Other water purification techniques

Other popular methods for purifying water, especially for local private supplies are listed below. In some countries some of these methods are also used for large scale municipal supply. Particularly important are Distillation (de-salination of sea-water) and reverse osmosis

Boiling: Water is heated to its boiling point long enough to inactivate or kill microorganisms that normally live in water at room temperature. Near sea level, bring the water to a vigorous rolling boil for at least one minute. At high altitudes (greater than a kilometer or a mile) boil for three minutes. Source: US EPA (http://www.epa.gov/safewater/faq/emerg.html) In areas where the water is "hard", (containing dissolved calcium salts), boiling decomposes the bicarbonate ion, resulting in some (but not all) of the dissolved calcium being precipitated in the form of calcium carbonate. This is the so-called "fur" that builds up on kettle elements etc. in hard water areas. With the exception of calcium, boiling does not remove solutes of higher boiling point than water, and in fact increases their concentration (due to some water being lost as vapour).
Carbon filtering: Charcoal, a form of carbon with a high surface area due to its mode of preparation, adsorbs many compounds, including some toxic compounds. Water is passed through activated charcoal to remove such contaminants. This method is most commonly used in household water filters and fish tanks. Household filters for drinking water sometimes also contain silver, trace amounts of silver ions having a bactericidal effect.
Distilling: Distillation involves boiling the water to produce water vapour. The water vapour then rises to a cooled surface where it can condense back into a liquid and be collected. Because the solutes are not normally vaporized, they remain in the boiling solution. Even distillation does not completely purify water, because of contaminants with similar boiling points and droplets of unvaporized liquid carried with the steam. However, 99.9% pure water can be obtained by distillation.
Reverse osmosis: Mechanical pressure is applied to an impure solution to force pure water through a semi-permeable membrane. The term is reverse osmosis, because normal osmosis would result in pure water moving in the other direction to dilute the impurities. Reverse osmosis is theoretically the most thorough method of large-scale water purification available, although perfect semi-permeable membranes are difficult to create.
Ion exchange: Most common ion exchange systems use a zeolite resin bed and simply replace unwanted Ca2+ and Mg2+ ions with benign (soap friendly) Na+ or K+ ions. This is the common water softener. A more rigorous type of ion exchange swaps H+ ions for unwanted cations and hydroxide (OH-) ions for unwanted anions. The result is H+ + OH- → H2O. This system is recharged with hydrochloric acid and sodium hydroxide, respectively. The result is essentially deionized water.
Electrodeionization: Water is passed between a positive electrode and a negative electrode. Ion selective membranes allow the positive ions to separate from the water toward the negative electrode and the negative ions toward the positive electrode. High purity de-ionized water results. The water is usually passed through a reverse osmosis unit first to remove nonionic organic contaminants.