Water sterilization

Boiling

Probably the most reliable and often the cheapest although stoves, kettles and heating coils may be inconvenient to use while travelling.
Ideally boil for 5 min, especially if at high altitude when water boils at a lower temperature.

Chlorine and silver

Chlorine and silver based tablets are available from chemists and specialised travel equipment shops.
When used correctly they destroy most bacteria (e.g. V.cholerae), but are less effective for viruses and cysts (e.g. hepatitis A virus, giardia and amoebic cysts).

Iodine

Iodine tincture, at a concentration of concentration of 8 mgs per litre and left for 10 minutes destroys bacteria, viruses and cysts. As with all halogens it is ineffective against cryptosporidia.
Adequate levels are normally achieved by using 4 drops of tincture of iodine to 1 litre of water or one drop to a glass Wait 20 min if water is very cold or contaminated with organic material.
Tablets and crystals are also available and the instructions should be followed carefully.
Iodine is best avoided for long term use, in pregnancy and in the very young.

Filters

A wide variety of filters are available. Some versions have cores which can be replaced. Filters remove sand, clay and other matter as well as organisms by means of small pore size, membranes, adsorption, exchange resins and osmosis.
They effectively remove bacteria, parasites and most viruses. Good filters are effective against cryptosporidia and giardia.

Filtration combined with iodine (or chlorine)

The removal of all enteric pathogens requires this combination.
This is usually achieved by passing water through iodine exchange resins. When negatively charged contaminants contact the iodine resin, iodine is instantly released so killing the microorganisms without large quantities of iodine being in solution.