Wilderness Medical Society Clinical Practice Guidelines for Water Disinfection for Wilderness, International Travel, and Austere Situations

Howard D.Backer MD, MPH, Robert W.Derlet MD, Vincent R.Hill PhD

To provide guidance to clinicians, the Wilderness Medical Society convened experts to develop evidence-based guidelines for water disinfection in situations where the potability of available water is not ensured, including wilderness and international travel, areas affected by disaster, and other areas without adequate sanitation. The guidelines present the available methods for reducing or eliminating microbiologic contamination of water for individuals, groups, or households; evaluation of their effectiveness; and practical considerations. The evidence evaluation includes both laboratory and clinical publications. The panel graded the recommendations based on the quality of supporting evidence and the balance between benefits and risks or burdens, according to the criteria published by the American College of Chest Physicians.

Keywords

• drinking water
• water purification
• water microbiology
• disaster planning
• pasteurization
• halogens

Introduction

Safe and efficient treatment of drinking water is among the major public health advances of the last century. Without treatment, waterborne diseases can spread rapidly, resulting in large-scale disease and death.^1,2 In industrialized nations, the population generally is protected from waterborne disease by sophisticated water supply systems that disinfect water and provide continuous monitoring. In contrast, travelers to wilderness and recreational areas anywhere in the world and to underdeveloped regions of some countries may be confronted with untreated or contaminated water that poses a risk of acquiring enteric disease. In addition, disaster situations, such as the 2017 hurricanes that affected Houston, Texas, and Puerto Rico, may result in a breakdown of municipal water systems, exposing victims to nonpotable water. These situations necessitate knowledge of how to disinfect water at the point-of-use, prior to drinking.

Methods of water treatment that can be applied in the field include the use of heat, ultraviolet light, clarification, filtration, and chemical disinfection. The choices for the wilderness hiker or international traveler are increasing as new technology is applied to field applications. Different microorganisms have varying susceptibilities to these methods. The risk of waterborne illness depends on the number and type of organisms consumed, host factors, and the efficacy of the treatment system.

Methods

To develop these guidelines, specialists with expertise in wilderness medicine, travel medicine, public health, and microbiology were chosen on the basis of their clinical or research experience. Relevant articles were identified through the PubMed database using the following keywords or phrases: water disinfection, waterborne illness, wilderness water, water filtration, emergency or disaster drinking water treatment. This was supplemented by a hand search of articles from references in the initial PubMed search. Conclusions from review articles were cited in an effort to provide background information and to augment reference selection.

The evidence base for water disinfection has substantial differences from other clinical guidelines. Most of the literature concerning the effectiveness of specific disinfectants and methods against various waterborne microorganisms is laboratory based. Evidence on the benefits of disinfection is either population-based public health research of disease outbreaks or randomized household trials of water disinfection that are influenced by compliance and hygiene. Therefore, the evidence grade is a combination of laboratory, population, and household- or community-level studies.

The panel used a consensus approach to develop recommendations for the disinfection of water. Water treatment techniques and recommendations were not evaluated for the removal of chemicals or toxins. Evidence grades were assigned according to methodology stipulated by the American College of Chest Physicians for grading of evidence and recommendations³ (online Supplementary Table 1). These recommendations are graded on the basis of the totality of supporting evidence and balance between the benefits and risks or burdens for each modality.

Etiology and Risk of Waterborne Infection

Wilderness Settings

Millions of people enter wilderness areas each year and drink surface water. Even in developed countries with low rates of diarrheal illness, regular waterborne disease outbreaks indicate that the microbiologic quality of the water, especially surface water, is not ensured.^4-7 Public health agencies regularly report outbreaks of disease associated with surface water from backcountry and parks as well as from campground water systems. The environment and activity upstream from the travelers' surface water source defines the risk. Side streams draining springs, snowmelt, and glaciers where there is no human or animal activity are lower risk. In contrast, upstream usage by humans, farm animals, or wildlife pose a major risk. Cattle excrete pathogenic strains of Escherichia coli and Salmonella and have been found in multiple studies to be the major animal species contributing to waterborne disease in North America.^8,9 Giardiasis is a zoonotic infection with numerous host species, including farm animals, deer and other wild ungulates, beavers, and even household animals; however, the extent of transmission to humans is less defined.¹⁰

Nonalpine wilderness areas in the United States may have streams and rivers that are contaminated with animal waste, including farm animal runoff, or may be contaminated with incompletely treated sewage from towns and urban areas. In many countries, wilderness areas are co-occupied by local populations and domesticated animals that pollute water sources. Because it is very difficult to exclude animal and human activity in the watershed, the Centers for Disease Control recommend treating surface water before ingestion as a precaution to protect health.

International Travel

Substantial progress has been made in the past 20 years toward the goal of safe drinking water and sanitation worldwide, particularly in Asia and Latin America¹¹; however, 780 million people (11% of world population) still lack a safe water source, and 2.5 billion people lack access to improved sanitation. Africa and Oceania are the regions with the greatest need for improvement. More than 890 million persons still practice open defecation, the largest number being in India and Africa.^11-18 Contamination of tap water commonly occurs because of antiquated and inadequately monitored waste disposal, water treatment, and distribution systems.¹⁹

In both developed and developing countries, after natural disasters such as hurricanes, tsunamis, and earthquakes, one of the most immediate public health problems is a lack of potable water. Wilderness visitors and international travelers have no reliable resources to evaluate local water system quality. Less information is available for remote surface water sources. Appearance, smell, and taste are not reliable indicators to estimate water safety.

Infectious agents with the potential for waterborne transmission include bacteria, viruses, protozoa, and nonprotozoan parasites. The list of microbial agents is similar to the list of microorganisms that can cause travelers' diarrhea, most of which can be waterborne as well as foodborne. Although the primary reason for disinfecting drinking water is to destroy microorganisms from animal and human biologic wastes, water may also be contaminated with toxins and chemical pollutants from industrial sources or from the environment. Escherichia coli and Vibrio cholerae may be capable of surviving indefinitely in tropical water. Enteric bacterial and viral pathogens survive in temperate water generally only several days; however, some species such as E coli O157: H7 can survive 12 weeks at 25°C.²¹ Most enteric organisms, including Shigella spp, Salmonella enterica serotype Typhi, hepatitis A, and Cryptosporidium spp, can retain viability for long periods in cold water and can even survive for weeks when frozen in water.

The risk of waterborne illness depends on the number of organisms consumed, which is in turn determined by the volume of water, concentration of organisms, and treatment system efficiency.^22,23 Additional factors include virulence of the organism and defenses of the host. Microorganisms with a small infectious dose (eg, Giardia, Cryptosporidium, Shigella spp, hepatitis A, enterohemorrhagic E coli, and norovirus-the leading viral disease risk in water contaminated with human waste) may cause illness even from inadvertent drinking during water-based recreational activities.¹⁰ Most diarrhea among travelers is probably foodborne; however, the capacity for waterborne transmission should not be underestimated. Because long-lasting immunity does not develop for most enteric pathogens, reinfection may occur.

The combined roles of safe water, hygiene, and adequate sanitation in reducing diarrhea and other diseases are clear and well documented. The World Health Organization (WHO) estimates that 94% of diarrheal cases globally are preventable through modifications to the environment, including access to safe water.¹ Recent studies of simple water interventions in households of developing countries clearly document improved microbiological quality of water, a 30 to 60% reduced incidence of diarrheal illness, enhanced childhood survival, and reduction of parasitic diseases, many of which are independent of other measures to improve sanitation.²⁴

General recommendations for drinking water disinfection:

• Treat water when traveling in developing countries. Evidence grade: 1A
• Treat water in wilderness areas with nearby agricultural use, animal grazing, or upstream human activity. Evidence grade: 1A
• Treat water in wilderness settings without evidence of domestic animal and little to no wildlife or human activity. Evidence grade: 2B
• Treat water in disaster situations affecting municipal or private drinking water sources. Evidence grade: 1A

Water Treatment Methods

Multiple techniques for improving the microbiologic quality of water are available to individuals and small groups while hiking or traveling. Bottled water may be a convenient and popular solution but creates ecologic problems. Furthermore, in underdeveloped countries, the quality of bottled water may not meet the standards of developed countries and may contain pathogenic microbes.²⁵

The term disinfection, the desired result of field water treatment, is used here to indicate the removal or destruction of harmful microorganisms, which reduces the risk of illness. This is sometimes used interchangeably with purification, but the latter term more accurately indicates the removal of organic or inorganic chemicals and particulate matter to improve color, taste, and odor. Unless specifically designed to remove chemical contaminants, disinfection techniques may not make water safe from chemical exposures. Potable implies drinkable water, but it technically means that a water source, on average, over a period of time, contains a minimal microbial hazard so that the statistical likelihood of illness is acceptably low. All standards, including water regulations in the United States, acknowledge the impracticality of trying to eliminate all microorganisms from drinking water. Generally, the goal is a 3 to 5 log reduction (99.9%-99.999%), allowing a small risk of enteric infection. Newer standards from the US Environmental Protection Agency (US EPA) and the WHO set target goals to reduce some organisms to zero; however, all enforceable standards allow a small risk for enteric infection.²⁶

Product Testing and Rating

Filters are rated by their ability to retain particles of a certain size, which is described by 2 terms. Absolute rating means that 100% of a certain size of particle is retained by the filter (ie, filtered-out). Nominal rating indicates that 90% of a given particle size will be retained. Filter efficiency is generally determined with hard particles (beads of known diameter), but microorganisms are soft and compressible under pressure. The US EPA and NSF International are the primary agencies that set standards for disinfection products and protocols for testing to meet these standards.

The US EPA does not endorse, test, or approve mechanical filters; it merely assigns registration numbers that distinguish between 2 types of filters: those that use mechanical means only and those that use a chemical designated as a pesticide. Portable water treatment device claims for microbiologic reduction are based on consensus performance standards that serve as a guideline for testing.²⁷ Testing is done or contracted by the manufacturer; the US EPA neither tests nor specifies laboratories. Testing must be done with bacteria (Klebsiella), viruses (poliovirus and rotavirus), and protozoa (Cryptosporidium has replaced Giardia). A 3-log reduction (99.9%) is required for protozoan cysts, 4-log reduction (99.99%) for viruses, and 5- to 6-log reduction for bacteria. To be called a microbiologic water purifier, the unit must remove, kill, or inactivate all types of disease-causing microorganisms from the water, including bacteria, viruses, and protozoan cysts, so as to render the processed water safe for drinking. An exception for limited claims may be allowed for units removing specific organisms to serve a definable environmental need, for example, removal of protozoan cysts.²⁷

Clarification Techniques

Clarification refers to techniques that reduce the turbidity or cloudiness of water caused by natural organic and inorganic material. (Turbidity is measured in nephelometric turbidity units [NTU].) These techniques can markedly improve the appearance and taste of water. They may reduce the number of microorganisms, but not enough to ensure potable water; however, clarifying the water facilitates disinfection by filtration or chemical treatment. Cloudy water can rapidly clog filters designed to remove microorganisms. Moreover, cloudy water requires increased levels of chemical treatment, and the combined effects of the water contaminants plus chemical disinfectants results in unpleasant taste.

Adsorption

Granular activated carbon (GAC) is widely used in water treatment. When activated, charcoal's regular array of carbon bonds is disrupted, making it highly reactive for adsorbing dissolved chemicals.^28,29 GAC is the best means to remove toxic organic and inorganic chemicals from water (including disinfection byproducts) and to improve odor and taste.^30,31 Thus, it is widely used in municipal disinfection plants, in household under-sink devices, and in portable water filters. In field water treatment, GAC is best used after chemical disinfection to make water safer and more palatable by removing disinfection byproducts and pesticides, as well as many other organic chemicals and some heavy metals. It removes the taste of chemical disinfectants such as iodine and chlorine.

GAC does not kill microorganisms and is not designed for microbial removal; in fact, bacteria attach to charcoal, where they are resistant to chlorination because the chlorine is adsorbed by the GAC.^30-32

Sedimentation

Sedimentation is the separation of suspended particles such as sand and silt that are large enough to settle rapidly by gravity. Most microorganisms, especially protozoan cysts, also settle eventually, but this takes much longer.³³ Simply allowing the water to sit undisturbed for about 1 h or until sediment has formed on the bottom of the container and then decanting or filtering the clear water from the top through a coffee filter or finely woven cloth will remove many larger particles from the water. A second method of disinfection must then be used to obtain potable water.

Coagulation-flocculation

Coagulation-flocculation (C-F) is a technique that has been in use since 2000 bc and remains a routine step in municipal water treatment.^34,35 C-F can remove smaller suspended particles and chemical complexes too small to settle by gravity (colloids). Coagulation is achieved with the addition of a chemical that causes particles to stick together by electrostatic and ionic forces. Flocculation is a physical process that promotes the formation of larger particles by gentle mixing. Alum (an aluminum salt), lime (alkaline chemicals principally containing calcium or magnesium with oxygen), or iron salts are commonly used coagulants. Alum is nontoxic and used in the food industry for pickling. It is readily available in most chemical supply stores and some grocery stores. C-F removes 60 to 98% of microorganisms, heavy metals, and some chemicals and minerals.^36,37 The tendency of microorganisms to clump with small particles or clump together to form larger aggregates enhances their removal by C-F. C-F also has the benefit of reducing the amount of chemical disinfectant needed because turbidity increases demand for disinfectants such as hypochlorite.^37-39

The amount of alum added in the field, approximately 1 large pinch (1 mL or 1/8 tsp) per 4 L (approximately 1 gal) of water, need not be precise. Stir or shake briskly for 1 min to mix, and then agitate gently and frequently for at least 5 min to assist flocculation. If the water is still cloudy, add more flocculent and repeat mixing. After at least 30 min for settling, pour the water through a fine-woven cloth or paper filter. Although most microorganisms are removed with the floc, a final process of microbiologic filtration or chemical disinfection (below) should be completed to ensure disinfection. Several products combine C-F with halogen disinfection, which allows a single-step process.^40-43

Improvisational techniques for clarification

Many inorganic and organic compounds can be used as a coagulant, including lime (calcium oxide) or potash (from wood ash).⁴⁴ In an emergency, bleaching powder, baking powder, or even the fine white ash from a campfire can be used.⁴⁵ Other C-F agents used traditionally by native peoples include seed extracts from the nirmali plant in southern India, moringa plants in Sudan, crushed almonds, dried and crushed beans, and rauwaq (a form of bentonite clay).⁴⁶

Adsorbents such as charcoal, clay, and other types of organic matter have been used for water treatment since biblical times.³² These substances are used as the filter media and also can act as coagulants.⁴⁷ Clays can decrease turbidity and microbes in water by about 90 to 95%, but adsorption is not the main action of ceramic or clay filters.

Adsorbents such as charcoal, clay, and other types of organic matter have been used for water treatment since biblical times.³² These substances are used as the filter media and also can act as coagulants.⁴⁷ Clays can decrease turbidity and microbes in water by about 90 to 95%, but adsorption is not the main action of ceramic or clay filters.

Assessment of supporting evidence:

• Clarification reduces cloudiness, particulate matter, and waterborne microorganisms; improves the taste and esthetics of water; and improves the effectiveness of chemical disinfectants, filtration, and ultraviolet disinfection. However, it does not reliably disinfect if used alone. Evidence grade: 1A
• GAC is highly effective at removing taste and odor compounds but is not adequate for microbial removal. Evidence grade: 1A
• Sedimentation is effective for removing large particles such as sand and dirt but will not remove suspended or dissolved substances (see C-F). Evidence grade: 2B

Clarification reduces cloudiness, particulate matter, and waterborne microorganisms; improves the taste and esthetics of water; and improves the effectiveness of chemical disinfectants, filtration, and ultraviolet disinfection. However, it does not reliably disinfect if used alone. Evidence grade: 1A

GAC is highly effective at removing taste and odor compounds but is not adequate for microbial removal. Evidence grade: 1A

Sedimentation is effective for removing large particles such as sand and dirt but will not remove suspended or dissolved substances (see C-F). Evidence grade: 2B

C-F removes most microorganisms, but it does not reliably disinfect if used alone. Evidence grade: 1A

Traditional or improvisational C-F techniques (other than alum or those used in municipal disinfection plants) have empiric evidence but do not have robust scientific evidence or practical use guidance and should be used with caution to protect the health of consumers. Evidence grade: 2C

Traditional or improvisational C-F techniques (other than alum or those used in municipal disinfection plants) have empiric evidence but do not have robust scientific evidence or practical use guidance and should be used with caution to protect the health of consumers. Evidence grade: 2C

Disinfection Methods

Heat

Heat is the oldest and most reliable means of water disinfection. Heat inactivation of microorganisms is a function of time and temperature (exponential function of first-order kinetics). Thus, the thermal death point is reached in a shorter time at higher temperatures, whereas lower temperatures are effective if applied for a longer time. Pasteurization uses this principle to kill food pathogens and spoiling organisms at temperatures well below boiling, generally between 60°C (140°F) and 70°C (158°F). Flash pasteurization occurs within 30 s at 70 to 72°C (158 to 162°F).^48,49

All common enteric pathogens are readily inactivated by heat at pasteurization temperatures, although microorganisms vary in heat sensitivity, with protozoan cysts being the most sensitive to heat, bacteria intermediate, and viruses less sensitive (Table 1^50-62).^50,51 Only bacterial spores are more resistant, but they are not generally enteric pathogens.⁵²

Table 1. Heat inactivation of microorganisms

_Organism_	_Lethal temperature/Time_	_Reference_
Protozoan cysts, including _Giardia, Entamoeba histolytica_	50°C (122°F) for 10 min 55°C (131°F) for 5 min 100°C (212°F) immediately	53-55
_Cryptosporidium oocysts_	55°C (131°F) warmed over 20 min 64°C (148°F) within 2 min	50,56
Parasitic eggs, larvae, and cercariae	50°C-55°C (122-131°F)	57
Common bacterial enteric pathogens (_E coli_, _Salmonella_, _Campylobacter_, _Shigella_)	55°C (131°F) for 30 min or 65°C (149°F) for less than 1 min (standard pasteurization temperatures)	48,51
Viruses	56°C-60°C (133-140°F) in less than 20-40 min	52,58,59
Hepatitis A virus	98°C (208°F) for 1 min 75°C (167°F) for less than 0.5 min 85°C (185°F) for 1 min or less (in various food products)	60-62

As enteric pathogens are killed within seconds by boiling water rapidly at temperatures > 60°C (140°F), the traditional advice to boil water for 10 min to ensure potable water is excessive. The time required to heat water from 55°C (131°F) to a boil works toward disinfection; therefore, any water brought to a rapid boil should be adequately disinfected.⁶³ Boiling for 1 min is recommended by the US CDC to account for user variability in identifying boiling points and adds a margin of safety. The boiling point decreases with increasing altitude, but this is not significant compared with the time required for thermal death at these temperatures (Table 2).

Table 2. Boiling temperatures at various altitudes

_Altitude (ft)_	_Altitude (m)_	_Boiling point_
5000	1524	95°C (203°F)
10,000	3048	90°C (194°F)
14,000	4267	86°C (187°F)
19,000	5791	81°C (178°F)

Improvisational techniques

In wilderness or travel environments, the main limitation for using heat is availability of fuel. Although attaining boiling temperature is not necessary to kill microorganisms, boiling is the only easily recognizable endpoint without use of a thermometer. Based on microbiologic testing, hot tap water has been proposed as a means of heat disinfection.^64,65

Most water from hot water taps measured in countries outside the United States measured 55 to 60°C (131 to 140°F).⁵¹ As a rule of thumb, water too hot to touch fell within the pasteurization range, but tolerance to touch is too variable to be reliable.⁶⁶

If no reliable method of water treatment is available, tap water that has been kept hot in a tank for at least 30 min and is too hot to keep a finger immersed for 5 s (estimated 55 to 65°C; 131 to 149°F) is a reasonable alternative. However, this improvisational measure is less useful for hotels that use on-demand water heaters without a hot water tank. Travelers with access to electricity can boil water with either a small electric heating coil or a lightweight electric beverage warmer brought from home. In austere and desperate situations with hot, sunny climate, pasteurization temperature can be achieved with a solar oven or simple reflectors^67,68 (see the Solar UV Disinfection [UV-SODIS] section).

If no reliable method of water treatment is available, tap water that has been kept hot in a tank for at least 30 min and is too hot to keep a finger immersed for 5 s (estimated 55 to 65°C; 131 to 149°F) is a reasonable alternative. However, this improvisational measure is less useful for hotels that use on-demand water heaters without a hot water tank. Travelers with access to electricity can boil water with either a small electric heating coil or a lightweight electric beverage warmer brought from home. In austere and desperate situations with hot, sunny climate, pasteurization temperature can be achieved with a solar oven or simple reflectors^67,68 (see the Solar UV Disinfection [UV-SODIS] section).

Assessment of supporting evidence:

• Bringing water to boil (100°C/212°F) will kill pathogenic microorganisms. Evidence grade: 1A
• Bringing water at 5000 m (16,000 ft) elevation to boil (83°C/181°F) will kill pathogenic organisms. Evidence grade: 1B
• Tap water that has been tanked for 30 min or longer and is too hot to touch (60°C) has a significantly reduced number of pathogenic microorganisms, but this cannot be relied on as the sole means of disinfection. Such water may contain increased amounts of lead or other chemicals from the water heater and piping. Evidence grade: 2B
• Pasteurization temperatures can be achieved with a solar oven. Evidence grade: 2B

Ultraviolet Light

Ultraviolet (UV) radiation and UV lamp disinfection systems are widely used to disinfect drinking water at the community and household levels. At sufficient doses, all waterborne enteric pathogens are inactivated by UV radiation (UVR). UVC light in the range of 200 to 280 nm is the most effective. The germicidal effect of UV light is the result of action on the nucleic acids of microorganisms and depends on light intensity and exposure time. In sufficient doses of energy, all waterborne enteric pathogens are inactivated by UVR.⁶⁹ The UV waves must strike the organism, so the water must be free of particles that could act as a shield.⁷⁰ The UV waves do not alter the water, but they also do not provide any residual disinfecting power.⁷¹ Bacteria and protozoan parasites generally require lower doses than do enteric viruses and bacterial spores. However, all viruses, including hepatitis A and norovirus, are susceptible, with relatively minor differences, and follow similar kinetics. The vegetative cells of bacteria are significantly more susceptible to UVR than are bacterial spores or viruses. Giardia and Cryptosporidium are susceptible to practical doses of UVR and may be more sensitive because of their relatively large size.^72-74 Both large high-volume units and portable, lightweight battery-operated units are available for disinfection of small quantities of water.

Ultraviolet (UV) radiation and UV lamp disinfection systems are widely used to disinfect drinking water at the community and household levels. At sufficient doses, all waterborne enteric pathogens are inactivated by UV radiation (UVR). UVC light in the range of 200 to 280 nm is the most effective. The germicidal effect of UV light is the result of action on the nucleic acids of microorganisms and depends on light intensity and exposure time. In sufficient doses of energy, all waterborne enteric pathogens are inactivated by UVR.⁶⁹ The UV waves must strike the organism, so the water must be free of particles that could act as a shield.⁷⁰ The UV waves do not alter the water, but they also do not provide any residual disinfecting power.⁷¹ Bacteria and protozoan parasites generally require lower doses than do enteric viruses and bacterial spores. However, all viruses, including hepatitis A and norovirus, are susceptible, with relatively minor differences, and follow similar kinetics. The vegetative cells of bacteria are significantly more susceptible to UVR than are bacterial spores or viruses. Giardia and Cryptosporidium are susceptible to practical doses of UVR and may be more sensitive because of their relatively large size.^72-74 Both large high-volume units and portable, lightweight battery-operated units are available for disinfection of small quantities of water.

Improvisational technique: UV-SODIS

UV irradiation by sunlight can substantially improve the microbiologic quality of water and reduce diarrheal illness in developing countries.^75-86 The optimal procedure for the SODIS technique is to use transparent bottles (eg, clear plastic beverage bottles), preferably lying on a dark surface and exposed to sunlight for a minimum of 4 h with intermittent agitation.⁸⁶ UV and thermal inactivation are strongly synergistic for the solar disinfection of drinking water.^67,87-88

Assessment of supporting evidence:

• UV light is an effective means of water disinfection. Evidence grade: 1A
• Full sunlight exposure of clear water in a clear plastic bottle for at least 4 h significantly reduces and possibly eliminates microorganism contamination (Evidence grade: 1B); however, studies evaluating this technique for reduction of childhood diarrhea show mixed results. Evidence grade: 2B

Filtration

Filters are appealing because of their simplicity and suitability for commercial production. Portable water treatment products are the third highest intended purchase of outdoor equipment, after backpacks and tents.⁸⁹ Filtration is a standard step in municipal water treatment and widely used in the food and beverage industry and in many other industrial processes. Many different types of media, from sand to vegetable products to fabric have been used for water filtration throughout history in various parts of the world.⁹⁰ Filters have the advantages of being simple and requiring no holding time. They do not add any unpleasant taste and may improve taste and appearance of water. All filters eventually clog from suspended particulate matter (present even in clear streams), requiring cleaning or replacement of the filter. As a filter clogs, it requires increasing pressure to drive the water through it, which can force microorganisms through the filter or damage the filter. A crack or eroded channel in a filter will allow passage of unfiltered water. Bacteria can grow on filter media and potentially result in some bacteria in filtered water, but pathogenic bacteria and illness have not been demonstrated.⁹¹ Silver is often incorporated into the filter media to prevent this growth, but it is not totally effective.

Filters are appealing because of their simplicity and suitability for commercial production. Portable water treatment products are the third highest intended purchase of outdoor equipment, after backpacks and tents.⁸⁹ Filtration is a standard step in municipal water treatment and widely used in the food and beverage industry and in many other industrial processes. Many different types of media, from sand to vegetable products to fabric have been used for water filtration throughout history in various parts of the world.⁹⁰ Filters have the advantages of being simple and requiring no holding time. They do not add any unpleasant taste and may improve taste and appearance of water. All filters eventually clog from suspended particulate matter (present even in clear streams), requiring cleaning or replacement of the filter. As a filter clogs, it requires increasing pressure to drive the water through it, which can force microorganisms through the filter or damage the filter. A crack or eroded channel in a filter will allow passage of unfiltered water. Bacteria can grow on filter media and potentially result in some bacteria in filtered water, but pathogenic bacteria and illness have not been demonstrated.⁹¹ Silver is often incorporated into the filter media to prevent this growth, but it is not totally effective.

The primary determinant of a microorganism's susceptibility to filtration is its size (Table 3; Figure 1). Portable filters for water treatment can be divided into microfilters with pore sizes down to 0.1 μm, ultrafilters that can remove particles as small as 0.01 μm, nanofilters with pore sizes as small as 0.001 μm or less, and reverse osmosis filters with pore sizes of 0.0001 μm or less.⁶⁹ All filters require pressure to drive the water through the filter element. The smaller the pore size, the more pressure required. Waterborne pathogens often adhere to larger particles or clump together, making them easier to remove by physical processes. Therefore, observed reductions are often greater than expected based on their individual sizes.

Table 3. Microorganism susceptibility to filtration

Table 3. Microorganism susceptibility to filtration

_Organism_	_Approximate size (μm)_	_Recommended filter rating (μm)_
Viruses_a_	0.03	Ultrafilter, nanofilter, reverse osmosis
_Escherichia coli_	0.5 by 3-8	0.2-0.4 (microfilter)
_Campylobacter_	0.2-0.4 by 1.5-3.5
_V cholerae_	0.5 by 1.5-3.0
_Cryptosporidium_ oocyst	2-6	1 (microfilter)

_Organism_	_Approximate size (μm)_	_Recommended filter rating (μm)_
Viruses_a_	0.03	Ultrafilter, nanofilter, reverse osmosis
_Escherichia coli_	0.5 by 3-8	0.2-0.4 (microfilter)
_Campylobacter_	0.2-0.4 by 1.5-3.5
_V cholerae_	0.5 by 1.5-3.0
_Cryptosporidium_ oocyst	2-6	1 (microfilter)
_Giardia_ cyst	6-10 by 8-15	3-5 (microfilter)
_Entamoeba histolytica_ cyst	5-30 (average 10)
Nematode eggs	30-40 by 50-80	20 (microfilter)
Schistosome cercariae	50 by 100	Coffee filter or fine cloth, or double thickness closely woven cloth
_Dracunculus_ larvae	20 by 500

Microfilters (includes most filters with pore size of 0.1-0.2 μm) can filter bacteria and protozoan cysts, but are not effective for virus removal unless designed to rely on electrostatic trapping of viruses. Hollow fiber filters with 0.02 μm pores and reverse osmosis filters are capable of filtering viruses.

Most portable filters are microfilters that can readily remove protozoan cysts and bacteria but may not remove all viruses, which are much smaller than the pore size of most field filters.^92,93 Viruses often clump together and to other larger particles or organisms, resulting in an aggregate large enough to be trapped by the filter; in addition, electrochemical attraction may cause viruses to adhere to the filter surface.^47,94,95 Through these mechanisms, mechanical filters using ceramic elements with a pore size of 0.2 μm can reduce viral loads by 2 to 3 logs (99-99.9%), but they are not adequate for complete removal of viruses.⁹⁶ Ultrafiltration membranes are required for complete microbial removal, including viruses; they can also remove colloids and some dissolved solids.⁹⁷

Recently, hollow-fiber technology has been adapted for field use; this technology uses bundles of tube fibers whose pore size can be engineered to achieve ultrafiltration with viral removal.⁹⁸ The large surface area allows these hollow-fiber filters to have relatively high flow rates at low pressure. Small group and individual gravity or hand pump filters are available through several vendors.

Some filters on the market combine the porous filter material with other substances to help the disinfection process. This may include activated charcoal, iodine, silver, and other substances. Iodine molecules can be bound in a resin engineered into field products, but the effectiveness of the resin is highly dependent on the product design and function. Most companies have abandoned iodine resin-containing portable hand-pump filters due to excess iodine or viral breakthrough in the effluent. Only one drink-through bottle remains on the US market, but other products may still be available outside the United States. (GAC was discussed earlier, and silver is addressed later.)

Several factors influence the decision of which filter to buy: 1) flow volume sufficient for the number of persons relying on the filter; 2) whether the filter functional claims matches the microbiologic demands that will be put on the filter; 3) the preferred means of operation (eg, hand pump or gravity); and 4) cost.

Improvisational filtration techniques

Filtration using simple, available products, such as rice hull ash filters, crushed charcoal, sponges, and various fabrics and paper, have all been used in developing countries and in emergency situations. Typically, bacteria and viruses can be reduced by as much as 50 to 85% and larger parasites by 99%, depending on the media. The effectiveness for decreasing turbidity may be used as an indicator that a filter material will reduce microbiologic contamination.^38,99,100

Ceramic filters are a common component in portable water pump filters, but they are also a cost-effective means of household disinfection in developing countries. Ceramic clay is widely available and very inexpensive to locally manufacture in the shape of a sink or flower pot that is set into a larger container that collects the filtered water.^{101,102,103,104,105,106,107}

Biosand filters use a technology that has been used over centuries and is still used widely in municipal plants and at the household and community level.^{108,109,110,111} Sand filters can be highly effective at removing turbidity (in 1 study, from 6.2 NTU to 0.9 NTU) and improving microbiologic quality (99% efficacy), depending on their design and operation.^112,113 Sand filters are constructed by forming layers of aggregate increasing in size from the top to the bottom. The top layer is very fine sand and the bottom layer consists of large gravel. The container needs an exit port on the bottom. The top layer forms a biolayer that is important for the function of the filter. The optimum depth of a community or household sand filter is 2 m, with diameter determined by the volume of water needed. An emergency sand filter can be made in a 20 L (5.3 gal) bucket, composed of a 10 cm (3.9 in) layer of gravel beneath a 23 cm (9.1 in) layer of sand; a layer of cotton cloth, sandwiched between 2 layers of wire mesh, separates the sand and gravel layers.³⁸ A sand filter also can be improvised with stacked buckets of successive filter layers with holes in the bottom to allow water passage. Many websites provide design and assembly instructions, but there are no data for comparative function.

Assessments of supporting evidence:

• Filtration is effective as a primary or adjunctive means of water treatment. Evidence grade: 1A
• Standard commercially available microfilters with a pore size of 0.2 microns are effective in removing protozoa and bacteria. Evidence grade: 1A
• Ultrafiltration with pore size of less than 0.01 is needed to completely remove pathogenic viruses. Evidence grade: 1A -Filters may clog, so users should know how to clean them or consider carrying a backup method of disinfection. Evidence grade: 1C
• Biosand filters are a reasonable improvised technique for filtration. Evidence grade: 1B

Chemical Disinfection: Halogens (Iodine and Chlorine)

Worldwide, disinfection with chemicals, chiefly chlorine, is the most commonly used method for improving and maintaining the microbiologic quality of drinking water and can be used by individuals and groups in the field.¹¹⁴ The germicidal activity of chlorine and other halogens is well established and results from oxidation of essential cellular structures and enzymes.^115,116 Disinfection effectiveness is determined by characteristics of the microorganism, the disinfectant, contact time, and environmental factors. Both chlorine and iodine are widely available worldwide in multiple formulations. The most commonly available form of chlorine is hypochlorite (household bleach [5 to 8%] or concentrated swimming pool granules or tablets [70%]).

Both chlorine and iodine have been used for water disinfection for more than a century. Hypochlorite, the major chlorine disinfectant, is currently the preferred means of municipal water disinfection worldwide. Both calcium hypochlorite (Ca[OCl]²) and sodium hypochlorite (NaOCl) readily dissociate in water to form hypochlorite, the active disinfectant.

Iodine is also effective in low concentrations for killing bacteria, viruses, and some protozoan cysts; in higher concentrations, it is effective against fungi and even bacterial spores. However, it is a poor algaecide. Elemental iodine (I²) and hypoiodous acid (HOI) are the major germicides in an aqueous solution. Iodine is the only halogen that is a solid at room temperature.

Iodine is also effective in low concentrations for killing bacteria, viruses, and some protozoan cysts; in higher concentrations, it is effective against fungi and even bacterial spores. However, it is a poor algaecide. Elemental iodine (I²) and hypoiodous acid (HOI) are the major germicides in an aqueous solution. Iodine is the only halogen that is a solid at room temperature. Given

Chlorine is still advocated by the WHO and the CDC as a mainstay of large-scale community, individual household, and emergency use.^118,119 There are extensive data on effectiveness of hypochlorite in remote settings.^{69,120,121,122} The CDC/WHO safe water system for household disinfection in developing countries provides a dosage of 1.875 or 3.75 mg/ L^-1 of sodium hypochlorite with a contact time of 30 min, which is sufficient to inactivate most bacteria, viruses, and some protozoa that cause waterborne diseases.¹²³ Another advantage of hypochlorite is the ease of adjusting the dose for large volumes of water.^45,99

Vegetative bacteria (nonspore forming) are very sensitive to halogens.^116,124 Viruses, including hepatitis A, have intermediate sensitivity, requiring higher concentrations or longer contact times.^{125,126,127,128,129,130} Protozoan cysts are more resistant than enteric bacteria and enteric viruses but some cysts (eg, Giardia) can be inactivated by field doses of halogens.^{131,132,133,134,135} Cryptosporidium oocysts, however, are much more resistant to halogens, and inactivation is not practical with common doses of iodine and chlorine used in field water disinfection.^136,137 Little is known about Cyclospora, but it is assumed to be similar to Cryptosporidium. Certain parasitic eggs, such as those of Ascaris, are also resistant, but these are not commonly spread by water. (All of these resistant cysts and eggs are susceptible to heat or filtration.) Bacterial spores, such as Bacillus anthracis, are relatively resistant to halogens. With chlorine, however, spores are not much more resistant than are Giardia cysts; furthermore, they do not normally cause waterborne enteric disease. Relative susceptibility between organisms is similar for iodine and chlorine (Table 4).

Table 4. Disinfection data for chlorine and iodine to achieve 99.9% kill or inactivation^a of select microorganisms

_Organism_	_Concentration (mg/ L-1 )_	_Time (min)_	_pH_	_Temp_	_Disinfection constant (Ct)__a_	_Reference_
Chlorine
_Escherichia coli_	0.1	0.16	6.0	5°C (41°F)	0.016	116
_Campylobacter_	0.3	0.5	6.0-8.0	25°C (77°F)	0.15	124
20 enteric virus	0.5	60	7.8	2°C (36°F)	30	138
6 enteric viruses	0.5	4.5	6.0-8.0	5°C (41°F)	2.5	125
Norovirus	1 5	10 20 sec	6.0	5°C	10 1.66	126
Hepatitis A virus	0.5	1	6.0	25°C (77°F)	0.5	127
Amebic cysts	3.5	10		25°C (77°F)	35	139
_Giardia_ cysts	2.5	60	6.0-8.0	5°C (41°F)	150	140
_Giardia lamblia_ cysts	0.85	90	8.0	2-3°C (36-37°F)	77	135
_Giardia muris_ cysts	3.05	50	7.0	5°C (41°F)	153	134
_Cryptosporidium_ (2 strains)	20 20	755 501	7.5 7.5	23°C 23°C	15,300 10,400	141
Iodine
_Escherichia coli_	1.3	1	6.0-7.0	2-5°C (36-41°F)	1.3	31
Hepatitis A‡	8	.4	7.0	25°C	3	142
Coxsackie virus	0.5	30	7.0	5°C (41°F)	15	143
Amebic cysts	3.5	10		25°C (77°F)	35	139
_Giardia_ cysts	4	15	5.0	30°C (86°F)	60_b_	131
_Giardia_ cysts	4	45	5.0	15°C (59°F)	170_b_	131
_Giardia_ cysts	4	120	5.0	5°C (41°F)	480_b_	131

99.9% is for comparison of disinfection potency and microorganism susceptibility. The standard for potable water is 99.99% kill for viruses and 99.999% for bacteria. This would be achieved in each example with a higher concentration of disinfectant or a longer contact time.

100% kill; viability tested only at 15, 30, 45, 60, and 120 min.

Understanding factors that influence the disinfection reaction allows flexibility with greater reassurance. The primary factors of the first-order chemical disinfection reaction are concentration and contact time.¹³³ To achieve microbial inactivation in aqueous solution with a chemical agent, a residual concentration must be present for a specified contact time. Lower concentrations can be used with longer contact times. In field disinfection, this can be used to minimize halogen dose and improve taste or, conversely, to minimize the required contact time.

Cold water slows chemical reactions; the reaction rate can be adjusted by longer contact times or higher concentration of disinfectant chemical. Another important factor in chemical disinfection is the presence of organic and inorganic contaminants, mainly nitrogen compounds from decomposition of organisms and their wastes, fecal matter, and urea. These contaminants react, especially with chlorine, to form compounds with little or no disinfecting ability, effectively decreasing the concentration of available halogen.^26,115 Halogen demand is the amount of halogen reacting with impurities. Residual concentration is the amount of active disinfectant remaining after demand of the water is met. Halogen demand is associated with turbidity (cloudiness).³⁹ Typical recommendations for field treatment double the amount of chlorine or iodine in cloudy water; however, it is preferable to use clarification techniques prior to chemical disinfection in cloudy water to improve efficacy and taste.^144,145

Because of the difficulty of estimating halogen demand, it is prudent to use 3 to 4 mg/ L^-1 as a target halogen concentration range for clear surface water. Lower concentrations (eg, 2 mg/ L^-1 ) can be used for back-up treatment of questionable tap water or high-quality well water (Table 5,Table 6).

Table 5. Halogen disinfection products and recommended doses

	_Add to 1 L or qt of water_
_Iodination techniques__a_	_Amount to achieve 4 mg/ L__-1_	_Amount to achieve 8 mg/ L__-1_
Iodine tabs_b_	0.5 tab (or 1 tab in 2 L)	1 tab
Tetraglycine hydroperiodide
Emergency drinking water germicidal tablet
Potable aqua
Globaline
2% iodine solution (tincture)	0.2 mL	0.4 mL
	5 drops_c_	10 drops
10% povidone-iodine solution_d_	0.35 mL	0.70 mL
	8 drops	16 drops
Saturated solution: iodine crystals in water_e_	13 mL	26 mL
Chlorination techniques_f_	Amount to achieve 2 mg/ L-1	Amount to achieve 5 mg/ L-1
Sodium hypochlorite (household bleach 5%)	1 drop	0.1 mL 2 drops
Sodium hypochlorite (household bleach 8.25%)	1 drop (in 2 L)	1 drop
1% bleach (CDC-WHO Safe Water System)_g_	4-5 drops	8-10 drops
Calcium hypochlorite_h_ (Redi Chlor [0.1-g tab])	Cannot use in small quantities for low concentrations	0.25 tab
Sodium dichloroisocyanurate (NaDCC)_i_ (Aquatab, Kintab)	0.25 tab of 8.5 mg NaDCC (may be impractical)	0.5 tab (8.5 mg NaDCC)
Chlorine plus flocculating agent (C-F)	Not practical for small volumes	0.5 tablet per gal yields 5 mg/ L-1

^aWorld Health Organization recommends only for short-term emergency use.

^bIodine tablets were developed by the military with the criteria that they will disinfect water, including for Giardia, with a short contact (holding) time of 10 min because troops in the field may not wait longer. This high concentration is not necessary for field disinfection of clear water; it is preferable to target 4 mg/ L^-1 and wait longer. Additionally, the recommendation to use 8 mg/ L^-1 for cloudy water will result in poor taste, so it is recommended to clarify the water first.

^cMeasure of a drop varies from 16-24 gtt/ mL^-1 , standard 20 gtt/ mL^-1 is used here.

^dPovidone-iodine solutions release free iodine in levels adequate for disinfection, but scant data are available (see text).

^eA small amount of elemental iodine goes into solution (no significant iodide is present); the saturated solution is used to disinfect drinking water. Water can be added to the crystals hundreds of times before they are completely dissolved.

^fCan easily be adapted to large or small quantities of water. Simple field test kits or swimming pool test kits with color strips are widely available to ensure adequate residual chlorine. In usual situations, EPA recommends a target residual of 4 mg/ L^-1 . For household use, the CDC recommends < 2 mg/ L^-1 . Many of the recommended emergency doses exceed this threshold.⁹⁷ For treatment of large volumes, see formula to calculate in Lantagne (2008).²⁰

^gSafe water system for long-term routine household point-of-use water disinfection recommends a hypochlorite dose of about 2 mg/ L^-1 in clear water and 4 mg/ L^-1 in slightly turbid water. This results in a low yet effective target residual concentration but requires testing in a particular water source to ensure sufficient residual.

^hStable, concentrated (70%), dry source of hypochlor

ⁱAvailable in different strengths to treat different volumes of water. Check packaging to determine proper dose.

Table 6. Recommendations for contact time using halogen disinfection in the field

_Concentration of halogen_	_Contact time (min) at various water temperatures_
	_5°C (41°F)_	_15°C (59°F)_	_30°C (86°F)_
2 ppm	240	180	60
4 ppm	180	60	45
8 ppm	60	30	15

Concentration and contact time are based on the most resistant organism, which is the Giardia cyst. These are well beyond the time needed to kill bacteria and viruses. These contact times have been extended from the usual recommendations in cold water to account for the extended inactivation time required in very cold water and for the uncertainty of residual concentration.

Halogen toxicity

Chlorine has no known toxicity at the concentrations used for water disinfection. Sodium hypochlorite is not carcinogenic; however, reactions of chlorine with certain organic contaminants yield chlorinated hydrocarbons, chloroform, and other trihalomethanes, which are considered to have carcinogenic potential in animal models. Nevertheless, the risk of severe illness or even death from infectious diseases if disinfection is not used far exceeds any risk from byproducts of chlorine disinfection.¹⁴⁶

Despite several advantages over chlorine disinfection, iodine has not gained general acceptance because of concern for its physiologic activity. Some older data indicate that iodination of water with a low residual concentration of ≤1 to 2 mg/ L^-1 appears safe, even for long periods of time, in people with normal thyroid function.^147,148 This is not the current recommendation of major agencies. Recently, the European Union stopped the sale of iodine products used for water disinfection. The WHO did not set a guideline value for iodine in drinking water because of a paucity of data and because it is not recommended for long-term disinfection. If the typical wilderness or international traveler disinfected 3 L of water a day using 2 to 4 mg/ L^-1 of iodine, the ingested amount of iodine would be 6 to 12 mg/ d^-1 , well above US Institute of Medicine recommended dietary allowance levels. Levels produced by the recommended doses of iodine tablets are even higher (16 to 32 mg/ d^-1 ). Therefore, the use of iodine for water disinfection should be limited to short periods of ≤ 1 mo. Individuals planning to use iodine for prolonged periods should have their thyroid examined and thyroid function tests done to ensure they are initially euthyroid. Certain groups should not use iodine for water treatment: pregnant women (because of concerns of neonatal goiter); those with known hypersensitivity to iodine; persons with a history of thyroid disease, even if controlled on medication; persons with a strong family history of thyroid disease (thyroiditis); and persons from countries with chronic iodine deficiency.¹⁴⁹

Improving halogen taste

Objectionable taste and smell limit the acceptance of halogens, but taste can be improved by several means. One method is to use the minimum necessary dose with a longer contact time, as in the CDC safe water system. Another method is to use higher doses and remove the taste through chemical reduction of chlorine to chloride and iodine to iodide; these have no color or taste. The best and most readily available agent is ascorbic acid (vitamin C), available in crystalline or powder form. A small pinch in a liter, mixed after the required contact time, will usually suffice. Ascorbic acid is a common ingredient of flavored drink mixes, accounting for their effectiveness in removing the taste of halogens. GAC (see above) adsorbs organic and inorganic chemicals, including iodine and chlorine byproducts, thereby improving odor and taste-the reason for its common inclusion in field filters.

Improvisational techniques

There is no comparable substitute for proven chemical disinfectants, but there are many common substances that contain halogens. Household bleach is available in most parts of the world. The active disinfectant is sodium hypochlorite. Products for disinfection of swimming pools and spas generally contain calcium hypochlorite that provides much higher concentrations than bleach. Hypochlorite is readily released from different products formulated in liquid, powder, granules, and tablets. Iodine is also available in liquid or tablets; a common household source is tincture of iodine or similar topical disinfectants with an iodine concentration of 2 to 8%. These products also contain iodide, which has no disinfecting power but does contribute to iodine toxicity. Colorless iodine solution contains only iodide and should not be used. Povidone-iodine, a topical disinfectant commonly used in medical settings, contains active iodine bound to a neutral polymer of high molecular weight that gives the iodine greater solubility and stability. In dilute aqueous solution, povidone-iodine provides a sustained-release reservoir, releasing free iodine in a concentration of 2 to 10 mg/ L^-1 .¹⁵⁰

Mixed species disinfectant (electrolysis)

Passing a current through a simple brine salt solution generates free available chlorine and other mixed species disinfectants that have been shown to be effective against bacteria, viruses, Cryptosporidium, and bacterial spores.^151,152 The process is well described and can be used on both large and small scales. The main disinfectant effect is probably attributable to a combination of chlorine dioxide, ozone, superoxides, and hypochlorous acid, giving the resulting solution greater disinfectant ability than a simple solution of sodium hypochlorite. Small units are now available commercially that use salt, water, and a 12-volt direct current (automobile) battery to create 60 mL of a 0.75% chlorine solution over a 5-min.

Other common substances, including hydrogen peroxide and citrus juice that have some disinfectant activity, are discussed later. Assessments of supporting evidence:

Halogens chlorine and iodine are an effective means of disinfecting water of bacteria, viruses, and Giardia in the field or household when using appropriate contact time and halogen concentration. Evidence grade: 1A

Usual field concentrations of iodine and chlorine are not effective for other protozoa including Cryptosporidium and Cyclospora. Evidence grade: 2A

•

Extended use of iodine should be weighed against risks of iodine toxicity. **Evidence grade: 1B**

•

Simple techniques for improving taste of halogenated water are available for field use. **Evidence grade: 1B**

•

Mixed species electrolytic disinfection techniques are effective for water disinfection of microbes that are susceptible to halogens. **Evidence grade: 1B**

MISCELLANEOUS DISINFECTANTS

Chlorine dioxide

Chlorine dioxide (ClO²), a potent biocide, has been used for many years to disinfect municipal water and in numerous other large-scale applications. Until recently, the benefits of chlorine dioxide have been limited to large-scale applications because standard formulations must be made on-site and are associated with a risk for producing volatile gas. Newer methods enable cost-effective and portable ClO² generation and distribution for use in an ever-widening array of small-scale applications. ClO²-production tablets contain 6.4% sodium chlorite as the active ingredient. After a tablet is added to water, a series of complex chemical reactions occurs, generating chlorine dioxide. Some of the intermediary chemical compounds may also have antimicrobial activity. ClO² has no taste or odor in water. It is capable of inactivating most waterborne pathogens, including Cryptosporidium parvum oocysts.¹⁵³, ¹⁵⁴, ¹⁵⁵ It is at least as effective a bactericide as chlorine and far superior for virus and parasite inactivation. Several commercial point-of-use applications use ClO² in liquid or tablet form, but relatively few data are available on product testing these products.¹³⁷ A major disadvantage for field use of tablets is the long reaction or contact time required, with upward of 2 to 4 h needed to achieve dependable disinfection. ClO² does not produce a lasting residual, and water undergoing chlorine dioxide disinfection must be protected from sunlight.

Assessment of supporting evidence:

•

Chlorine dioxide is a widely used and potent water disinfectant, including efficacy against the protozoan parasites _Cryptosporidium_. **Evidence grade: 1A**

•

Individual use products have limited data demonstrating effective concentration and contact time. **Evidence grade: 2B**

Silver

Silver ion has bactericidal effects in low doses and some attractive features, including absence of color, taste, and odor. Scant data for disinfection of viruses and protozoan cysts indicate limited effect, even at high doses. Moreover, the concentrations are strongly affected by adsorption onto the surface of any container. Silver is physiologically active but not likely to cause a problem in concentrations found in drinking water. The EPA has not approved silver for primary water disinfection in the United States, but silver is approved as a water preservative to prevent bacterial growth in previously treated and stored water. In Europe, silver tablets are sold for field water disinfection. One rational combination product combines silver with hypochlorite for both disinfection and preservation. There is some promise in steady release products and incorporation into nanoparticles.¹⁵⁶

Assessment of supporting evidence:

•

Use of silver in wilderness settings should be limited to water preservation and not as a primary disinfectant. **Evidence grade: 1B**

Hydrogen peroxide

Hydrogen peroxide is a strong oxidizing agent that is widely used as a preservative in food, as a sterilant for medical and food equipment, and in many other applications. Although hydrogen peroxide can sterilize water, it is not widely used as a field water disinfectant, perhaps because high concentrations known to be effective are very caustic, and there is a lack of data for protozoal cysts and quantitative data for dilute solutions. It can be used to remove the taste of hypochlorite and in combination with other processes.¹⁵⁷

Assessment of supporting evidence:

•

Hydrogen peroxide in typical concentration of 3% cannot be used as a primary drinking water disinfectant, and effective concentrations are not practical for field use. **Evidence grade: 1B**

Citrus and potassium permanganate

Both citrus juice and potassium permanganate have some demonstrated antibacterial effects in an aqueous solution.¹⁵⁸ However, data are few and not available for effect on cysts. In municipal water disinfection, potassium permanganate is used primarily for reducing contaminants to improve taste and odor.¹⁵⁹ Either substance could be used in an emergency to reduce bacterial and viral contamination or as an adjunct in combination with another technique, but they cannot be recommended as a primary means of water disinfection.

Assessment of supporting evidence:

• Citrus juice and potassium permanganate have limited applications for drinking water disinfection. Evidence grade: 1C

Nanoparticles: solar photocatalytic disinfection

Several nanomaterials have been shown to have strong antimicrobial properties and are being evaluated for use in water disinfection and purification.¹⁶⁰, ¹⁶¹ The metals are of particular interest for water disinfection applications because they can be activated by UV light to produce 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 photocatalytic substance identified to date. Recent work demonstrated inactivation of Cryptosporidium by titanium dioxide.¹⁶¹, ¹⁶² These methods are widely used in industry, but few products have incorporated the technology into individual or small group point of use products.¹⁶³, ¹⁶⁴

Assessment of supporting evidence:

• New technology using nanoparticles and photocatalytic disinfection is highly promising for translation into point-of-use water disinfection. Evidence grade: 2A

PREFERRED TECHNIQUE

The optimal water treatment technique for an individual or group will depend on the number of persons to be served, space and weight accommodations, quality of source water, personal taste preferences, and fuel availability. Because halogens are not effective for killing Cryptosporidium at drinking water concentrations and common microfilters are not reliable for virus removal, optimal protection for all situations may require a 2-step process of 1) filtration or C-F, followed by 2) halogenation. Heat (boiling) is effective as a 1-step process in all situations but will not improve the esthetics of the water. Table 7 summarizes effects of major water disinfection methods on categories of microorganisms. Persons living or working in communities where sanitation and water treatment are lacking are at higher risk than the average international traveler. Sobsey et al reviewed data for point-of-use methods for household disinfection in developing countries¹⁶⁵ (Table 8).