Desalination

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Humans cannot drink saline water, but, saline water can be made into freshwater, for which there are many uses. The process is called "desalination", and it is being used more and more around the world to provide people with needed freshwater. 

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Thirsty? How 'bout a cool, refreshing cup of seawater?

A floating solar still to desalinate seawater

A floating solar still is used to desalinate small amounts of seawater, using evaporation and condensation. (Credit: Landfall Navigation)

No, don't take us literally! Humans cannot drink saline water. But, saline water can be made into freshwater, which is the purpose of this portable, inflatable solar still (it even wraps up into a tiny package). The process is called desalination, and it is being used more and more around the world to provide people with needed freshwater. Most of the United States has, or can gain access to, ample supplies of freshwater for drinking purposes. But, freshwater can be in short supply in many parts of the Nation and world. And, as the population continues to grow, shortages of freshwater will occur more often, if only in certain locations. In some areas, salt water (from the ocean, for instance) is being turned into freshwater for drinking.

The "simple" hurdle that must be overcome to turn seawater into freshwater is to remove the dissolved salt in seawater. That may seem as easy as just boiling some seawater in a pan, capturing the steam and condensing it back into water (distillation). Other methods are available but these current technological processes must be done on a large scale to be useful to large populations, and the current processes are expensive, energy-intensive, and involve large-scale facilities.

What makes water saline?

What do we mean by "saline water?" Water that is saline contains significant amounts (referred to as "concentrations") of dissolved salts. In this case, the concentration is the amount (by weight) of salt in water, as expressed in "parts per million" (ppm). If water has a concentration of 10,000 ppm of dissolved salts, then one percent of the weight of the water comes from dissolved salts.

Here are our parameters for saline water:

  • Freshwater - Less than 1,000 ppm
  • Slightly saline water - From 1,000 ppm to 3,000 ppm
  • Moderately saline water - From 3,000 ppm to 10,000 ppm
  • Highly saline water - From 10,000 ppm to 35,000 ppm

By the way, ocean water contains about 35,000 ppm of salt.

 

A view across a reverse osmosis desalination plant.

A view across a reverse osmosis desalination plant in Barcelona, Spain. (Credit: James Grellier)

The worldwide need for freshwater

The scarcity of freshwater resources and the need for additional water supplies is already critical in many arid regions of the world and will be increasingly important in the future. Many arid areas simply do not have freshwater resources in the form of surface water such as rivers and lakes. They may have only limited underground water resources, some that are becoming more brackish as extraction of water from the aquifers continues. Solar desalination evaporation is used by nature to produce rain, which is the main source of freshwater on earth.

Another method: Reverse osmosis

Another way saline water is desalinized is by the "reverse osmosis" procedure. In most simplistic terms, water, containing dissolved salt molecules, is forced through a semipermiable membrane (essentially a filter), in which the larger salt molecules do not get through the membrane holes but the smaller water molecules do. 

Reverse osmosis is an effective means to desalinate saline water, but it is more expensive than other methods. As prices come down in the future the use of reverse osmosis plants to desalinate large amounts of saline water should become more common.

Desalination is not modern science

Distillation desalination is one of mankind's earliest forms of water treatment, and it is still a popular treatment solution throughout the world today. In ancient times, many civilizations used this process on their ships to convert sea water into drinking water. Today, desalination plants are used to convert sea water to drinking water on ships and in many arid regions of the world, and to treat water in other areas that is fouled by natural and unnatural contaminants. Distillation is perhaps the one water treatment technology that most completely reduces the widest range of drinking water contaminants.

In nature, this basic process is responsible for the water (hydrologic) cycle. The sun supplies energy that causes water to evaporate from surface sources such as lakes, oceans, and streams. The water vapor eventually comes in contact with cooler air, where it re-condenses to form dew or rain. This process can be imitated artificially and more rapidly than in nature, using alternative sources of heating and cooling.

do-it-yourself solar still schematic

You can try this at home!

  • Dig a pit in the ground
  • Place a bowl at the bottom of the pit that will be used to catch the condensed water
  • Cover the pit loosley with a plastic sheet (you can use stones or other heavy objects to hold it in place over the pit)
  • ​​​​​​​Be sure that the lowest part of the plastic sheet hovers directly over the bowl
  • Leave your water "trap" overnight and water can be collected from the bowl in the morning

(Credit: Daniele Pugliesi)

Your own personal desalination plant

Remember looking at the picture at the top of this page of a floating solar still? The same process that drives that device can also be applied if you find yourself in the desert in need of a drink of water.

The low-tech approach to accomplish this is to construct a "solar still" which uses heat from the sun to run a distillation process to cause dew to form on something like plastic sheeting. The diagram to the right illustrates this. Using seawater or plant material in the body of the distiller creates humid air, which, because of the enclosure created by the plastic sheet, is warmed by the sun. The humid air condenses water droplets on the underside of the plastic sheet, and because of surface tension, the water drops stick to the sheet and move downward into a trough, from which it can be consumed.

Some desalination facts

  • It is estimated that some 30% of the world's irrigated areas suffer from salinity problems and remediation is seen to be very costly.
  • According to the International Desalination Association, in June 2015, 18,426 desalination plants operated worldwide, producing 86.8 million cubic meters per day, providing water for 300 million people. This number increased from 78.4 million cubic meters in 2013, a 10.71% increase in 2 years.
  • The most important users of desalinated water are in the Middle East, (mainly Saudi Arabia, Kuwait, the United Arab Emirates, Qatar and Bahrain), which uses about 70% of worldwide capacity; and in North Africa (mainly Libya and Algeria), which uses about 6% of worldwide capacity.
  • Among industrialized countries, the United States is one of the most important users of desalinated water, especially in California and parts of Florida. The cost of desalination has kept desalination from being used more often.

 

Some of this information came from the Water Education Foundation and from the Corpus Christi TAMU-CC Public Administration.