Treatment of Drinking Water

Travellers should be advised of the following methods for treating water to make it safe for drinking and other purposes.

Boiling

Boiling is by far the most reliable method to make water of uncertain purity safe for drinking. Water should be brought to a vigorous rolling boil for 1 minute and allowed to cool to room temperature; ice should not be added. This procedure will kill bacterial and parasitic causes of diarrhoea at all altitudes and viruses at low altitudes. To kill viruses at altitudes >2,000 m (6,562 ft), water should be boiled for 3 minutes or chemical disinfection should be used after the water has boiled for 1 minute. Adding a pinch of salt to each quart or pouring the water several times from one clean container to another will improve the taste.

Chemical Disinfection

Chemical disinfection with iodine is an alternative method of water treatment when it is not feasible to boil water. However, this method cannot be relied on to kill Cryptosporidium. Two well-tested methods for disinfection with iodine are the use of tincture of iodine (Table 2-2) and tetraglycine hydroperiodide tablets (e.g., Globaline, Potable-Aqua, or Coghlan's). These tablets are available from pharmacies and sporting goods stores. The manufacturer's instructions should be followed. If water is cloudy, the number of tablets used should be doubled; if water is extremely cold (<5°C; <41°F]), an attempt should be made to warm the water, and the recommended contact time should be increased to achieve reliable disinfection. Cloudy water should be strained through a clean cloth into a container to remove any sediment or floating matter, and then the water should be boiled or treated with iodine. Iodine treatment of water is intended for short-term use only. When the only water available is iodine treated, it should be used for only a few weeks.

Table 2-2. Treatment of water with tincture of iodine

1One drop = 0.05 mL. Water must stand for a minimum of 30 minutes before it is safe to use.
2Very turbid or cold water can require prolonged contact time; if possible, such water should be allowed to stand several hours before use. To ensure that Cryptosporidium is killed, water must stand for 15 hours before drinking.

Chlorine, in various forms, can also be used for chemical disinfection. However, its germicidal activity varies greatly with the pH, temperature, and organic content of the water to be purified; therefore, it can produce less consistent levels of disinfection in many types of water.

Water Filters

Portable filters currently on the market will provide various degrees of protection against microbes. Reverse-osmosis filters provide protection against viruses, bacteria, and protozoa, but they are expensive and larger than most filters used by backpackers, and the small pores on this type of filter are rapidly plugged by muddy or cloudy water. In addition, the membranes in some filters can be damaged by chlorine in water. Microstrainer filters with pore sizes in the 0.1- to 0.3-μm range can remove bacteria and protozoa from drinking water, but they do not remove viruses. To kill viruses, travellers using microstrainer filters should be advised to disinfect the water with iodine or chlorine after filtration, as described previously. Some filtration kits come with an additional filter effective against viruses. Filters with iodine-impregnated resins are most effective against bacteria, and the iodine will kill some viruses; however, the contact time with the iodine in the filter is too short to kill the protozoa Cryptosporidium and, in cold water, Giardia.

Filters that are designed to remove Cryptosporidium and Giardia carry one of the four messages below—verbatim—on the package label.

• Reverse osmosis
• Absolute pore size of 1 micron
• Tested and certified by NSF International (formerly the National Sanitation Foundation) Standard 53 or NSF Standard 58 for cyst removal
• Tested and certified by NSF Standard 53 or NSF Standard 58 for cyst reduction

Filters may not be designed to remove Cryptosporidium and Giardia if they are labeled only with these words:

• Nominal pore size of 1 micron
• One-micron filter
• Effective against Giardia
• Effective against parasites
• Carbon filter
• Water purifier
• Environmental Protection Agency (EPA)-approved (Caution: EPA does not approve or test filters.)
• EPA-registered (Caution: EPA does not register filters for Cryptosporidium removal)
• Activated carbon
• Removes chlorine
• Ultraviolet light
• Pentiodide resins
• Water softener

Filters collect organisms from water. Anyone changing cartridges should wash hands afterwards. Filters may not remove Cryptosporidium as well as boiling does, because even good brands of filters may sometimes have manufacturing flaws that allow small numbers of organisms to pass through the filter. In addition, poor filter maintenance or failure to replace filter cartridges as recommended by the manufacturer can cause a filter to fail.

A travellers' guide to buying water filters for preventing cryptosporidiosis and giardiasis can be found at www.cdc.gov/ncidod/dpd/parasites/cryptosporidiosis/factsht_crypto_prevent_water.htm. These two organisms are either highly (Cryptosporidium) or moderately (Giardia) resistant to chlorine; so conventional halogen disinfection may be ineffective. Boiling water or filtration can be used as an alternative to chemical disinfection.

Proper selection, operation, care, and maintenance of water filters are essential to producing safe water. The manufacturers' instructions should be followed. NSF International, an independent testing company, tests and certifies water filters for their ability to remove protozoa, but not for their ability to remove bacteria or viruses. Few published scientific reports have evaluated the efficacy of specific brands or models of filters against bacteria and viruses in water. Until such information becomes available, CDC cannot identify which specific brands or models of filters are most likely to remove bacteria and viruses. To find out if a particular filter is certified to remove cryptosporidia, contact NSF International by calling 1-877-867-3435; by fax to 313-769-0109; or by writing to 789 North Dixboro Road, P.O. Box 130140, Ann Arbor, Michigan 48113-0140; or online at http://www.NSF.org/certified/DWTU/. Under "Reduction claims for drinking water treatment units—health effects," check the box in front of the words "Cyst Reduction."

As a last resort, if no source of safe drinking water is available or can be obtained, tap water that is uncomfortably hot to touch might be safer than cold tap water; however, proper disinfection, filtering, or boiling is still advised.