Water purification is the process of changing undrinkable water to drinkable (potable) water.

Water is a very good solvent, it is this one fact which makes it difficult to obtain pure water without treatment. One solute commonly found in water is sodium chloride, or common table salt. Because of the high salt content of seawater, it is generally unfit for human consumption. In the United States, the legal limit for the salt content of municipal water supplies is set at 500 ppm. This is much lower than the 3.5% of salt in seawater or the 0.5% found underground in brackish water in some regions. The removal of salt from these waters is a particular type of water purification called desalination. Water and dissolved salts can be separated by distillation (water is volatile, the salts are not).

On a large scale, distillation is problematical and expensive. A more efficient desalination...