Process — How drinking water is produced from seawater

The diagram sets out the six stages through which seawater is turned into water that is safe to drink at a desalination plant.

Overall, the process is a linear one which begins offshore, where seawater is taken in, and ends with treated water being piped to homes. The decisive stage is the point at which the salt is separated out, everything before it being preparation and everything after it refinement.

Initially, seawater is drawn in through a screened pipe some distance from the shore, the screen preventing larger debris from entering the plant. The water then passes to pre-treatment, where sand and algae are filtered out and chemicals are added to protect the equipment further along the line. Once clean, it reaches a high-pressure pump, whose sole function is to raise the water to the pressure needed for the following stage.

The water is now forced through a bank of fine membranes. Because these allow water molecules to pass but not salt, dissolved salts are retained on one side while fresh water emerges on the other; the concentrated brine left behind is diluted and returned to the sea as a by-product. The fresh water, meanwhile, has minerals added back and is disinfected before being stored in tanks and distributed to households.