Using various methods for removing salts and other impurities in effluents, brackish and other marginal water to result in fresh water. It is done through either reverse osmosis or cross-flow filtration. There are a number of desalination plants in commercial production around the world. There are plants in:

  • Israel
  • Jordan
  • Bahrain
  • Cyprus
  • United Arab Emirates
  • Puerto Rico
  • Korea
  • Cape May, NJ
  • Kuwait
  • Saudi Arabia

One of the first major Desalination plants was build in Saudi Arabia in 1989 called Assir Power and Desalination Plant. It produces over 96,000 Cubic meters per day of fresh water.

Desalinization (also called desalination and desalinisation) refers to any process that removes excessive amounts of salt and other minerals from water.

There are many methods of desalinizing water and while it’s beyond the scope of this write-up to completely describe each technique, the following list briefly explains (and likely oversimplifies) the most common. Many of these methods result in desalinization as a by-product of the process, with the main purpose being to remove all impurities, not just salt.

Reverse Osmosis
In this process, water is pumped at high pressure through a semi-permeable membrane, which allows water to pass through while catching and filtering out other particles. This system has the advantage of being quite simple and the quality of the water produced can easily be increased by passing the water through more than one layer of membrane. However, large particles in the source water can clog and/or damage the membrane.

Nanofiltration
Nanofiltration is nearly the same process as reverse osmosis, except the pores or holes in the filter membrane are slightly larger than those of the RO membrane. NF membranes are sometimes called "loose" RO membranes. Although still highly effective, NF does not provide the same level of purity as RO. Nanofiltration is cheaper and easier to maintain than RO equipment.

Multi-Stage Flash Distillation
The Multi-Stage Flash (MSF) method is based on the principle that water will boil and evaporate at relatively low temperatures in a low pressure environment. Hot water is injected into a low pressure container where it boils and evaporates nearly instantly, hence the "flash". Not all of the water will evaporate, since the process of evaporation increases the pressure in the vessel and thus increases the boiling point. The remaining water is therefore injected into another vessel, which is kept at an even lower pressure than the previous, and the water is flashed again. Anywhere between 4 and 40 different pressure stages may be used. This process uses no chemicals or membranes and consumes nearly half the energy of reverse osmosis.

Multiple Effect Distillation
MED systems utilize the simplest form of distillation: water is heated until it boils and the steam is captured and condensed back into its liquid state. However, this process attempts to increase the efficiency of the system by reusing the heat energy of the steam.

An MED system will consist of multiple boiling chambers, or “effects”. Each chamber contains source water and is heated to produce steam, which is then captured and condensed. However, only the first chamber is directly heated by an external source of energy. The second chamber is heated using the steam produced by the first chamber. That steam condenses when in contact with the cooler exterior of the second chamber and is captured as fresh water. The energy transferred into the second chamber will boil its water and that steam is used to heat a third chamber, and so on.

Multiple Effect Multi-Stage Distillation
The MEMS system is a combination of the MED and MSF methods of desalinization.

Vapour Compression Distillation
VCD is similar to MED in that the steam resulting from boiling water is reused to heat additional source water before it is condensed and collected as fresh water, although there are a few differences. Source water enters a heat exchanger where the water is heated and some of it boils into steam. The water and steam is then ejected into a separator, which separates the steam and the water.

The water is returned to the heat exchanger to be processes again. The steam is compressed, which increases its temperature. The superheated water vapour is then reused by the heat exchanger to reheat the remaining water, which now contains a higher density of sediment and impurities. The superheated water vapour then condenses and is collected as fresh water.

The source water will be reprocessed until the desired level of concentrated impurities is reached, at which point it is discharged as waste and new source water is introduced.

Freezing
When the temperature of salt water is lowered to a certain point, ice crystals of fresh water will form. Since ice is less dense than water, these ice crystals will float and can then be harvested from the top of the source water. A common disadvantage to freezing desalinization systems is that some salt is collected along with the ice crystals, requiring the salt to be washed off the ice using fresh water, reducing the efficiency of the system.

Vacuum Freezing
This method is identical to the above Freezing method, except the water is frozen in a low pressure environment and therefore less energy is required to cool the water. Cool saline water is sprayed into a vacuum chamber where some of it instantly boils and the vapour condenses and forms pure ice crystals, which can then be washed and harvested.

Membrane Distillation
This method of desalinization uses a hydrophobic membrane, which allows water vapour to pass through it but not liquid water. Hot salt water comes into contact with one side of the membrane and the warm water vapour passes through the membrane and is then cooled and condensed by fresh water on the other side. The advantage of this system is that the membrane isn’t filtering anything out of the water and therefore won’t become clogged, resulting in maintenance-free operation. Also, the temperature of the water can be significantly lower than the boiling point, thus requiring less energy.

Hydrate Crystallisation
When hydrocarbon gases such as ethane and methane come into contact with water at the right temperature and pressure, the water forms a crystal lattice that is similar to ice. The gas becomes trapped in this crystal lattice, called clathrate, making the crystal buoyant and causing it to float like ice. Once the clathrate is harvested and the correct combination of temperature and pressure is removed, the gas and water part ways, leaving fresh water and hydrocarbon gas.


Sources
http://www.coastal.ca.gov/desalrpt/dchap1.html
http://en.wikipedia.org/wiki/Desalination
http://cape.uwaterloo.ca/che100projects/sea/msf.html
http://www.discover.com/issues/oct-04/features/hydrates-hydrates-everywhere/
http://www.takenaka.co.jp/takenaka_e/techno/n02_kaisui/n02_kaisui.htm

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