Heat.Heat kills microorganisms, and virtually all enteropathogens are readily killed at temperatures well below the boiling point. The process of heating water to a boil makes it hot enough long enough to disinfect it, even at elevations as high as Everest Base Camp (references 1,2). There is no need to boil water for 5 minutes, 10 minutes, or 20 minutes, as some guide books recommend! Bringing water to a boil is adequate for disinfection.
Chemical Treatments.Chemical disinfection of water depends on the killing of bacteria, Giardia and amoeba cysts, and viruses by the chemical. Halogens (chlorine and iodine) are most commonly used. The important points are that the killing effectiveness of the chemical is dependant on concentration of the chemical, temperature of the water, and contact time. Decreased concentration (better flavor) or decreased temperature (inevitably the case in the mountains) requires a longer contact time for disinfection. Sediment (cloudy water) increases the need for halogen. Bear in mind that adding flavor crystals to your water will use up the halogen and should only be done AFTER the recommended contact time for disinfection. Remember: "Add Flavor Later."
Chlorine.Chlorine has been used for several centuries for water disinfection. The most common objection to it is the flavor, though there have been some suggestions that it is unreliable in killing Giardia cysts in the commonly used concentrations.
Halazone tablets.These are convenient and inexpensive, but have several disadvantages. Due to its chemical formulation, reliable disinfection in all conditions requires 6 tablets per liter for 1 hour contact, resulting in poor flavor. The tablets rapidly lose effectiveness when exposed to warm, humid air.
Superchlorination-dechlorination.This two-step method is somewhat inconvenient, and the chemicals needed are destructive to clothing and gear if spilled, but it is highly effective and results in nearly flavorless water. High concentrations of chlorine are initially developed, and then in a second step removed by the addition of peroxide.
Iodine.Iodine has been used to disinfect water for nearly a century. It has advantages over chlorine in convenience and probably efficacy; many travellers find the taste less offensive as well. It appears safe for short and intermediate length use (3-6 months), but questions remain about its safety in long-term usage. It should not be used by persons with allergy to iodine, persons with active thyroid disease, or pregnant women.
Note that Iodine and other halogens appear to be relatively ineffective at killing cyclospora, a troublesome diarrhea-causing bacteria seen in Nepal only in the late Spring and Summer months. At these times it may be reasonable to pre-filter water to remove the large cyclospora (about the size of Giardia cysts), and then treating with iodine.
Iodine is available in numerous forms, which can be confusing to the traveller.