Desalination

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Desalination refers to any of several processes (e.g. reverse osmosis) that remove the excess salt and other minerals from water in order to obtain fresh water suitable for animal consumption or irrigation, and if almost all of the salt is removed, for human consumption, sometimes producing table salt as a by-product. Desalination of brackish water is already commonplace in the U.S., where it is used to meet treaty obligations for river water entering Mexico. Indeed, desalination has spread into use in over a hundred countries, with Saudi Arabia accounting for about 24% of total world capacity. Kuwait built the world's first large-scale desalination plant in the 1960s. Kuwait's energy reserves are so great that Kuwait is unique in using desalinated water for agriculture. The world's largest desalination plant is the Shoaiba Desalination Plant in Saudi Arabia. It uses multi-stage flash distillation, and it is capable of producing 150 million cubic meters of water per year. [1]

1 Methods
2 Considerations
3 Experimental techniques and other developments
4 See also
5 External links

[edit] Methods

Image:Shevchenko BN350 desalinati.jpg

Distillation
1. Multi-stage flash (MSF)
2. Multiple-effect (MED|ME)
3. Vapor compression (VC)
4. Evaporation/condensation
Membrane processes
1. Electrodialysis reversal (EDR)
2. Reverse osmosis (RO)
3. Nanofiltration (NF)
4. Forward osmosis (FO)
5. Membrane distillation (MD)
Freezing
Geothermal
Solar humidification (HDH, MEH)
Methane hydrate crystallisation
High grade water recycling

As of July 2004, the two leading methods were Reverse Osmosis (47.2% of installed capacity world-wide) and Multi Stage Flash (36.5%). (Source: 2004 IDA Worldwide Desalting Plants Inventory Report No 18; published by Wangnick Consulting: [2].

Desalination of ocean water is common in the Middle East and the Caribbean, and is growing fast in the USA, North Africa, Spain, Australia and China. It is also used on ships, submarines and islands.

The traditional process used in these operations is distillation — essentially the boiling of water at less than atmospheric pressure, and thus a much lower temperature than normal. Due to the reduced temperature, energy is saved.

In the last decade, membrane processes have grown very fast, and Reverse Osmosis (R.O.) has taken nearly half the world's installed capacity. Membrane processes use semi-permeable membranes to filter out dissolved material or fine solids. The systems are usually driven by high-pressure pumps, but the growth of more efficient energy-recovery devices has reduced the power consumption of these plants and made them much more viable; however, they remain energy intensive and, as energy costs rise, so will the cost of R.O. water.

Forward Osmosis (F.O.) employs a passive membrane filter that is hydrophylic (attracts water), slowly permeable to water, and blocks a portion of the solutes. Water is driven across the membrane by osmotic pressure created by food grade concentrate on the clean side of the membrane. Forward osmosis systems are passive in that they require no energy inputs. They are used for emergency desalination purposes in seawater and floodwater settings.

[edit] Considerations

[edit] Cogeneration

There are circumstances in which it may be possible to use the same energy more than once. With cogeneration this occurs as energy drops from a high level of activity to an ambient level. Distillation processes, in particular, can be designed to take advantage of co-generation. In the Middle East and North Africa, it has become fairly common for dual-purpose facilities to produce both electricity and water. The main advantage being that a combined facility can consume less fuel than would be needed by two separate facilities.

[edit] Economics

A number of factors determine the capital and operating costs for desalination: capacity and type of facility, location, feed water, labor, energy, financing and concentrate disposal. Desalination stills now control pressure, temperature and brine concentrations to optimize the water extraction efficiency. Nuclear-powered desalination might be economical on a large scale, and there is a pilot plant in the former USSR.

[edit] Environmental

Regardless of the method used, there is always a highly concentrated waste product consisting of everything that was removed from the created "fresh water". These concentrates are classified by the U.S. Environmental Protection Agency as industrial wastes. With coastal facilities, it may be possible to return it to the sea without harm if this concentrate does not exceed the normal ocean salinity gradients to which osmoregulators are accustomed. Reverse osmosis, for instance, may remove 50% or more of the water, doubling the salinity of ocean waste.

The hypersaline brine has the potential to harm ecosystems, especially marine environments in regions with low turbidity and high evaporation that already have elevated salinity. Examples of such locations are the Persian Gulf, the Red Sea and, in particular, coral lagoons of atolls and other tropical islands around the world. Because the brine is more dense than the surrounding sea water due to the higher solute concentration, discharge into water bodies means that the ecosystems on the bed of the water body are most at risk because the brine sinks and remains there long enough to damage the ecosystems. Careful re-introduction avoids this problem.

The benthic community cannot accommodate such an extreme change and many filter feeding animals are destroyed when the water is returned to the ocean. It is more of a problem as you move inland, as one needs to avoid ruining existing fresh water supplies such as ponds, rivers and aquifers. As such, proper disposal of "concentrate" needs to be investigated during the design phase.

[edit] Experimental techniques and other developments

In the past many novel desalination techniques have been researched with varying degrees of success. Some are still on the drawing board now while others have attracted research funding. For example, to offset the energetic requirements of desalination, the U.S. Government is working to develop practical Solar Desalination. This development has much potential, since the regions in which desalination is most needed often have an abundance of solar energy. The problems with solar are energy density and time limited exposures. Solar will never be able to meet the needs of large users, the square footage of the panels needed is simply too great according to Adj. Professor Ronald A. Newcomb. Also, this method may be costly because of the large need for heat energy.

Other approaches involve the use of geothermal energy. An example would be the work being done by SDSU CITI International Consortium for Advanced Technologies and Security. [3] From an environmental and economic point of view, in most locations geothermal desalination can be preferable to using fossil groundwater or surface water for human needs, as in many regions the available surface and groundwater resources already have long been under severe stress.