Surface waters may also contain domestic sewage and industrial wastes. Groundwater from shallow wells may contain nitrogen compounds and chlorides, but water from deep wells generally contains only dissolved minerals. Seawater contains many soluble compounds in addition to salt. Impurities are removed from water by screening, sedimentation, filtration, chlorination, or irradiation. See also Sewage Disposal.
Aeration saturates water with air, usually by spraying fountains of water into the air.
Aeration removes odors and tastes caused by decomposing organic matter, industrial wastes, and some gases. Various salts and metals cause hardness in water. Hardness may be removed by boiling, by adding sodium carbonate and lime, or by filtering through natural or artificial zeolites. Freezing is another desalinization method, based on the different freezing points of fresh and salt water. Ice crystals are separated from the salt brine, washed, and melted into fresh water.