Desalination is a process whereby seawater can be converted into a usable water resource for human consumption, irrigation and industrial needs. In desalination seawater salts are removed to attain approximately 0.5 grams of salt per liter of drinking water.
The term “water” refers to varying natural dilutions that range from very fine mountain spring waters containing 0.2 grams of salt per liter, to seawater, which contains as much as 35 grams of salt per liter. Water for human consumption should neither have more than one gram per liter, nor should it be distilled water. Water with an extremely low salt content is not convenient either. Desalination can be performed through different techniques such as distillation, freezing, instantaneous evaporation or hydrate formation, but reverse osmosis is presently the most widely used method.
Reverse osmosis desalinators have a very simple mode of operation. The desalinator is placed near the sea, and pipes are used to absorb seawater and carry it to a number of channels where sand and impurities are removed by means of filters and chemicals. This is followed by desalination by reverse osmosis. Osmosis is a natural phenomenon of living cells whereby two solutions of differing concentrations will tend to equal their concentrations across a semi-permeable membrane, shifting from the most diluted solution to the most concentrated one, until a balance is attained.
Reverse osmosis consists in applying pressure to the more concentrated solution so as to obtain a larger amount of the diluted solution, i.e., the salt-free solution in our case. To do this we need a turbine-driven feed pump capable of exerting considerable pressure on the membrane, so as to enable salt separation on membrane racks and thus hydrogen bridge breaking and salt crystal reduction.
Two products emerge after water has gone through the membranes:
Salt water is returned to the pump to enable recovery of part of the energy used in desalination, and is then poured back into the sea, usually in areas with considerable water motion, such as a jetty, so that it may mix well with seawater and so as to prevent residual salt from causing environmental damage.
Drinking water is sent to a storage facility where minerals are added to make it more amenable to irrigation and to improve its quality for human consumption in accordance with Health Authority regulations.
A large number of serious water shortage problems have been solved thanks to desalination. Currently over 24 million cubic meters of desalinated water are produced every day worldwide, enough to cater to the water needs of more than 120 million inhabitants.
The geography of desalination is widespread across the globe and is particularly significant in countries such as Saudi Arabia, the United Arab Emirates and the US, as well as in Europe and more particularly Spain, where desalination has been used since the 1970s in the Canary Islands first, the Balearic Islands and the Peninsula later, and Ceuta and Melilla more recently. Desalination in Spain amounts to approximately 1,200,000 m3/day, with 700,000 m3/day pertaining to seawater and 500,000 m3/day corresponding to groundwater.
Water purification consists in removing all impurities from water, so that the resulting water may be reused for industrial and agricultural purposes. Given its usefulness, this approach to water treatment is widespread across Spain.
The purification process begins with the arrival of the wastewater to the treatment plant, where, first, water is passed through a number of filters designed to remove all waste substances so that subsequent purification processes may be carried out successfully.
Then in a series of pools (decanters and reactors) detritus are separated by physical and biological means. The resulting sludge is absorbed and taken to treatment plants where they are transformed into fertilizers.
At the end of the process the water is ready for its agricultural and industrial reuse.