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It involves a large structure located close to the sea which has several tanks around it. The closer it is to the coast, the less energy will be required to pump the water. This is why it is so important to find an appropriate site. This technology, although controversial in some circles, is a means of acquiring good quality water in areas which are affected by water shortages.

Reverse osmosis 

In 1965, the first Spanish desalination plant was opened in Lanzarote (Canary Islands). In those days, it used evaporation-based technology, which is now barely used, since a few years later this was replaced by methods based on reverse osmosis. Nowadays, Spain is one of the countries with the largest number of desalination plants and its construction companies are world leaders in this technology.

Osmosis is a natural process through which plants absorb salts from soil. Reverse osmosis involves precisely the opposite, desalinating or removing the salt, from water in this case. Technological progress has meant that the cost of building this kind of facility, which is designed to operate for about a decade, has dropped.

Parts of the plant

All desalination plants have collection wells, usually by the sea or next to a salt water lake. A pump sucks the water from them, which is subsequently passed through several filters in order to remove the sand and other impurities. Once it is stored in a tank, the water is disinfected by means of a chemical treatment. This process removes, in not much more than a minute, any organic and inorganic contamination. 

Then the water is separated from the salt. This is achieved by using high pressure that requires significant energy consumption, which is necessary in order to drive the fluid through semi-permeable membranes, a kind of micro-filter. In order to improve the energy efficiency, desalination plants usually install cogeneration systems along with the plant itself.

Approximately 45 litres of fresh water are obtained from every 100 litres of sea water. The percentage increases significantly if the water comes from underwater sources or rivers. Furthermore, some three kilogrammes of salt are obtained from every 100 litres of sea water. Finally, once it has been desalinated the water goes through a remineralisation system –by means of a dose of calcium hydroxide – and it is disinfected once again.

The time it takes varies depending on the distances between the collection and storage points. It can be said that it takes about 60 minutes from when the water is collected in the well to when it is delivered to the tank. Desalination plants can work continually, 24 hours per day.

The saline waste, which also contains certain chemical products becomes pollutant waste if it ends up in the soil or in the sea close to the coast, since it can damage the ecosystems. Consequently, it is returned to the sea, but it is taken via pipes to a point far from the coast. People who are against desalination plants highlight the environmental impact that this process implies, since removing salt represents a direct attack on marine biodiversity. 

Desalinated water was even labelled "bottled electricity" due to the excessive energy consumption that desalination initially represented. Nevertheless, the energy consumption per cubic metre desalinated has reduced significantly. In fact, in order to reduce the high energy consumption of these installations, technology which can create energy from salt water itself is starting to be used: a device called a PE (pressure exchanger) transfers energy directly from saline waste to the feed stream. 

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