Heat is the best way to make water safe to drink, according to the CDC. Bringing the water to an actual boil is not entirely necessary to kill most common intestinal pathogens, but it is the easiest and safest indicator that you have indeed reached appropriate levels of heat to achieve sanitation---particularly if you do not have a thermometer with you. Boil the water vigorously for one minute, then allow it to cool to room temperature, but do not add ice. If you are at an altitude of greater than 6,562 feet (more than 2,000 meters), use chemical disinfection after boiling the water for one minute or boil the water for three minutes instead of only one. You can add a pinch of salt to each quart to improve the taste of boiled water if you wish.
The CDC notes that certain portable water filters will purify water, but you must be sure of the filter's pore size. Filters that state an "absolute" pore size of 0.1 to 0.4 micrometers can often effectively remove most parasites and bacteria that cause diarrhea. (Make certain the filter uses the term "absolute" in the description, not "nominal.") Viruses, however, tend to be smaller than 0.1 micron, so they may not be adequately removed with such filters. Therefore, in conditions where the water might have fecal or other biologic contamination, you should chemically treat the water before filtering it to avoid intestinal viruses.
Chemical disinfection with chlorine or iodine, also called halogens, is another option for making water safer to drink. Although iodine is recommended over chlorine for purifying water, the parasite Cryptosporidium and other coccidian parasites, like Cyclospora and Toxoplasma, might not be destroyed with iodine, so filtering the water afterward is recommended. If the filter has a charcoal stage, this will give you the added benefit of removing the taste of the halogens from the water. You can also improve the taste by adding a small pinch of ascorbic acid (vitamin C) after chemical treatment. The ascorbic acid will convert the iodine to iodide or the chlorine to chloride, which have no color or taste.
Keep in mind that iodine is utilized by your thyroid gland, so you should not use this method for extended periods of time. If you have unstable thyroid disease, an iodine allergy or are pregnant, you should not use iodine as a water purification method.
