From the 4th century B.C. a method was known for this, when Aristotle discovered that by boiling seawater, the steam was free of salt.
The simplest desalination plant is a still where water is boiled and steam is condensed. A solar still can be improvised by placing a glass dome over a saltwater pond. The water is heated, then evaporated, condensed into the glass and finally drained into collection channels. A still of just under 1m² can produce 4-5 litres of drinking water per day.
A much larger alembic is required to desalinate usable quantities of water. The water is first heated above its boiling point in a pressure chamber so that it does not boil. It then flows into a lower pressure chamber, where part of the water is instantly converted to steam. The water that did not boil in the first chamber passes into the second, slightly lower pressure chamber, where it partially evaporates and condenses.
A modern desalination method, called reverse osmosis, is more profitable than the one described. It consists of using plastic membranes with tiny perforations, which allow water molecules to pass but not salt molecules. The membranes are placed in tubes, and the salt water is passed through them, pumping it under pressure. The desalinated water then drips out of the tube.
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