Desalination is the process of converting seawater into freshwater. Seawater contains about 3.5% salt, and one cubic meter of sea water contains around 40kg of salt. To produce ‘freshwater’ the salt content must be reduced to less than 0.05%.

As well as being costly in terms of energy, desalination also has environmental impacts: discharge of very salty water may locally kill plants and animals intolerant to salt and alter the habitat and local ecosystems.

Figure 4.4.1 Flow representation of desalination

Figure 4.4.1 sourced from The ImpEE Project, The Cambridge-MIT institute. The ImpEE wesbite is designed as an educational resource. It may be reproduced, modified and used freely for educational purposes
http://www-g.eng.cam.ac.uk/impee/?section=topics&topic=water&page=slideshow
 

The reverse osmosis method, the most widely used for desalination of sea water, requires large amounts of energy in order to push source water through a membrane at a pressure of around 7,000 bar (100,000psi)[see reference 15]. The high-tech membranes are themselves expensive, although costs are reducing as the technology matures and the market grows. They require cleaning with chemicals, which then contribute to the problem of waste disposal together with the excessively salty wastewater produced by the process.

Only those countries which are water-poor but energy-rich, such as oil-producing nations in the Middle East, have the necessary combination of “desperation, wealth, and cheap energy”[15] that make desalination worth consideration. Desalination is currently limited to locations with a specific concentration of factors. Desalination plants on a large scale have high capital costs and high running costs. Many of the existing plants have been built adjacent to coastal power plants in order to consolidate impact and reduce costs associated with power transmission and water intake pipe work[see reference 15].

It is possible to purify salty or brackish water using the power of the sun, through a process known as Solar Distillation. A technical brief of the process can be found here: http://practicalaction.org/solar-distillation-1

Postel has the following insights about desalination:

“Desalinating brackish water – which is too salty to drink but much less salty than ocean water – is among the most rapidly growing uses of desalination. ... it typically costs less than half as much as seawater desalination.” [see reference 16]

“...desalination holds out the unrealistic hope of a supply-side solution, which delays the onset of the water efficiency revolution so urgently needed.” [see reference 16]