Ingestion of contaminated drinking water is a common mode of transmission of infection in travelers to lesser-developed countries. Parasitic, viral, and bacterial pathogens found in contaminated water can cause life-threatening infections, diarrheal syndromes, systemic disease, hepatitis, and parasitic infestations. In all lesser-developed countries, water from streams, ponds, wells, and irrigated areas should be considered unsafe. Tap water may not be filtered and chemically treated on a reliable basis - so should not be trusted.
Charcoal Resins
Granular activated charcoal (AC) "purifies" by removing organic pollutants, and chemicals by adsorption. This improves color, taste, and smell. AC does not remove all microorganisms so it does not disinfect. Some filters incorporate activated charcoal resins.
Filtration & Clarification
Filters mechanically remove the larger microorganisms such as protozoans (eg Giardia), bacteria, and sediment from water, but are unable to remove viruses due to their minute size. Further treatment with either activated carbon, boiling, or chlorine/iodine is necessary to remove viruses (see below). Prior to filtering the water, particularly turbid water can be "clarified" with the addition of a pinch of alum (aluminum potassium sulfate) per gallon: mix well, stir occasionally for 30 minutes, allow 30-60 minutes to settle - then pour through a paper filter to remove the clumps of floc. The addition of an iodine-resin component (see below) to the filter device turns it into a purification device.
Halogens
Halogens (chlorine and iodine) are excellent disinfectants for bacteria, viruses, Giardia and amebic cysts, excluding the cysts of the bacterium: Cryptosporidium and some parasites. Vegetative bacteria (non-cyst forms) are extremely sensitive to halogens. Viruses and Giardia require higher concentrations or longer contact times. In order to achieve reliable results, it is important to attain a target concentration capable of killing most enteric pathogens, and to maintain a "contact time" adequate to achieve killing. Advantages of halogens are that they are easily obtainable and inexpensive. Since halogens impart a "taste" to the water, the water may be dehalogenated after appropriate contact time with acetic acid (vitamin C). Halogens should be used with caution with pregnant women, patients with iodine allergies or thyroid disorders, and should not be used for periods longer than several weeks. Both chlorine and iodine are available in liquid and table form. Iodine has some advantage over chlorine for field disinfection: it is less affected by pH or nitrogenous wastes, and the taste is better tolerated at effective levels. Chorine may not kill Giardia (a parasite) spores. Some filters incorporate a halogen resin.
Heat
Heat is the best know method of water disinfection. Virtually all enteric pathogens can be killed with boiling. Water should be safe to drink by the time it reaches a full boil. To be safe, either keep the water covered for several minutes after reaching the boiling point, or boil the water for one minute.