Purification

The removal of contaminants from raw water to produce drinking water that is pure enough for human consumption

techniques

boiling

Water is heated hot enough and long enough to inactivate or kill microorganisms that normally live in water at room temperature. Near sea level, a vigorous rolling boil for at least one minute is sufficient. At high altitudes (greater than two kilometers or 5000 feet) three minutes is recommended. US EPA emergency disinfection recomendations In areas where the water is "hard" (that is, containing significant dissolved calcium salts), boiling decomposes the bicarbonate ions, resulting in partial precipitation as calcium carbonate. This is the "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). Boiling does not leave a residual disinfectant in the water. Therefore, water that has been boiled and then stored for any length of time may have acquired new pathogens.

carbon filtering

Charcoal, a form of carbon with a high surface area, absorbs many compounds including some toxic compounds. Water passing through activated charcoal is common in household water filters and fish tanks. Household filters for drinking water sometimes contain silver to release silver ions which have a bactericidal effect. Portable pump filters are also commercially available with ceramic filters that filter 5000 to 50,000 litres per cartridge

disenfection

Iodine is added to water as a solution, crystallised, or in tablets. The iodine kills many-but not all-of the most common pathogens present in natural fresh water sources. Carrying iodine for water purification is a light weight but imperfect solution for those in need of field purification of drinking water. There are kits available in camping stores that include an iodine pill and a second pill that will remove the iodine taste from the water after it has been disinfected.

Chlorine-based bleach may be used for emergency disinfection. Add 2 drops of 5% bleach per litre or quart of clear water, then let stand covered for 30 to 60 minutes. After this it may be left open to reduce the chlorine smell and taste. Guidelines are available online for safe and effective use of bleach. EPA, BC Health

Neither chlorine (e.g. bleach) nor iodine alone is considered effective against Cryptosporidium, and they are limited in effectiveness against Giardia. Chlorine is slightly better than iodine against Giardia.

solar treatment

In SODIS (Solar Water Disinfection) microbes are destroyed by temperature and UVA radiation provided by the Sun. Water is placed in a transparent plastic bottle, which is oxygenated by shaking. It is placed for six hours in full sun, which raises the temperature and gives an extended dose of solar radiation, killing any microbes that may be present. The combination of the two provides a simple method of disinfection for tropical developing countries.