Desalination

Desalination

Approximately 97% of Earth's water is either sea water or brackish (salt water contained in inland bodies), both of which are undrinkable by humans. Desalination is the process of removing salt from seawater. Natural desalination occurs as a part of the hydrologic cycle as seawater evaporates. Manipulated desalinization—desalting, or saline water reclamation—is an energy expensive alternative to natural desalination.

Sea water contains 35,000 parts per million (ppm) (3.5% by weight) of dissolved solids, mostly sodium chloride, calcium and magnesium salts. Brackish water typically contains 5,000-10,000 ppm dissolved solids. To be consumable, or potable, water must contain less than 500 ppm dissolved solids. The method used to reach this level depends on the local water supply, the water needs of the community, and economics. Growing populations in arid or desert lands, contaminated groundwater , and sailors at sea all created the need for desalting techniques.

In the fourth century b.c., Aristotle related tales of Greek sailors desalting water using evaporation techniques. Sand filters were also used. Another technique used a wool wick to siphon the water. The salts were trapped in the wool. During the first century a.d., the Romans employed clay filters to trap salt. Distillation was widely used from the fourth century on—salt water was boiled and the steam collected in sponges. The first scientific paper on desalting was published by Arab chemists in the eighth century. By the 1500s, methods included filtering water through sand, distillation, and the use of white wax bowls to absorb the salt. The techniques have become more sophisticated, but distillation and filtering are still the primary methods of desalination for most of the world. The first desalination patent was granted in 1869, and in that same year, the first land-based steam distillation plant was established in Britain, to replenish the fresh water supplies of the ships at anchor in the harbor. A constant problem in such a process is scaling. When the water is heated over 160°F (71°C), the dissolved solids in water will precipitate as a crusty residue known as scale. The scale interferes with the transfer of heat in desalting machinery, greatly reducing the effectiveness. Today, the majority of desalting plants use a procedure known as multistage flash distillation to avoid scale. Lowering the pressure on the sea water allows it to boil at temperatures below 160°F (71°C), avoiding scaling. Some of the water evaporates, or flashes, during this low pressure boiling. The remaining water is now at a lower temperature , having lost some energy during the flashing. It is passed to the next stage at a lower temperature and pressure, where it flashes again. The condensate of the previous stage is piped through the water at the following stage to heat the water. The process is repeated many times. The water vapor is filtered to remove any remaining brine, then condensed and stored. Over 80% of land-based desalting plants are multistage flash distillation facilities.

A host of other desalinization processes have been developed. An increasingly popular process, reverse osmosis, essentially filters water at the molecular level, by forcing it through a membrane. The pressures required for brackish water range from 250 to 400 pounds per square inch (psi), while those for seawater are between 800 to 1,200 psi. The pressure required depends on the type of membrane used. Membranes have been steadily improving with the introduction of polymers. Membranes were formerly made of cellulose acetate, but today they are made from polyamide plastics. The polyamide membranes are more durable than those of cellulose acetate and require about half the pressure. Solar distillation is used in the subtropical regions of the world. Seawater is placed in a black tray and covered by a sloping sheet of glass or plastic. Sunlight passes through the cover. Water evaporates and then condenses on the cover. It runs down the cover and is collected. The salts are left behind in the trays.

Modern desalination technology allows use of desalinated water to supplement regular drinking water. The state of Florida, for example, is using dozens of reverse-osmotic plants to treat undrinkable brackish water and then mixing the treated water with the regular water supply. The intent is to extend the local water supply. Another approach is to make traditional methods, like distillation, more economically feasible.

See also Hydrologic cycle; Saltwater encroachment