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Desalination

Desalination of water (also called desalination or desalinization) is a process that provides the fresh water ( drinking or, more rarely because of cost, usable for irrigation ) from a brackish or saline ( salt water in particular). Despite the name, this does not remove the salt from the water , but, conversely, to extract freshwater.

Very generally, it is easier and cheaper to seek sources of fresh water to be treated (surface water such as lakes and rivers or groundwater) that desalinate seawater However, in many regions of the world, fresh water sources are unavailable or become inadequate in view of population growth or industrial production.

On the other hand, it is often profitable to combine the production of fresh water with another activity (including the production of energy , as the steam available at the output of the turbines, and lost in a factory standard, is reusable in a called thermal desalination plant or operating on the principle of evaporation) Sea water is salty at about 35 gl ^-1 in general. In regions like the Persian Gulf, the salinity reached 42 gl ^-1. To separate the salt must be of a purely theoretical and without loss of energy (desalination isentropic), about 563 Wh.m ^-3^{Different desalination systems}

The most frequently used systems are:

Reverse osmosis : This technique "membrane" is based on a pressurized ultrafiltration through membranes with pores are 'holes' so small that even the salts are retained. This technique, growing (average energy cost ( 4-5 kWh / m) ) is a proven system has shown its reliability.
Multi-effect distillation. This system provides a very pure water, high energy cost ( 15 kWh / m).
Multistage flash or flash system: it is used in the Gulf countries, providing water with a residual salt is not negligible energy cost rate ( 10 kWh / m).
Vapor compression: it provides clean water; average energy cost ( 5 kWh / m).
Distillation vacuum: this system based on the fact that the temperature of evaporation depends on the pressure provides a very pure water, low energy cost ( 2 to 3 kWh / m). It is used for smaller units.
Distillation by solar furnace : the solar oven concentrates in a small area of the heat rays through a parabolic mirror, to bear the high temperature component that contains the water to be evaporated.
Electrolysis ( Electrodialysis ?) is applied electric current which migrating ions to electrodes. System very cost effective for low concentrations, the energy put into play depends on the salt concentration.

In any case, desalination produces brine that must be discarded, which is not a problem aboard Wed where the current is important, but may be inland, and in some ecosystems such as lagoons , bays, lagoons, mangroves ...

Mobile desalination units: The city of Yokohama in June 2009 introduced a truck equipped with a device with reverse osmosis membrane , able to desalinate sea water or fresh drinkable water after a river or a lake. Fresh water is purified 15 times faster than ozone, by microbes (the same as those that produce natto , fermented bean popular in Japan). Treatment with chlorine and the ozone enters the process. The energy needed for desalination is fully furnished with a small wind turbine and solar panels that power also batteries for a range of 24 hours. 3 liters of seawater to provide drinking a liter. The truck can provide water to 3,500 people per day. The brine can be used for thalassotherapy or human consumption ^,

Economics of desalination

Desalination of seawater is an important issue for the future of regions arid. At a cost of production could fall to about 0.5 per cubic meter for recent projects (reverse osmosis and all charges included: operating cost, depreciation of plant, profit for the operator ^, ...), it is possible to solve the problems of lack of potable water in many countries. In the case of a use for human consumption, desalination of seawater is a technique now reliable and less expensive than the so-called recycling wastewater. It becomes profitable even in developed countries do not generally lacking water in certain specific situations (eg tourist islands).

Thus, this activity is growing strongly. The annual installed capacity increasing on average by over 10% per year. The technique called thermal (evaporative) accounted for a few years ago the main technology used, but reverse osmosis, due to increased reliability and low power consumption (4-5 kWh / m) can cost very low, which gives it today 50% market share.

Some regions, particularly the Canary Islands , and many others depend totally on these technologies for their drinking water.

The plant of Ashkelon in Israel produces 320 000 cubic meters / day, for an estimated 250 liters per capita per day consumption, the plant can meet the drinking water needs of more than one million people. This country is pursuing a goal of increasing the quantity of drinking water produced by desalination of 25% per year, or 57 million cubic meters more to pass the milestone of 300 million cubic meters of desalinated water by the end of 2010 .

However, desalination for subsistence agriculture is not profitable yet, by far the price of water used greatly exceeds the value of agricultural production would make it possible. For example, the production of one tonne of wheat, worth about 100 , requires approximately 1000 sqm of land irrigated by about 500 mm rainfall, 500 m ³ of water, costing not less than 250 in the best case. But some cultures at a very high value added may bear such costs.

Pharaonic projects have been proposed to desalinate water, especially with nuclear plants , to the agriculture.

The disadvantages of desalination

Energy costs still high
Rejection of brines concentrated to twice the natural salinity in the sea or injected into the soil;
Rejection of warm waters in the case of distillation;
Use of chemicals to clean the membranes (chlorine);
Traces of copper escaped facilities;
No specific legislation concerning the potability of water from these treatments.

Distribution

Countries using the technology of desalination of sea water are mainly located in the Middle East ( United Arab Emirates : Jebel Ali facility, the largest in the world with a capacity of about 900 000 m ³ / day plant in Fujairah , Saudi Arabia : Plant Jubail ). These states have significant fossil fuel resources mainly use the method of spraying.

Other countries have more developed reverse osmosis. Israel : plant of Ashkelon ; Syria : plant of Amman ; Tunisia : Plant Djerba ), Latin America ( Mexico , Chile : plant Minera Escondida ), in Spain (factory of Carboneras , Balearic Islands : Palma Baya plant). The United States are placed in second position behind the Middle East for filtering brackish water . A desalination plant of seawater was built in 1993 in Tampa Bay in Florida to address the lack of water .

The Algeria has built across its territory and plans to build one of the largest desalination of seawater in the world to Magtaa ( Oran ) with a capacity of 500,000 m / day. The system manufacturer will be the Singaporean group See also