and condensed on cold surfaces, whereas the concentrated

brines are run off. Vapor compression condensation is

often used in combination with other processes or by itself

for small-scale operations. Water is boiled, and the steam is

ejected and mechanically compressed to collect freshwater.

Membrane processes operate on the principle of membranes

being able to selectively separate salts from water.

Reverse osmosis, commonly used in the United States, is a

pressure driven process in which water is pressed through a

membrane, leaving the salts behind. Electrodialysis uses electrical

potential, driven by voltage, to selectively move salts

through a membrane, leaving freshwater behind. Electrodialysis

operates on the principle that most salts are ionic and carry

an electrical charge, so they can be driven to migrate toward

electrodes with the opposite charge. Membranes are built that

allow passage of only certain types of ions, typically either

positively (cation) or negatively (anion) charged. Direct current

sources with positive and negative charge are placed on

either side of the vessel, with a series of alternate cation and

anion selective membranes placed in the vessel. Salty water is

pumped through the vessel, the salt ions migrate through the

membranes to the pole with the opposite charge, and freshwater

is gathered from the other end of the vessel. Reverse osmosis

only appeared technologically feasible in the 1970s. The

main energy required for this process is for applying the pressure

to force the water through the membrane. The salty feed

water is preprocessed to remove suspended solids and chemically

treated to prevent microbial growth and precipitation.

As the water is forced through the membrane, a portion of the

salty feed water must be discharged from the process to prevent

the precipitation of supersaturated salts. Presently membranes

are made of hollow fibers or spiral wound.

Improvements in energy recovery and membrane technology

have decreased the cost of reverse osmosis, and this trend may

continue, particularly with the use of new nanofiltration

membranes that can soften water in the filtration process by

selectively removing Ca+2 and Mg+2 ions.

Several other processes have been less successful in

desalination. These include freezing, which naturally excludes

salts from the ice crystals. Membrane distillation uses a combination

of membrane and distillation processes, which can

operate at low temperature differentials but require large

fluxes of saltwater. Solar humidification was used in World

War II for desalination stills in life rafts, but these are not

particularly efficient because they require large solar collection

areas, have a high capital cost, and are vulnerable to

weather-related damage.

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