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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 2-09 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. Although any water that is brought to a boil should be adequately disinfected, to allow for a margin of safety, boil for 1 minute. Although the boiling point decreases with altitude, at common terrestrial travel elevations it is still above temperatures required to inactivate enteric pathogens. To conserve fuel, the same results can be obtained by bringing water to a boil and then turning off the stove but keeping the container covered for several minutes.

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 2-10). 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 filter water under pressure through semipermeable membranes and can physically remove particles smaller than 0.1 μm, including viruses, proteins, metal ions, and even salt molecules. One new portable filter design incorporates hollow fiber technology, which is a cluster of tiny tubules with variable pore sizes that can remove virus-size particles. Reverse-osmosis filters achieve nanofiltration levels that can remove all microbiologic contamination and desalinate water. The high price and slow output of small hand-pump reverse-osmosis units prohibit use by land-based travelers; however, they are important survival aids for ocean voyagers, and larger powered devices are used for military and refugee situations.

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]).

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. Worldwide, chemical disinfection with chlorine is the most commonly used method for improving and maintaining the microbiologic quality of drinking water, though boiling may be more common than chemical disinfection in low- and middle-income countries. 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 household water treatment.

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.

Since many factors for halogen disinfection in the field are uncontrolled, extending the contact time when feasible is always recommended to add a margin of safety. 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 of emergency use. Iodine use is not recommended for people with unstable thyroid disease or known iodine allergy. In addition, iodine should not be used by pregnant women because of the potential effect on the fetal thyroid.

The taste of halogens in water can be improved by several means:

  • Reduce concentration and increase contact time proportionately.
  • After the required contact time, run water through a filter that contains activated carbon.
  • After the required contact time, add a 25-mg tablet of vitamin C (or a “tiny pinch” of powdered ascorbic acid), which reduces the disinfectant to tasteless and colorless forms of chloride or iodide.
Iodine Resins

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 many different 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, but some models are still available for international use.

Salt (Sodium Chloride) Electrolysis

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

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.

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. Transparent bottles (such as clear plastic PET beverage bottles), preferably lying on a reflective surface, are exposed to sunlight for a minimum of 6 hours. UV and thermal inactivation are synergistic for solar disinfection of drinking water. Use of a simple reflector or solar cooker can achieve temperatures of 149°F (65°C), which will pasteurize the water after 4 hours. 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 conditions, water must be placed in the sun for 2 consecutive days.

Photocatalytic Disinfection

Advanced oxidation processes use UV light or natural sunlight to catalyze the production of potent oxidizers that are excellent disinfectants for microorganisms and can break down complex organic contaminants and even most heavy metals into nontoxic forms. Titanium dioxide (TiO2) is the most effective substance. For field water disinfection, a product is commercially available that uses nanoparticles coated with TiO2 that are integrated into a plastic bag and remain active for hundreds of uses.

Silver and Other Products

Silver ion has bactericidal effects in low doses and some attractive features, including absence of color, taste, and odor. The use of silver as a drinking water disinfectant is popular in Europe, but it is not approved for this purpose in the United States, because silver concentration in water is strongly affected by adsorption onto the surface of the container and there has been limited testing on viruses and cysts. In the United States, silver is approved for maintaining microbiologic quality of stored water.

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 2-09. 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
  • Difficult to know if devices are delivering required UV doses
  • No persistent residual concentration, so does not prevent recontamination during storage

Table 2-10. 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


Table 2-11 summarizes field water disinfection techniques. The optimal technique for a person or group depends on personal preference, size of the group, water source, and the style of travel. Boiling is the most reliable single-step treatment, but certain filters, UV, and chlorine dioxide are also effective in most situations. Optimal treatment of highly contaminated or cloudy water may require CF followed by chemical disinfection. On long-distance, ocean-going boats where water must be desalinated during the voyage, only reverse-osmosis membrane filters are adequate. Halogens are used to maintain microbiological quality of stored water because of their capacity to maintain a persistent residual concentration.

Table 2-11. Summary of field water disinfection techniques

Chlorine dioxide and photocatalytic

1Most filters make no claims for viruses. Reverse osmosis is effective. Virus removal is based on company claims of electrostatic attraction between viruses and filter media.
2Require higher concentrations and contact time.
3Eggs are not very susceptible to halogens but risk of waterborne transmission is very low.


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