Three methods are generally available for purifying water on an individual or domestic scale. These methods can be used singly or in combination
Boiling is the most satisfactory method of water purification at household level. To be effective the water must be brought to a "rolling boil" for 5 to 10 minutes. It kills all bacteria, spores, cysts and ova and yields sterilized water. Boiling also removes temporary hardness by driving off carbon dioxide and precipitating the calcium carbonate. The taste of water is altered but this is harmless. While boiling is excellent method of purifying water, it does not offer "residual protection" against subsequent microbial contamination. Water should be boiled preferably in the same container in which it is to be stored to avoid contamination during storage.
Bleaching Powder: Bleaching powder or chlorinated lime (CaOCl2) is a white amorphous powder with a pungent smell of chlorine. When freshly made it contains about 3 per cent of "available chlorine
Chlorine solution: Chlorine solution may be prepared from bleaching-powder. If 4 kg of bleaching powder is mixed with 20 liters of water it will give 5 per cent solution of chlorine.
High test hypochlorite: High test hypochlorite (HTH) or perchloron is a calcium compound containing 60-70 per cent available chlorine. It is more stable than bleaching powder and deteriorates much less on storage. Solutions prepared from HTH are also used for water disinfection.
Chlorine tablets: Under various trade names (viz. halazone tablets) are available in the market. They are quite good for disinfecting small quantities of water, but they are costly. The National Environmental Engineering Research Institute, Nagpur has formulated a new type of chlorine tablet which is 15 times better than ordinary halogen tablets.
Iodine: Iodine may be used for emergency disinfection of water. Two drops of 2 per cent ethanol solution of iodine will suffice for one litre of clear water
PotassiumPermanganate: Once widely used it is no longer recommended for water disinfection.
Water can be purified on a small scale by filtering through ceramic filters such as Pasteur Chamberland filter, Berkefeld filter and Katadyn filter. The essential part of a filter is the "candle" which is made of porcelain in the Chamberland type and of kieselgurh or infusorial earth in the Berkefeld filter. In the Katadyn filter, the surface of the filter is coated with a silver catalyst so that bacteria coming in contact with the surface are kiIled by the "oligodynamic" action of the silver ions, which are liberated into the water. Filter candles usually remove bacteriae found in drinking water but not the filter passing viruses. Filter candles are liable to be logged with impurities and bacteriae. They should be cleaned by scrubbing with a hard brush under running water and boiled at least once a week. Only clean water should be used with ceramic filters. Although ceramic filters are effective in purifying water, they are not quite suitable for widespread use under Indian conditions.