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| Dubai's massive desalination plant |
Approximately 97% of Earth’s water is either sea water or brackish water (a mixture of salt and fresh water). Humans and other animals cannot drink salt water and to do so can bring on dehydration (the loss of the body’s existing water) that can lead to illness and in extreme cases, death.
Desalination is the process of removing salt from seawater to make it drinkable (drinkable water is also called potable water) or to make it useable for irrigation (watering fields and crops).
Natural desalination occurs everyday as a part of the world’s hydrologic cycle. As salt water from the oceans evaporates (changes from liquid to gas), the salt is left behind and the water that moves into the atmosphere is fresh water. Thus, the water in clouds that eventually falls as rain is fresh water.
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Salt can also be removed from water by a series of processes known as manipulated desalinization, desalting, or saline water reclamation (salt water reclamation). All of these man made processes are expensive in terms of how much money and energy they each require to produce a gallon of water.
Salt is composed (made up) of sodium and chorine atoms (the smallest particles of each element). Seawater contains the same kind of salt (sodium chloride) used everyday on food and in cooking. In addition, seawater also contains many small particles of the chemicals such as calcium and magnesium that also form chemicals called salts. Some of these salts come from chemicals used by industry, others from natural processes.
Between three and four pounds out of every 100 pounds of atoms in saltwater (the hydrogen and oxygen atoms that together form water plus the atoms of all chemicals dissolved in the water) are combined into salts. Public health officials who test water use a different scale and label the salt in water as parts (particles) per million (ppm).
Using this scale, seawater contains 35,000 ppm of dissolved salts. Brackish water typically contains less than half the amount of salt that is found in sea-water, about 5,000-10,000 ppm of salt. Safe drinking water for humans, and water for most types of crops, must contain only 5,000-10,000 ppm of salt.
Methods to remove salt
There are several ways to remove salt from seawater and the method used is determined by the intended use of the water. Salt can also be removed from groundwater contaminated with saltwater.
For example, if the water is to be drinkable then more salt needs to be removed than if the water is to be used for crops. Cost is also an important consideration because the more salt that needs to be removed, the greater the cost.
Stories from ancient Greece tell of how sailors obtained fresh water by first removing salt from seawater by evaporating the seawater, and then condensing (changing from a gas to a liquid) the air carrying the evaporated water.
This process, because it uses the heat of the Sun is now called solar distillation. Solar distillation is similar to the natural process of the heat of the Sun evaporating water from the oceans that later condense into fresh water drops in clouds. When the water evaporates, only fresh water moves into the surrounding air because the salts are too heavy and are left behind in the ocean.
Only fresh water went into the surrounding air (for example, the air over a bucket of sea-water). As the air came into contact with cooler sheets or sails spread over the bucket, drops of fresh water would form and could then be collected in a separate bucket.
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Other, but far less efficient ways to obtain fresh water included the use of filtering seawater. One method of filtering included the use of a wool wick (a length of rope made of wool) to absorb (siphon) the water. The salts were trapped in the wool and fresh water dripped out. Water was also poured through sand or clay to remove salts.
By the fourth century (400 a.d.) onward, people obtained fresh water by boiling salt water and using sponges to absorb the fresh water in the air above the pot. The first scientific paper on desalting was published by Arab scientists in the eighth century.
The first desalination efforts for industry started in 1869, as land-based steam distillation plants were established in Britain to prepare fresh water for ships going to sea.
Methods of distillation and filtering are still the most widely used methods of desalination used in most areas of the world.
Other modern techniques use complex machines that change the temperature at which water boils away by lowering the pressure of the atmosphere over a sealed container of water.
This methods reduces the formation of crusty white salts, which appear similar to the sticky white powder found at the bottom of a pan from which all water has boiled away. These crusty white salts can clog machinery and make it more difficult to heat water. In industry, the crusty residue is called scaling, and the method of lowering the temperature at which water boils is called multistage distillation (multiple stages of distillation).
The goal of multistage distillation is to reduce the boiling point of water to a temperature where it will still boil (evaporate) into a collection flask, but that it will not form a crusty salt residue.
The residue forms at about 160°F (71°C) so the goal is to reduce the boiling point of water to less than 160°F In some desalinization plants, distilled water is also filtered of other pollutants to make it ready to drink.
A process called reverse osmosis can also be used to remove salt from water. Water molecules are forced through a plastic membrane (a barrier) with very small pores (openings) that allow the passage of water, but not of salts.
