Chapter 67 Field Water Disinfection

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Waterborne disease is a risk for international travelers who visit countries that have poor hygiene and inadequate sanitation and for wilderness users relying on surface water in any country, including the United States. Natural water may be contaminated with organic or inorganic material from land erosion, dissolution of minerals, decay of organic vegetation, biologic organisms that reside in soil and water, industrial chemical pollutants, and microorganisms from animal or human biologic waste.^61,¹⁰⁶ The main reason for treating drinking water is to prevent gastrointestinal illness from fecal pollution with enteric pathogens.²⁷⁰ Appearance, odor, and taste are not reliable to estimate water safety. Natural organic and inorganic material may not cause illness but can impart unpleasant turbidity, color, and taste to the water (Box 67-1).

BOX 67-1 Chapter Outline and Shortcuts

• For summary of each technique, see boxes in specific sections.

• For comparison of various techniques, see text and Tables 67-16 to 67-18.

• For details of products, see the appendixes.

Methods of Water Disinfection That Can Be Applied in the Field	Page
Heat	1330
Physical Removal	1332
Sedimentation	1333
Coagulation–flocculation	1333
Granular activated carbon	1333
Filtration	1334
Reverse osmosis	1335
Chemical Disinfectants	1336
Chlorine	1339
Iodine	1341
Chlorine Dioxide	1345
Silver	1345
Potassium Permanganate	1346
Hydrogen Peroxide	1346
Citrus	1346
Ultraviolet Light and Solar	1346
Copper-zinc	1347

Of the 1700 million square miles of water on Earth, less than 0.5% is potable.²⁹⁵ Global warming is accelerating deterioration of the remaining potable supplies,¹⁶¹ and emerging waterborne pathogens create the need to reevaluate detection and disinfection methods.²⁷⁰ Chemical contamination of groundwater is increasing at an alarming rate in the United States and worldwide from industrial, agricultural, and individual sources. Except for certain pristine alpine or other remote water sources, virtually none of the surface water in the United States is drinkable without treatment.¹⁶¹ According to the National Water Quality Inventory Report by the U.S. Environmental Protection Agency (EPA), as of 2004, bacterial contamination was the leading cause of river and stream impairment; mercury contamination was the leading cause of impairment in lakes, ponds, and reservoirs. Overall, about 40% of the nation’s surveyed rivers, lakes, and estuaries are too polluted for basic uses such as fishing and swimming.²⁸⁵ Disasters, such as floods and hurricanes, often overwhelm treatment facilities and contaminate ground water, requiring point-of-use disinfection.²⁵⁰ The World Health Organization (WHO) reports that 1.1 billion people lack access to an improved drinking water supply, and 2.4 billion people lack access to improved sanitation.³⁰³

Benefits of Water Treatment

Methods for treating water are found in Sanskrit medical lore, and pictures of apparatus used to purify water appear on Egyptian walls from the 15th century BC. Boiling and filtration through porous vessels, sand, and gravel have been known for thousands of years. The Greeks and Romans also understood the importance of pure water.¹⁸⁶ Sanitation, including water treatment, is considered one of the ten great public health achievements that helped conquer infectious disease as a main cause of mortality in the United States.⁴³ As the percentage of the U.S. urban population served by water treatment utilities increased after 1900, the annual death rate from typhoid decreased.²⁴⁹

Drinking-water treatment processes provide enormous benefits with minimal risk. Without filtration and disinfection, waterborne disease would spread rapidly in most public water systems served by surface water.^69,²⁸⁶ The combined roles of safe water, hygiene, and adequate sanitation in reducing diarrhea and other diseases are clear and well documented. WHO estimates that 94% of diarrheal cases globally are preventable through modifications to the environment, including access to safe water.³⁰³ Recent studies of simple water interventions in households of developing countries clearly document improved microbiologic quality of water, 30% to 60% reduced incidence of diarrheal illness, enhanced childhood survival, and reduction of parasitic diseases; much of this progress is independent of other measures to improve sanitation.* Moreover, from a global health perspective, water and sanitation improvements are cost beneficial in all developing world regions.^118,¹³⁵ Although the combination of improved water quality and sanitation has the greatest effect, improvement in water quality alone has a beneficial effect on health and can reduce incidence of diarrheal disease by more than one-third.^260,²⁶²

In contrast to extensive evidence from developing areas of the world, there are few data to demonstrate benefits of water disinfection in the U.S. wilderness. Boulware ²⁷ demonstrated that drinking untreated water correlated with higher rates of diarrhea among Appalachian Trail hikers.

Risk and Etiology

Infectious agents with the potential for waterborne transmission include bacteria, viruses, protozoa, and parasites (Box 67-2).^101,²⁷⁰ The long list of pathogenic microorganisms capable of waterborne transmission is similar to that of potential etiologic agents of traveler’s diarrhea; almost all enteric pathogens and opportunistic pathogens that are transmissible by the fecal–oral route can be transmitted through water. Separating the contribution of waterborne transmission of these pathogens from food-borne and person-to-person transmission is impossible; the latter two are probably more common. In developing countries, 15% to 20% of diarrhea is estimated to be waterborne. Surprisingly, this is similar in developed countries, where as much as 15% to 30% is attributed to municipal drinking water.

BOX 67-2

Waterborne Enteric Pathogens

Bacterial

Yersinia enterocolitica

Aeromonas

Viral

Miscellaneous viruses (more than 100 types: e.g., adenovirus, enterovirus, calicivirus, echovirus, astrovirus, coronavirus)

Protozoal

Giardia lamblia

Entamoeba histolytica

Cryptosporidium

Blastocystis hominis

Data from references 101, 106, 228, 249, and 280.

Risk for waterborne illness depends on the number of organisms consumed, which is determined by the volume of water, concentration of organisms, and treatment system efficiency.^33,^67,¹³³ Additional factors include virulence of the organism and defenses of the host. Infection and illness are not synonymous; the overall likelihood of illness for all three categories of microorganisms (bacterial, viral, protozoan) is 50% to 60%. Death from enteric pathogens is unlikely in healthy, well-nourished persons, except with a few specific organisms (e.g., Escherichia coli O157:H7, E. coli: 0104:H4, or, in pregnant women, hepatitis E). In malnourished individuals, especially young children, many other pathogens, including Vibrio cholerae and Cryptosporidium, can lead to infectious causes of death. Total immunity does not develop for most enteric pathogens, and reinfection may occur.¹³³ Several nonenteric waterborne organisms have high case fatality rates. These include Legionella (respiratory) and Acanthamoeba (neurologic).

Waterborne outbreaks do not give a complete picture of the potential for waterborne illness. Most outbreaks of waterborne diseases are not identified because not enough people become ill, providing an insensitive mechanism for detecting water contamination. When an outbreak is identified, it is very difficult to prove conclusively that the source was waterborne. The supply may have been only transiently contaminated, water samples from the time of exposure are seldom available, some organisms are difficult to detect, and almost everyone has some exposure to water.^274,³⁰⁵

The data on concentration of microorganisms in surface water show widely varying values, but the testing is insufficient for risk assessment and dose-response models. Instead, infectious dose data and statistical techniques have been used to devise models for determining risk (Table 67-1).¹³³ These models cannot be applied unless the microbial content of water is known. Pathogenic microorganisms clearly exist in most raw source waters, especially in surface waters.²⁸⁶ Most microbiologic testing is done on community water intake sources and sewage treatment effluent. Less information is available for more remote water sources.^68,^241,³⁰⁰

TABLE 67-1 Estimated Infectious Dose of Enteric Organisms

Organism	Infectious Dose
Salmonella	10⁵
Shigella	10²
Vibrio	10³
Enteric viruses	1-10
Giardia	10-100
Cryptosporidium	10-100

Data from Hurst C, Clark R, Regli S: Estimating the risk of acquiring infectious disease from ingestion of water. In Hurst C, editor: Modeling disease transmission and its prevention by disinfection, Melbourne, 1996, Cambridge University Press, pp. 99-139.

Detection techniques using enzyme immunoassay and polymerase chain reaction may give a much more accurate picture of the specific microbes and degree of water contamination.²⁷⁰ But testing may not be representative, because excretion and loading of microbial contaminants are dynamic and change over time.

Surface water is subject to frequent, dramatic changes in microbial quality as a result of activities on a watershed. Storm water causes deterioration of source water quality by increasing suspended solids, organic materials, and microorganisms. Some of these contaminants are carried by rain from the atmosphere, but most come from ground runoff. In water sources downstream from towns or villages, storms may overload sewage facilities and cause them to discharge directly into the receiving water.

The source of fecal contamination in water may be either human or animal. Some bacterial pathogens (Shigella, Salmonella typhosa) occur exclusively in human feces, whereas others (Yersinia, Campylobacter, nontyphoidal Salmonella) may be present in wild or domestic animals. No enteric viruses excreted by animals have been shown to be pathogenic to humans.²²² Derlet and colleagues^84,⁸⁵ found that wilderness water sources in the California Sierra Nevada with significant amounts of human or animal activity are more likely to be contaminated with bacteria. Some organisms (e.g., Legionella pneumophila and Vibrio cholerae) exist as natural organisms in water.²⁶⁴

Developing Countries

In tropical areas and developing countries, water has a complex relationship with spread of disease. Table 67-2 presents a useful classification,²⁸ and Steiner and associates²⁶⁵ proposed adding the category “water carried” for infections resulting from accidental ingestion in recreational water. Globally, 88% of the 4 billion annual cases of diarrheal disease are attributed to unsafe water and inadequate sanitation and hygiene. About 2 million people, most children under 5 years of age, die annually from illnesses associated with unsafe drinking water or inadequate sanitation.³⁰³

TABLE 67-2 Water and Spread of Disease

WHO estimates that worldwide 25% of the population (1.5 billion rural and 200 million urban people) lack safe drinking water and 50% lack adequate sanitation. In Africa, about 40% of the population does not have access to an improved water supply, and 52% are without access to improved sanitation. In Asia, 20% are without improved water, and nearly 50% without sanitation. In both Africa and Asia, nearly 20% of rural water supplies are not functioning at any one time. In Latin America, 15% are without improved water and 20% without sanitation.³⁰² Only 30% of waste from India’s cities is treated before disposal, the rest flowing into surface or ground waters. Eight hundred million Indians lack a household water connection, relying on public taps, wells, or surface sources.⁵⁸

An estimated 80% of the world’s diseases are linked to inadequate water supply and sanitation. Of the four diseases that may cause the most morbidity and death in developing parts of the world—cholera, hepatitis, malaria, and typhoid—malaria is the only one that is not waterborne.¹³³ In developing countries, these illnesses account for 1 billion cases of diarrhea every year and 95% of deaths in children under 5 years of age.^101,^121,²⁹³ Another estimate puts the disease burden from water-related diseases, sanitation, and hygiene at 4% of all deaths and 5.7% of the total disease burden occurring worldwide.²¹⁶

In certain tropical countries, the influence of high-density population, rampant pollution, and absence of sanitation systems means that available raw water is virtually wastewater.⁵³ Contamination of tap water in many urban areas should be assumed because of antiquated and inadequately monitored disposal, disinfection, and distribution systems.⁷⁰ Recent studies of household point-of-use water treatment in developing countries and refugee camps provide evidence of extensive coliform contamination in these settings.^55,^72,^87,²²⁵

Water from springs and wells and even commercial bottled water may be contaminated with pathogenic microorganisms.

the United States and Developed Countries

Developed countries like the United States still face substantial challenges to ensure potable water in households and to maintain quality of water sources.^101,^161,^249,²⁷⁰ Community water supplies treated by modern methods are responsible for outbreaks of gastrointestinal illness. Based on studies comparing rates of diarrheal illness in households with and without effective water filters, Colford and associates⁶²^–⁶⁴ have estimated that 4 to 11 million cases of acute gastrointestinal illness annually are attributable to public drinking water systems in the United States.

The etiology of waterborne disease outbreaks from drinking water is different from untreated sources, with more respiratory disease due to Legionella and a mix of bacterial, virus, and protozoan pathogens resulting in gastroenteritis.³⁰⁵ Waterborne pathogens account for most outbreaks of infectious diarrhea acquired in U.S. wilderness and recreation areas (Box 67-3).⁶⁹ In both public and surface water supplies there has been an increase in the number of outbreaks caused by protozoan pathogens. Giardia is one of the most common waterborne infections, but Cryptosporidium epidemics have been identified with increasing frequency.^69,^120,^122,²³⁸ Enteric bacteria are responsible for a relatively small but consistent proportion of waterborne outbreaks in the United States,^122,³⁰⁵ but less commonly than are protozoa. Bacteria linked to water ingestion include Salmonella, Campylobacter, Shigella sonnei, Plesiomonas shigelloides, and E. coli O157:H7.^41,^42,^45,³⁰⁵

In recreational areas, a distinct seasonal variation is seen, with the majority of cases from recreational areas occurring during summer months.⁶⁹ This is probably a result of both increased contamination and number of persons at risk.

Risk for gastrointestinal disease to wilderness users is determined primarily by indirect evidence based on surveys.^27,³⁰⁵ Derlet and colleagues⁸⁰^–⁸⁵ have performed extensive testing for bacterial contamination of wilderness waters in the California Sierra Nevada. Human pathogenic bacteria were uncommon, and other enteric bacteria as indicators of contamination were associated with animal grazing, pack animals, and high levels of human activity.

Recreational Contact

Inadvertent ingestion during recreational activities of water not intended for drinking is a risk for swimmers and white-water boaters. The microorganisms that cause infection are those that require only a small dose. Recreational water activities have resulted in giardiasis, cryptosporidiosis,^40,^44,^48,³⁰⁶ typhoid fever, salmonellosis, shigellosis, E. coli O157 infection,⁹⁶ viral gastroenteritis, and hepatitis A, as well as in wound infections, septicemia, and aspiration pneumonia due to Legionella.⁶⁹ Between 2005 and 2006, a total of 78 outbreaks associated with recreational water were reported, involving 4412 persons, resulting in 116 hospitalizations and 5 deaths.³⁰⁶ Most outbreaks occurred in treated water venues such as pools and water parks. Of these, the majority were caused by parasites, predominantly Cryptosporidium, and a only few by Giardia, reflecting Cryptosporidium resistance to chlorine disinfection.⁴⁸ Bacterial contamination in treated water sources (including hospitals and hotels) most often resulted in respiratory disease caused by Legionella rather than in gastrointestinal illness. In untreated recreational water, most commonly lakes and ponds, bacteria (Shigella and E. coli O157:H7) and norovirus were much more common than were protozoa. These were predominantly small inland water bodies without external sources of contamination, suggesting swimmers were the source. At least one case of usually fatal neuroinvasive disease caused by Naegleria fowleri is reported each year from untreated recreational water.

Specific Etiologic Agents

Viruses

The infectious dose of enteric viruses is only a few infectious units in the most susceptible people.^174,^292,³⁰⁰ The risk for infections from enteric viruses is estimated to be 10 to 10,000 times higher than bacteria at the same level of exposure.²⁷⁰ Hepatitis A virus (HAV), norovirus, and rotavirus are the main viruses of concern for potable water supplies. Norovirus was the most commonly identified enteric virus in waterborne outbreaks in Finland.¹⁷³ All serotypes of adenovirus (besides enteric alone) are excreted in feces, so contaminated water could be a source of exposure for any type, either through ingestion, inhalation, or direct contact with the eyes. Outbreaks of adenovirus have been associated with drinking water and contact with recreational water, including swimming pools.¹⁸⁰ In addition to HAV, waterborne transmission of hepatitis E is suspected in outbreaks among travelers from Asia.^37,^144,²⁵² Many other viruses are capable and suspected of waterborne transmission, and more than 100 different virus types are known to be excreted in human feces.^112,²⁴¹The most frequent waterborne illness (acute infectious nonbacterial gastroenteritis of unknown etiology) in the United States may be caused by undetected viruses; estimates of viral waterborne illness are 6.5 million cases annually.^69,^101,^174,²⁵⁶ Enteroviruses have been detected even in finished water from sewage treatment plants with measurable levels of free residual chlorine.¹¹³

All surface water supplies in the United States and Canada contain naturally occurring human enteroviruses.²⁹⁵ Even remote surface lakes and streams tested in California showed disturbing levels of viral contamination.¹¹² Widespread enteric viral contamination was found at multiple sites in a popular recreational canyon in Arizona. Viruses included poliovirus, echovirus, coxsackievirus, rotavirus, and other unidentifiable viruses and exceeded the recommended state level for recreational water use in several areas. Virus levels correlated with human activity but not with excess levels of standard coliform indicators.²⁴¹ In 2002, a series of outbreaks involving more than 130 individuals was reported on 17 different Colorado River rafting trips. Laboratory evaluation of effluent from portable toilets found norovirus, which was also isolated from river water at Lee’s Ferry. No specific risk factors were identified in individuals or trips, and it was concluded that risk was likely from the river water.¹²⁵

No evidence exists of human immunodeficiency virus (HIV) transmitted via a waterborne route, and no epidemiologic evidence exists of casual transmission by fomites or by any environmentally mediated mode.²³² There has never been a documented case of influenza virus infection associated with water exposure. Free chlorine levels of 1 to 3 mg/L are adequate to disinfect avian, H1N1, or other influenza viruses.

Protozoa

Six protozoa that cause enteric disease and may be passed via waterborne transmission are Giardia lamblia, Cryptosporidium parvum, Entamoeba histolytica, Cyclospora cayetanensis, Isospora belli, and the microsporidia.^101,¹⁷² The first two are the most important for wilderness travelers. Cryptosporidium is an emerging enteric pathogen that has overtaken Giardia as the most common waterborne protozoa.^38,^101,³⁰⁶ Many aspects of the epidemiology and transmission appear similar to Giardia; 10 G. lamblia cysts may result in infection, and the infectious dose of Cryptosporidium is also on the order of 10² oocysts.^88,²²⁷ Waterborne transmission of E. histolytica is common in developing countries. Cyclospora has been epidemiologically linked to waterborne transmission in the United States and in Nepal, but the reservoir and host range are not known. Unlike Giardia and Cryptosporidium, Cyclospora is not infectious when passed in feces and requires up to 2 weeks in the laboratory to sporulate.²⁵⁴Surface water is a common environmental source for microsporidia; however, the route of infection is unknown. N. fowleri is a waterborne protozoan that enters the body through the nasal epithelia during swimming in contaminated surface water and causes meningoencephalitis.

Giardia and Cryptosporidium

Giardia cysts have been found as frequently in pristine water and protected sources as in unprotected waters.^123,^136,^233,^239,^240,²⁶³ Repeated sampling of “negative” sources invariably produced positive results.

A zoonosis with Giardia is known, with at least three different species; the extent of cross-species infection is not clear.^15,²⁹⁸ Many of the species apparently capable of passing Giardia cysts to humans, including dogs, cattle, ungulates (deer), and beaver, are present in wilderness areas. Forty percent of beavers in Colorado were infected and shedding 1 × 10⁸ cysts per animal per day. All 386 muskrats found were infected. Up to 20% of cattle examined were infected.¹²³ Beaver have been implicated in multiple municipal outbreaks of giardiasis. Samples from Rocky Mountain National Park^154,¹⁸⁷ and the California Sierra Nevada^263,²⁶⁷ show a direct correlation between numbers of cysts and levels of human use or beaver habitation. In Yukon, Canada, 13 of 61 scat samples from various wild animals yielded Giardia cysts.²³⁴

Even with a low infectious dose, environmental cyst recovery data indicate that the risk for ingesting an infectious dose of Giardia cysts is small.³⁰⁸ However, the likely model that poses a risk to campers is pulse contamination—a brief period of high cyst concentration from fecal contamination. Beaver stool and human stool may contain 1 × 10⁶ cysts/g. Stream contamination from a beaver has been calculated to reach 245 cysts/gal.¹³⁶ This is consistent with outbreaks in recreational water due to human contamination.^46,^48,²⁰⁹ In this instance, small amounts of water may cause infection, similar to an outbreak among lap swimmers from inadvertent water ingestion in a fecally contaminated pool.

Cryptosporidium oocysts are found widespread in surface water, and the cyst is durable in the environment. A large zoonosis is evident. Environmental occurrence appears ubiquitous.^159,^237,²³⁸ Cryptosporidium is now found more frequently than is Giardia in surface water, albeit in smaller numbers; it is the most frequent contaminant in treated recreational water; and it has resulted in large outbreaks in municipal water systems.^36,^77,^120,^169,³⁰⁶

Parasitic Organisms

Parasitic organisms other than protozoa are seldom considered in discussions of disinfection. Infectious eggs or larvae of many helminths are found in sewage, even in the United States.^224,²⁵¹ The frequency of infection by waterborne transmission is unknown, because food and environmental contamination or skin penetration is more prevalent.³⁰¹

The most obvious risk is from nematodes, with no intermediate hosts, that are infectious immediately or soon after eggs are passed in stool. Ascaris lumbricoides (roundworm) is transmitted by ingestion of the eggs in contaminated food or drink. In endemic areas, 85% of the population is infected; this leads to daily global environmental contamination by 9 × 10¹⁴ eggs.³⁰¹ Ancylostoma duodenale (hookworm) usually infects as larvae penetrate the skin of the foot, but it also may be acquired by mouth. Oral entry of the larvae causes pulmonary (Wakana) disease. Necator americanus (hookworm) does not appear to be infectious via the oral route.

Taenia solium (pork tapeworm) is infectious to humans in cyst or egg form. Eggs passed in stool are ingested in food or water and develop into tissue cysts, often in the brain, resulting in cysticercosis.

Echinococcus granulosus (dog tapeworm) can use humans as intermediate hosts. Eggs from the feces of an infected dog or other carnivore are ingested in food and water. Hydatid disease generates cysts in the liver, peritoneum, and other sites.

Fasciola hepatica (liver fluke of herbivores and humans) is normally acquired by ingestion of encysted metacercariae on water plants or free organisms in water.

Cercariae of schistosomiasis, which live in fresh water and normally enter through skin, can enter through the oral mucosa. The cercariae are killed by stomach acid.

Dracunculus medinensis (guinea tapeworm) is a tissue nematode of humans and causes the only such disease transmitted exclusively through drinking water.²⁹³ Dracunculus larvae are released in water from subcutaneous worms on the legs of infected bathers or water-gatherers. Larvae are ingested by a tiny crustacean (Cyclops species), which acts as the intermediate host and releases infectious larvae when ingested by humans. Worldwide eradication of dracunculosis has nearly been accomplished.

Bacterial Spores

Bacterial spores can cause serious wound and gut infections but are not likely to be waterborne enteric pathogens. Clostridium is ubiquitous in soil, lake sediment, tropical water sources, and the stool of animals and humans.^121,²⁴⁷ Clostridium botulinum and Clostridium perfringens type A food poisoning are not waterborne because they require germination of spores in food by inadequate cooking, then production of an enterotoxin, which is ingested. C. perfringens type C causes enteritis necroticans, probably through in vivo production of an enterotoxin, and thus has the potential for waterborne transmission in the tropics. However, the epidemiology of these infections in the United States, as in infant botulism, is related to food-borne sources.

Algae

Cyanobacterium, also known as blue-green algae, grow rapidly in water rich in organic matter during warm weather and produce toxins that pose a health risk to humans, pets, and wildlife. Effects depend on the type of cyanobacterium and the level of exposure through dermal contact or aspiration, as well as ingestion. Adverse effects include skin and eye irritation or gastrointestinal upset. At higher levels, exposure can result in serious illness, including hepatotoxicity (from microcystin toxins) or neurotoxicity. The algae are readily visible and would dissuade most humans from drinking, so it poses more of a threat to animals or small children due to recreational ingestion.³²

Chemical Hazards

Chemical hazards are also an alarming source of pollution in surface water. Wilderness users must consider removal of chemical, as well as microbiologic, contaminants. The greatest risks to wilderness travelers are pesticides from agricultural runoff and heavy metals from old mine tailings. Industrialization proceeds worldwide without adequate environmental protection. A vast array of toxins are sold with little concept of safe use and no means of safe disposal. Inorganic chemicals in drinking water include common salts, heavy metals, asbestos, fluorides, nitrates, radionuclides, and heavy metals (arsenic, copper, iron, lead, selenium). Natural organic chemicals predominate from soil runoff, forest canopy aquatic biota, and human and animal wastes. Synthetic organic matter includes pesticides, herbicides, and chemicals from industrial or human activities.²⁸⁶ Major underground aquifers are becoming contaminated. Streams and rivers in rural areas are contaminated by individual carelessness, leaching landfills, and agricultural runoff. Numerous pesticides have been found in runoff and rivers in agricultural areas of the Midwest.¹⁸³ Atmospheric spread has resulted in pesticides being found in remote wilderness lakes and in acid rain.

Persistence of Enteric Pathogens in the Environment

Once environmental contamination has occurred, a natural inactivation or die-off begins. However, enteric pathogens can retain viability for long periods (Table 67-3).^240,²⁸⁶ Factors promoting survival of microorganisms are pH near neutral (between 6 and 8) and cold temperatures, which contribute to the risk for transmission in mountain regions. In temperate and warm water, survival is measured in days, with densities of infectious agents decreasing by 90% every 60 minutes. However, tropical water differs from temperate water because it contains nutrients that create a microbiologically rich environment. Coliform bacteria can survive several months in natural tropical river water and may even proliferate. Survival of other bacteria is also prolonged (about 200 hours in tropical compared with 30 hours in temperate water). E. coli and V. cholerae may occur naturally in tropical waters and are capable of surviving indefinitely.^70,^121,²⁰⁵ As a rule, viruses persist longer than enteric bacteria in water.²⁹⁰

TABLE 67-3 Viability of Enteric Pathogens in Water

Most enteric organisms, including Shigella, resist freezing.⁸⁶ S. typhosa (typhi) can survive for up to 5 months in frozen debris and ice.²⁹⁵ HAV survives 6 months at below-freezing temperatures.²⁷³ Cryptosporidium may be able to survive a week or more in home freezers.²⁶⁵ Viruses persist well on chilled, acidified, frozen foods and foods packed under modified atmosphere or in dried conditions.¹⁰

Natural Purification Mechanisms

It is widely believed that streams purify themselves and that certain water sources are reliably safe for drinking. These concepts have some truth but do not preclude the need for disinfection to ensure water quality (Box 67-4).

Storms usually deteriorate surface water quality by washing solids, organic materials, and microorganisms into water sources; however, rainwater can also flush streams clean by dilution and by washing microbe-laden bottom sediments downstream.^106,¹²¹ Every stream, lake, or groundwater aquifer has limited capacity to assimilate waste effluents and storm water runoff entering the drainage basin. Self-purification is a complex process that involves settling of microorganisms after clumping or adherence to particles, sunlight providing ultraviolet (UV) destruction, natural die-off, predators eating bacteria, and dilution. Environmental factors include water volume and temperature, hydrologic effects, acid soil contact, and solar radiation. The process is time dependent and less active during wet periods and winter conditions. Hours needed in flow time downstream to achieve a 90% bacterial kill by natural self-purification vary with pollution inflow and rate of water flow. They have been measured at approximately 50 hours in the Tennessee River in summer, 47 hours in the Ohio River in summer, and 32 hours in the Sacramento River in summer.¹⁰⁶

Storage in reservoirs or lakes also improves microbiologic quality, with sedimentation as the primary process. A 100-to 1000-fold increase in fecal coliform bacteria can be found in bottom sediments compared with overlying water. This removal must be considered temporary, influenced by recirculation of organisms trapped in bottom sediments.^79,¹⁰⁶ In optimal conditions, 10 days of reservoir storage can result in 75% to 99% removal of coliform bacteria, and 30 days can produce safe drinking water. Generally, 80% to 90% of bacteria and viruses are removed by storage, depending on inflow and outflow, temperature, and no further contamination. Cysts, with a larger size and greater weight, should settle even faster than do bacteria and viruses.²

Groundwater is generally cleaner than surface water because of the filtration action of overlying sediments, but wells and aquifers can be polluted from surface runoff. Spring water is generally of higher quality than surface water, provided that the true source is not surface water channeling underground from a short distance above the spring.

Drawing conclusions from the preceding factors is difficult. The major factor governing the amount of microbe pollution in surface water is human and animal activity in the watershed. The settling effect of lakes may make them safer than streams, but care should be taken not to disturb bottom sediments when obtaining water.

Standards

Because coliforms originate primarily in the intestinal tracts of warm-blooded animals, including humans, they are used as indicators of possible fecal contamination.¹²¹ Although compelling reasons exist for testing other organisms before determining the safety of drinking water, cost and relative difficulty in testing for viruses and protozoa are major obstacles to expanding routine water testing, so coliforms remain the worldwide standard indicator organism. In the United States, bacterial indicators of water quality do not predict vulnerability to an outbreak. Contamination with other organisms has become common, resulting in expansion of U.S. regulations to test for organisms such as Cryptosporidium. In the future, molecular probes should make this process much easier.^186,²⁷⁰

The basic federal law pertaining to drinking water is the 1974 Safe Drinking Water Act, which was expanded and strengthened by amendments in 1977, 1986, and 1996.^186,³⁰⁵ Additional rules were added in 1989, 1996, 1998, and 2006.^45,²²⁸

The U.S. Public Health Service recommendations for potable water specify a mean of 1 coliform organism per 100 mL of water, or 10 organisms per liter. Absolute limits are 3 coliform bacteria per 50 mL, 4 per 100 mL, and 13 per 500 mL.²⁸⁸ In 1989, the standards for detection of fecal coliform bacteria in drinking water were relaxed slightly in recognition that coliform bacteria occur in large numbers in many water distribution systems that have no problem with waterborne disease.²⁸⁴ All standards acknowledge the impracticality of trying to eliminate all microorganisms from drinking water, allowing a small risk for enteric infection.^33,^228,²⁵⁶ Risk models are used to predict levels of illness and desired levels of reduction, with the reality that large numbers of people in the United States have increased susceptibility to enteric infections because of decreased immunologic competency.²²⁸ For example, EPA and Canadian guidelines suggest Giardia cyst removal with the goal of ensuring high probability that consumer risk is no more than one infection per 10,000 people per year.^33,^222,²²⁸

Generally, the goal of treatment is to achieve a 3- to 5-log reduction in the level of microorganisms: treatment must reduce Giardia by 99.9% (3 log) and enteric viruses by at least 99.99% (4 log).²²¹

The concept of risk is important for wilderness travelers as well, because it is impossible to know the risk of drinking the water in advance and may not be practical to eliminate all risk with treatment.

Standards for Portable Disinfection Products

The EPA and NSF International are the primary agencies that set standards for disinfection products and protocols for testing to meet these standards. NSF, The Public Health and Safety Company, is a not-for-profit, nongovernmental organization and world leader in standards development and product certification. EPA and NSF Protocol P231 (Microbiological Purifiers) were used to create NSF Protocol P248 (Emergency Military Operations Microbiological Water Purifiers) to test individual water purifiers for field military operations. The protocols are applicable to all types of individual water purifiers.

EPA Registration

Mechanical Filters

Until recently, no testing criteria were mandated for EPA registration. The EPA does not endorse, test, or approve mechanical filters; it merely assigns registration numbers. However, registration requirements distinguish between two types of filters: those that use mechanical means only and those that use a chemical, designated as a pesticide. Standards were developed to act as a framework for testing and evaluation of water purifiers for EPA registration, as a testing guide for manufacturers, to assist in research and development of new units, and as a guide for consumers.²⁸⁴

Filter Testing

Current registration of mechanical filters requires only that the product make reasonable claims and that the location of the manufacturer be listed; no disinfection studies are required. However, many companies now use the standards as their testing guidelines. For mechanical filters, the standards should be applied only for those microorganisms against which claims are made, such as protozoa and bacteria, excluding viruses. Despite criticisms of the methodology and inconsistencies and loopholes in the reporting process, EPA standards are currently the best means to compare filters.

EPA standards include performance-based microbiologic reduction requirements, chemical health limits for substances that may be discharged, and stability requirements for chemical(s) sufficient for the shelf life of the device. The unit should signal the end of effective lifetime (e.g., by terminating discharge of treated water) or give simple instructions for servicing or replacing within measurable volume, throughput, or time frame. There are currently no national guidelines for removal of chemicals by portable filters.

EPA standards require that challenge water seeded with specific amounts of microorganisms be pumped through the filters at given intervals during the claimed volume capacity of the filter. Between bacteriologic challenges, different test waters without organisms are passed through the unit. Water conditions are specified to include average and worst-case conditions; the latter are 5° C (41° F) with high levels of pollution, turbidity, and alkaline pH. 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 cysts, 4-log reduction (99.99%) for viruses, and 5- to 6-log reduction for bacteria. Testing is done or contracted by the manufacturer; the EPA neither tests nor specifies laboratories.

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.

Chemical Methods

Products that are used for treating municipal or private water supplies for drinking are considered pesticides and must be registered by the EPA Office of Pesticide Programs. Registration signifies the following:

• The composition is such as to warrant the proposed claims.

• The labeling and other material required to be submitted comply with the requirements of the act.

• The method will perform its intended function without unreasonable adverse effects on the environment.

• When used in accordance with widespread and commonly recognized practice, the method will not generally cause unreasonable adverse effects on the environment.

Thus EPA registration implies only that the “pesticide” agent is not released into the water at unsafe levels.^4,³⁵ This is less stringent than for filters that contain halogens.

Disinfection Methods

Definitions (Box 67-5)

Disinfection, the desired result of field water treatment, means the removal or destruction of harmful microorganisms. Technically it refers only to chemical means such as halogens, but the term is often applied to heat and filtration. Pasteurization is similar to disinfection but specifically refers to the use of heat, usually at temperatures below 100° C (212° F), to kill most pathogenic organisms. Disinfection and pasteurization should not be confused with sterilization, which is the destruction or removal of all life forms.¹⁵⁶ The goal of disinfection is to achieve potable water, indicating only that a water source, on average over a period of time, contains a “minimal microbial hazard,” so that the statistical likelihood of illness is acceptable. Water sterilization is not necessary, because not all organisms are enteric human pathogens.¹²⁸ Purification is the removal of organic or inorganic chemicals and particulate matter to remove offensive color, taste, and odor. It is frequently used interchangeably with disinfection, but purification as used here may not remove or kill enough microorganisms to ensure microbiologic safety.²⁹³

BOX 67-5 Definitions of Terms

Clarification	Techniques that reduce turbidity of water.
Coagulation–flocculation	Removes smaller suspended particles and chemical complexes too small to settle by gravity (colloids).
Contact time	The length of time that the halogen is in contact with microorganisms in the water.
Disinfection	A process that kills or destroys nearly all disease-producing microorganisms, with the exception of bacterial spores. As applied here, refers to pathogenic waterborne microbes and is the desired result of water treatment.
Enteric pathogen	Microorganism capable of causing intestinal infection after ingested; may be transmitted through food, water, or direct fecal–oral contamination.
Halogen	Oxidant chemical (primarily chlorine and iodine) that can be used for disinfection of water.
Halogen demand	The amount of halogen reacting with impurities in the water.
Potable	Implies “drinkable” water, but 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 acceptable.
Purification	The removal of organic or inorganic chemicals and particulate matter to improve offensive color, taste, and odor. Sometimes used by other sources to indicate microbiologic removal.
Residual halogen concentration	The amount of active halogen remaining after halogen demand of the water is met.
Reverse osmosis	A process of filtration that uses high pressure to force water through a nanopore semipermeable membrane that filters out dissolved ions, molecules, and solids.
Sterilization	A process by which all forms of microbial life, including bacteria, viruses, protozoa, and spores, are destroyed.

Heat

Heat is the oldest means of water disinfection. It is used worldwide by residents, travelers, and campers to provide safe drinking water. In countries with normally safe drinking water, it is often recommended as backup in emergencies or when water systems have become contaminated by floods or a lapse in water treatment plant efficacy. Fuel availability is the most important limitation to using heat. One kilogram of wood is required to boil 1 L of water.⁵³ For wilderness travelers without access to wood, liquid fuel is heavy (Box 67-6).

BOX 67-6 Heat

Advantages	Disadvantages
Does not impart additional taste or color to waterSingle-step process that inactivates all enteric pathogensEfficacy is not compromised by contaminants or particles in the water, as with chemical treatment and filtrationCan pasteurize water without sustained boiling	Does not improve the taste, smell, or appearance of poor-quality water
	Fuel sources may be scarce, expensive, or unavailable
	Does not prevent recontamination during storage

Relative susceptibility of microorganisms to heat: protozoa > bacteria > viruses.

Heat inactivation of microorganisms is exponential and follows first-order kinetics. Time plotted against temperature yields a straight line when plotted on a logarithmic scale.¹⁴¹ The thermal death point is reached in shorter time at higher temperatures, whereas lower temperatures are effective with a longer contact time. Pasteurization uses this principle to kill enteric food pathogens and spoiling organisms at temperatures between 60° and 74° C (140° and 165.2° F), well below boiling.¹⁰³ Pasteurization is not intended to kill all pathogenic microorganisms in the food or liquid. Instead, pasteurization aims to reduce the number of viable pathogens so they are unlikely to cause disease; for example, the high temperature–short time pasteurization standard for milk uses 71.7° C (161° F) for 15 to 20 seconds to achieve a 5-log reduction, killing 99.999% of viable microorganisms. Flash pasteurization subjects fruit or vegetable juice to temperatures of 71.5° C (160.7° F) to 74° C (165.2° F) for about 15 to 30 seconds. Therefore the minimum critical temperature is well below the boiling point at any terrestrial elevation.

Microorganisms have varying sensitivity to heat; however, all common enteric pathogens are readily inactivated by heat (Table 67-4). Bacterial spores (e.g., Clostridium spp.) are the most resistant; some can survive 100° C (212° F) for long periods but, as discussed, are not likely to be waterborne enteric pathogens. Boiling does not depend on water quality as does filtration or chemical disinfection. Heat kills or inactivates all enteric waterborne pathogens, regardless of whether they are freely suspended or present in particles.²⁴⁹ Protozoal cysts, including Giardia and E. histolytica, are very susceptible to heat. Cryptosporidium is also inactivated at these lower pasteurization levels.

TABLE 67-4 Data on Heat Inactivation of Microorganisms

Organism	Lethal Temperature/Time	Reference
Giardia	55° C (131° F) for 5 min	Jarroll, 1984¹³⁹
	100° C (212° F) immediately	Bingham, 1979²⁰
	50° C (122° F) for 10 min (95% inactivation)	Ongerth,1989²⁰¹
	60° C (140° F) for 10 min (98% inactivation)
	70° C (158° F) for 10 min (100% inactivation)
	55° C (131° F)	Aukerman, 1989⁶
Entamoeba histolytica	Similar to Giardia
Nematode cycsts, helminth eggs, larvae, cercariae	50°-55° C (122°-131° F)	Shephart, 1977²⁵¹
Nematode cycsts, helminth eggs, larvae, cercariae	65° C (149° F) for 1 min, 50° C (122°) for 30 min	Shephart, 1977²⁵¹
Cryptosporidium	45°-55° C (113°-131° F) for 20 min	Anderson, 1985³
Cryptosporidium	55° C (131° F) warmed over 20 min	Anderson, 1985³
	64.2° C (147.6° F) within 2 min	Fayer, 1994⁹⁵
	72° C (161.6° F) heated up over 1 min	Fayer, 1994⁹⁵
E. coli	55° C (131° F) for 30 min	Frazier, 1978¹⁰³ Neumann, 1969¹⁹⁴
E. coli	60°-62° C (140°-143.6° F) for 10 min
Salmonella and Shigella	65° C (149° F) for <1 min
V. cholerae	60°-62° C (140°-143.6° F) for 10 min	Rice, 1991²³⁰
V. cholerae	100° C (212° F) for 30 sec	Rice, 1991²³⁰
E. coli, Salmonella, Shigella, Campylobacter	60° C (140° F) for 3 min (3-log reduction)	Bandres, 1988¹¹
	65° C (149° F) for 3 min (all but a few Campylobacter)
	75° C (167° F) for 3 min (100% kill)
Escherichia coli	50° C (122° F) for 10 min ineffective	Groh, 1996¹¹⁶
	60° C (140° F) for 5 min
	70° C (158° F) for 1 min
Viruses	55°-60° C (131°-140°) within 20-40 min	Alder, 1992¹
Viruses	70° C (158° F) for <1 min	Alder, 1992¹
Hepatitis A	98° C (208.4° F) for 1 min	Krugman, 1970¹⁵³
	85° C (185° F) for 1 min	Thraenhart, 1991²⁷³
	61° C (141.8° F) for 10 min (50% disintegrated)	Thraenhart, 1991²⁷³
	60° C (140° F) for 19 min (in shellfish)	Peterson, 1978²⁰⁸
	60° C (140° F) for 10 min; 80° C (176° F) for 3 min; 85° C (185° F) for 1 min or less (in various food products)	Baert,2009¹⁰
Hepatitis E	60° C (140° F) for 30 min	Thraenhart, 1991²⁷³
Bacterial spores	>100° C (212° F)	Alder, 1992¹

Parasitic eggs, larvae, and cercariae are all susceptible to heat. For most helminth eggs and larvae, which are more resistant than cercariae and Cyclops (the copepod that carries Dracunculus), the critical lethal temperature is 50° to 55° C (122° to 131° F).²⁵¹

Common bacterial enteric pathogens (E. coli, Salmonella, Shigella) are killed by standard pasteurization temperatures of 55° C (131° F) for 30 minutes or 65° C (149° F) for less than 1 minute.^103,¹⁹⁴ Recent studies confirmed safety of water contaminated with V. cholerae and E. coli after 10 minutes at 60° to 62° C (140° to 143.6° F) or after boiling water for 30 seconds.^116,²³⁰

Viruses are more closely related to vegetative bacteria than to spore-bearing organisms ¹⁴¹ and are generally inactivated at 56° to 60° C (132.8° to 140° F) in less than 20 to 40 minutes.^1,^206,²⁶⁹ Inactivation at higher temperatures is similar to that of vegetative bacteria. Death occurs in less than 1 minute above 70° C (158° F), as confirmed in milk products.²⁶⁸

Given its environmental stability and clinical virulence, HAV is a special concern. Some data indicate that it has greater thermal resistance than do other enteric viruses, but a review of data from food industry studies confirms susceptibility of HAV and other enteric viruses to heat at pasteurization temperatures, even in milk and other products with solids that would shield the virus.¹⁰ Widely varying data probably result from different models for virus infectivity and destruction and from the use of various test media.