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Chapter 2 The Pre-Travel Consultation Counseling & Advice for Travelers

Water Disinfection for Travelers

Howard D. Backer


Waterborne disease is a risk for international travelers who visit countries that have poor hygiene and inadequate sanitation, and for wilderness visitors who rely on surface water in any country, including the United States. The list of potential waterborne pathogens is extensive and includes bacteria, viruses, protozoa, and parasitic helminths. Most of the organisms that can cause travelers’ diarrhea can be waterborne. Where treated tap water is available, most travelers’ intestinal infections are probably transmitted by food, but aging or inadequate water treatment infrastructure may not effectively disinfect water or maintain water quality during distribution. Where untreated surface or well water is used and there is no sanitation infrastructure, the risk of waterborne infection is high. Microorganisms with small infectious doses (such as Giardia, Cryptosporidium, Shigella, Escherichia coli O157:H7, and norovirus) can even cause illness through recreational water exposure, via inadvertent water ingestion.

Bottled water has become the convenient solution for most travelers, but in some places it may not be superior to tap water. Moreover, the plastic bottles create an ecological problem, since most developing countries do not recycle plastic bottles. All international travelers, especially long-term travelers or expatriates, should become familiar with and use simple methods to ensure safe drinking water. Several methods are scalable and some can be improvised from local resources, allowing adaptation to disaster relief and refugee situations. Table 2-10 compares benefits and limitations of different methods.



Common intestinal pathogens are readily inactivated by heat. Microorganisms are killed in a shorter time at higher temperatures, whereas temperatures as low as 140°F (60°C) are effective with a longer contact time. Pasteurization uses this principle to kill foodborne enteric pathogens and spoilage-causing organisms at temperatures between 140°F (60°C) and 158°F (70°C), well below the boiling point of water (212°F [100°C]).

Although boiling is not necessary to kill common intestinal pathogens, it is the only easily recognizable end point that does not require a thermometer. All organisms except bacterial spores, which are rarely waterborne enteric pathogens, are killed in seconds at boiling temperature. In addition, the time required to heat the water from 60°C to boiling works toward heat disinfection. Any water that is brought to a boil should be adequately disinfected; however, if fuel supplies are adequate, travelers may wish to boil for 1 minute to allow for a margin of safety. Although the boiling point decreases with altitude, at common terrestrial travel elevations it is still above temperatures required to inactivate enteric pathogens.

If no other means of water treatment is available, a potential alternative to boiling is to use tap water that is too hot to touch, which is probably at a temperature between 131°F (55°C) and 140°F (60°C). This temperature may be adequate to kill pathogens if the water has been kept hot in the tank for some time. Travelers with access to electricity can bring a small electric heating coil or a lightweight beverage warmer to boil water.

Filtration and Clarification

Portable hand-pump or gravity-drip filters with various designs and types of filter media are commercially available to international travelers. Filter pore size is the primary determinant of a filter’s effectiveness, but microorganisms also adhere to filter media by electrochemical reactions. Microfilters with “absolute” pore sizes of 0.1–0.4 µm are usually effective to remove cysts and bacteria but may not adequately remove viruses, which are a major concern in water with high levels of fecal contamination (Table 2-11). Filters that claim Environmental Protection Agency (EPA) designation of water “purifier” undergo company-sponsored testing that has demonstrated removal of 106 bacteria, 104 (9,999 of 10,000) viruses, and 103 Cryptosporidium oocysts or Giardia cysts. (EPA does not independently test the validity of these claims.)

Progressively finer levels of filtration known as ultrafiltration, nanofiltration, and reverse osmosis can remove particles of 0.01, 0.001, and 0.0001 µm, respectively. All of these filters can remove viruses. Nanofiltration will remove organic molecules, while reverse osmosis will remove monovalent salts, thus achieving desalination. One new portable filter design incorporates hollow fiber technology, which is a cluster of tiny tubules with variable pore sizes that can achieve nanofiltration and remove virus-size particles. These are now available in various designs at reasonable prices, including hand-pump, gravity drip, and drink through. All are effective, although drink-through is least practical due to the negative pressure required for flow. The high price and slow output of small hand-pump reverse-osmosis units prohibit use by land-based travelers; however, they are survival aids for ocean voyagers, and larger powered devices are used for military and refugee situations. Microfilters that commonly use ceramic, synthetic fiber, compressed carbon, or large-pore hollow-fiber filter elements are sufficient to remove bacteria and protozoan cysts in water with low levels of contamination (wilderness water with little human and animal activity), but hollow-fiber filters with ultrafiltration or nanofiltration should be used for water with high levels of human and animal activity in the watershed. A 2-step process of halogen (see below) and microfiltration can also assure virus removal.

Filters made from ceramic clay or simple sand and gravel (slow sand or biosand) filters are successfully used for households in developing countries. Gravel and sand filters can be improvised in remote or austere situations when no other means of disinfection is available.

Coagulation-flocculation (CF) removes suspended particles that cause a cloudy appearance and bad taste and do not settle by gravity; this process removes many but not all microorganisms. CF is easily applied in the field. Alum—an aluminum salt that is widely used in food, cosmetic, and medical applications— or one of several other natural substances is added to the water and stirred. The clumped particulates that form are allowed to settle, then poured through a coffee filter or fine cloth to remove the sediment. Tablets or packets of powder that combine flocculent and hypochlorite disinfection are available (products include Chlor-floc and PUR Purifier of Water [Proctor and Gamble—for humanitarian use, not sold commercially]). CF removes most microorganisms but should not be used as the sole means of disinfection. It also improves the effectiveness of filtration by causing microorganisms, including viruses, to clump with larger particles, facilitating their removal.

Granular-activated carbon (GAC) purifies water by adsorbing organic and inorganic chemicals and most heavy metals, thereby improving odor, taste, and safety. GAC is a common component of household and field filters. It may trap but does not kill organisms. In field water treatment, GAC is best used after chemical disinfection to remove disinfection byproducts as well as the taste of iodine and chlorine (see Halogens below).

Chemical Disinfection


The most common chemical water disinfectants are halogens, mainly chlorine and iodine, although bromine has similar qualities. Sodium hypochlorite, the active ingredient in common household bleach, is the primary disinfectant promoted by CDC and the World Health Organization Safe Water System at a 1.5% concentration for household use in the developing world. Other chlorine-containing compounds such as calcium hypochlorite and sodium dichloroisocyanurate, available in granular or tablet formulation, are also effective for water treatment. An advantage of halogens is flexible dosing that allows use by individual travelers, small or large groups, or communities.

Given adequate concentrations and length of exposure (contact time), chlorine and iodine have similar activity and are effective against bacteria and viruses ( Giardia cysts are more resistant to halogens; however, field-level concentrations are effective with longer contact times. For this reason, dosing and concentrations of halogen products are targeted to the cysts. Some common waterborne parasites, such as Cryptosporidium, are poorly inactivated by halogen disinfection at practical concentrations, even with extended contact times.

Chemical disinfection may be supplemented with filtration to remove resistant oocysts from drinking water. Cloudy water contains substances that will neutralize disinfectant, so it will require higher concentrations or contact times or, preferably, clarification through settling, CF, or filtration before disinfectant is added.

Both chlorine and iodine are available in liquid and tablet form. Because iodine has physiologic activity, WHO recommends limiting iodine water disinfection to a few weeks. Iodine use is not recommended for people with unstable thyroid disease or known iodine allergy. In addition, pregnant women should not use iodine to disinfect water over the long term because of the potential effect on the fetal thyroid. Pregnant travelers who have other options should use an alternative means such as heat, chlorine, or filtration.

Some prefer the taste of iodine to chlorine, but neither is appealing in doses often recommended for field use. The taste of halogens in water can be improved by:

  • Reducing concentration and increasing contact time;
  • Or, following contact time,
    • Running water through a filter containing activated carbon or
    • Adding a 25mg tablet of vitamin C, a tiny pinch of powdered ascorbic acid, or a small amount of hydrogen peroxide.

Iodine resins transfer iodine to microorganisms that come into contact with the resin but leave little iodine dissolved in the water. The resins have been incorporated into various filter designs available for field use. Most contain a 1-µm cyst filter, which should effectively remove protozoan cysts. Few models are sold in the United States because of inconsistent test results.


Passing a current through a simple brine salt solution generates oxidants, including hypochlorite, which can be used to disinfect microbes. This technique has been engineered into a pocket-sized, battery-powered product, which is commercially available.


Chlorine dioxide (ClO2) can kill most waterborne pathogens, including Cryptosporidium oocysts, at practical doses and contact times. Tablets and liquid formulations are available to generate chlorine dioxide in the field for small-quantity water treatment. Although extensive data show the efficacy of chlorine dioxide in municipal and industrial systems, fewer data are available to show the efficacy of small-quantity treatment comparable to other halogens—mainly concentrations achieved and contact time required.

Ultraviolet (UV) Light

UV light kills bacteria, viruses, and Cryptosporidium oocysts in water. The effect depends on UV dose and exposure time. Portable battery-operated units that deliver a metered, timed dose of UV are an effective way to disinfect small quantities of clear water in the field. Larger units with higher output are available where a power source is available. These units have limited effectiveness in water with high levels of suspended solids and turbidity, because suspended particles can shield microorganisms from UV rays.

Solar Irradiation and Heating

UV irradiation by sunlight in the UVA range can substantially improve the microbiologic quality of water and may be used in austere emergency situations. Recent work has confirmed the efficacy and optimal procedures of the solar disinfection (SODIS) technique. Solar disinfection is not effective on turbid water. If the headlines in a newspaper cannot be read through the bottle of water, then the water must be clarified before solar irradiation is used. Under cloudy weather conditions, water must be placed in the sun for 2 consecutive days.

Silver and Other Products

Silver ion has bactericidal effects in low doses, and some attractive features include lack of color, taste, and odor, and the ability of a thin coating on the container to maintain a steady, low concentration in water. Silver is widely used by European travelers as a primary drinking water disinfectant. In the United States, silver is approved only for maintaining microbiologic quality of stored water because its concentration can be strongly affected by adsorption onto the surface of the container, and there has been limited testing on viruses and cysts. Silver is available alone or in combination with chlorine in tablet formulation.

Several other common products, including hydrogen peroxide, citrus juice, and potassium permanganate have antibacterial effects in water and are marketed in commercial products for travelers. None have sufficient data to recommend them for primary water disinfection at low doses in the field.

Table 2-10. Comparison of water disinfection techniques

  • Does not impart additional taste or color
  • Single step that inactivates all enteric pathogens
  • Efficacy is not compromised by contaminants or particles in the water as for halogenation and filtration
  • Does not improve taste, smell or appearance of water
  • Fuel sources may be scarce, expensive, or unavailable
  • Does not prevent recontamination during storage
  • Simple to operate
  • Requires no holding time for treatment
  • Large choice of commercial product designs
  • Adds no unpleasant taste and often improves taste and appearance of water
  • Can be combined with halogens for to remove or kill all pathogenic waterborne microbes
  • Adds bulk and weight to baggage
  • Many do not reliably remove viruses
  • Channeling of water or high pressure can force microorganisms through the filter
  • More expensive than chemical treatment
  • Eventually clogs from suspended particulate matter and may require some field maintenance or repair
  • Does not prevent recontamination during storage
Halogens (chlorine, iodine)
  • Inexpensive and widely available in liquid or tablet form
  • Taste can be removed by simple techniques
  • Flexible dosing
  • Equally easy to treat large and small volumes
  • Will preserve microbiologic quality of stored water
  • Impart taste and odor to water
  • Flexible dosing requires understanding of principles
  • Iodine is physiologically active, with potential adverse effects
  • Not readily effective against Cryptosporidium oocysts
  • Efficacy decreases with low water temperature and decreasing water clarity
  • Corrosive and stain clothing
Chlorine dioxide
  • Low doses have no taste or color
  • Simple to use and available in liquid or tablet form
  • More potent than equivalent doses of chlorine
  • Effective against all waterborne pathogens
  • Volatile and sensitive to sunlight: do not expose tablets to air, and use generated solutions rapidly
  • No persistent residual concentration, so does not prevent recontamination during storage
Ultraviolet (UV)
  • Imparts no taste
  • Portable devices now available
  • Effective against all waterborne pathogens
  • Extra doses of UV can be used for added assurance and with no side effects
  • Solar UV exposure (SODIS) also provides substantial benefit
  • Requires clear water
  • Does not improve taste or appearance of water
  • Relatively expensive (except solar)
  • Requires batteries or power source
  • Cannot know if devices are delivering required UV doses
  • No persistent residual concentration, so does not prevent recontamination during storage


Table 2-11. Microorganism size and susceptibility to filtration

Viruses 0.03 Not specified (optimally 0.01, ultrafiltration)
Enteric bacteria (Escherichia coli) 0.5 × 3.0–8.0 0.2–0.4 (microfiltration)
Cryptosporidium oocyst 4–6 1 (microfiltration)
Giardia cyst 6.0–10.0 × 8.0–15.0 3.0–5.0 (microfiltration)
Nematode eggs 30 × 60 Not specified; any microfilter
Schistosome larvae 50 × 100 Not specified; any microfilter


Tables 2-10 and 2-12 summarize advantages and disadvantages of field water disinfection techniques and their bacteriological efficacy. It is advisable to test a method before travel.

Table 2-12. Summary of field water disinfection techniques

Chlorine dioxide and photocatalytic

1Most filters make no claims for viruses. Hollow-fiber filters with ultrafiltration pore size and reverse osmosis are effective.
2Require higher concentrations and contact time than for bacteria or viruses.
3Eggs are not very susceptible to halogens, but risk of waterborne transmission is very low.


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