Chapter 2The Pre-Travel ConsultationCounseling & Advice for Travelers
Water Disinfection for Travelers
RISK FOR TRAVELERS
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 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 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. Table 2-09 compares benefits and limitations of different methods.
FIELD TECHNIQUES FOR WATER TREATMENT
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 spoiling 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 point that doesn’t require a thermometer. All organisms except bacterial spores, which are not usually waterborne enteric pathogens, are killed in seconds at boiling temperature. Therefore, any water that is boiled for 1 minute (to allow for a margin of safety) should be adequately disinfected. Because the boiling point decreases with increasing altitude, water should be boiled for 3 minutes at altitudes above 6,562 ft (2,000 m). 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
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). The Environmental Protection Agency (EPA) designation of water “purifier” indicates that company-sponsored testing has substantiated claims for removing 106 bacteria, 104 (9,999 of 10,000) viruses, and 103 Cryptosporidium oocysts or Giardia cysts, although EPA does not independently test the validity of these claims. NSF International is a nonprofit, nongovernmental organization that develops standards and product certification for public health and safety. They are a designated collaborating center of the World Health Organization (WHO) for Food and Water Safety and Indoor Environment. They are accredited by the American National Standards Institute (ANSI), the International Accreditation Service (IAS), and the Standards Council of Canada (SCC) for third-party certification. Some water filtration units have been evaluated by NSF International under the NSF/ANSI Standard 53. A listing of certified water filtration products can be found at www.nsf.org/consumer.
New portable filter designs include hollow fiber technology, which is a cluster of tiny tubules with variable pore sizes that can remove virus-size particles. Reverse-osmosis filters can both remove 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.
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, or one of several other substances, is added to the water, stirred well, 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 (commercial products such as Chlor-floc or PUR).
The most common chemical water disinfectants are chlorine and iodine (halogens). Worldwide, chemical disinfection with chlorine is the most commonly used method for improving and maintaining the microbiologic quality of drinking water. Sodium hypochlorite, the active ingredient in common household bleach, is the primary disinfectant promoted by CDC and the WHO 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, which are both solids, 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, viruses, and Giardia cysts (www.cdc.gov/safewater/about_pages/CTfactor-final.pdf) (PDF). Because many factors in the field are uncontrolled, extending the contact time adds a margin of safety. However, some common waterborne parasites, such as Cryptosporidium, are poorly inactivated by halogen disinfection, even at practical extended contact times. Therefore, chemical disinfection should be supplemented with adequate filtration to remove these microorganisms 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 or filtration before disinfectant is added. Tablets that combine flocculent and disinfectant are available.
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. 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 tiny pinch of ascorbic acid.
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.
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
Extensive data show that UV light can kill bacteria, viruses, and Cryptosporidium oocysts in water. The effect depends on UV dose and exposure time, and requires clear water because suspended particles can shield microorganisms from UV rays. These units have limited effectiveness in water with high levels of suspended solids and turbidity. They also have no disinfection residual. Portable battery-operated units that deliver a metered, timed dose of UV may be an effective way to disinfect small quantities of clear water in the field; however, more testing is needed for conclusive evidence.
Solar Irradiation and Heating
UV irradiation by sunlight in the UVA range can substantially improve the microbiologic quality of water. Recent work has confirmed the efficacy and optimal procedures of the solar disinfection technique. Transparent bottles (such as clear plastic beverage bottles), preferably lying on a dark surface, are exposed to sunlight for a minimum of 4 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 can’t be read through the bottle of water, then the water must be filtered before solar irradiation is used. If more than half the sky is clouded over, then solar irradiation is not effective and should be repeated before using the water. Solar disinfection of drinking water may be acceptable for austere emergency situations.
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 have antibacterial effects in water and are marketed in commercial products for travelers, including hydrogen peroxide, citrus juice, and potassium permanganate. None have sufficient data to recommend them for water disinfection in the field.
Table 2-09. Comparison of water disinfection techniques
|Halogens (chlorine, iodine)||
Table 2-10. Microorganism size and susceptibility to filtration
|ORGANISM||AVERAGE SIZE (µm)||MAXIMUM RECOMMENDED FILTER RATING (µm ABSOLUTE)1|
|Viruses||0.03||Not specified (optimally 0.01)|
|Enteric bacteria (Escherichia coli)||0.5 × 3.0–8.0||0.2–0.4|
|Giardia cyst||6.0–10.0 × 8.0–15.0||3.0–5.0|
1NSF 53 rating on a filter certifies for cyst/oocyst removal.
THE PREFERRED TECHNIQUE
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, oceangoing boats where water must be desalinated during the voyage, only reverse-osmosis membrane filters are adequate.
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