Wherever access to safe water, adequate sanitation, and hand hygiene is limited, water-borne diseases caused by enteric microbial pathogens are a risk for both visitors and residents. After natural disasters, even areas that usually provide effective sanitation and water treatment, lack of safe drinking water is one of the most common problems. Potential water-borne pathogens include bacteria, viruses, protozoa, and parasitic helminths. Effective water treatment methods that can be applied in the field are available for individuals, small groups, and households.

Introduction

Many people visit or live in areas that lack effective community-level sanitation and water treatment. These areas may include parts of low- or medium income countries, wilderness areas, refugee camps, disaster and war affected areas, or field research camps.  Wherever access to safe water, adequate sanitation, and hand hygiene is limited, water-borne diseases are a risk for both visitors and residents.

According to the World Health Organization/United Nations International Children’s Emergency Fund (WHO/UNICEF), steady progress has been made in the past 20 years toward the goal of safe drinking water and sanitation worldwide. However, they estimate that 25% of the world population still lacks safely managed drinking water in their homes. Nearly 2 billion people lack access to basic sanitation services, which can result in high levels of microbes in the environment and water sources. Those working in or visiting areas without adequate sanitation and water treatment may encounter highly contaminated water.  The consumer cannot estimate infectious potential by the water appearance, smell, or taste. The risk of water contamination is determined by activity upstream from the source. Usage by humans, farm animals, or wildlife pose a significant risk.

Infectious agents with the potential for water-borne transmission include bacteria, viruses, protozoa, and non-protozoan parasites. Common water-borne protozoa include Cryptosporidium, Entamoeba histolytica, and Giardia. Most organisms can remain infectious for prolonged periods in both tropical or cold water and even in ice. Although these guidelines focus on infectious risk, water may also be contaminated with toxins and chemical pollutants from industrial sources, from old mines, or from the environment.

Water treatment and disinfection methods that can be applied in the field include heat, clarification, filtration, chemical disinfection, and ultraviolet (UV) irradiation. Most techniques can be adapted to austere situations such as remote camps and households, disaster relief, and refugee camps. Extensive research supports the efficacy and effectiveness of field treatment techniques.

Field techniques for water treatment and disinfection

Heat

Common intestinal pathogens are readily inactivated by heat, including protozoan cysts and oocysts. Heat inactivation of microorganisms is determined by time and temperature. The thermal death point is reached in a shorter time at higher temperatures, but lower temperatures are effective with a longer exposure time. The food industry uses this principle to kill enteric pathogens and spoiling organisms at temperatures well below boiling, between 60°C (140° F) and 70°C (158° F) within 30 minutes, also known as pasteurization. 

Although lower temperatures may be effective, making boiling unnecessary, it is the only end point recognizable in the field or household that does not require a thermometer. Sustained boiling is not needed since the time required to heat water from 60°C to boiling temperature works toward heat disinfection. To save scarce fuel, heat water until first sign of simmering, reduce or remove heat, and leave the container covered for up 15-30 minutes.

Due to decreased atmospheric pressure, the boiling point for water decreases with increasing elevation; however, at common terrestrial elevations the temperature needed to achieve boiling is still well above the temperature required to inactivate enteric pathogens.

Clarification

Clarification refers to techniques that reduce the cloudiness (turbidity) of water caused by the presence of natural organic and inorganic material. Clarification can markedly improve both the appearance and taste of the water as well as facilitate disinfection by chemical treatment and help avoid filter clogging. Decreasing turbidity will reduce microbiological contamination but an additional treatment step should be used to ensure potable water.

Coagulation and flocculation (C-F)

Large particles like silt and sand will settle by gravity (i.e., sedimentation). Cloudiness due to dissolved substances or smaller particles that remain suspended in water can be improved through coagulation and flocculation, which cause them to clump into larger particles that can be more easily removed. The most common substance used is an aluminum salt (alum), but there are other substances from plants or even fine white ash from a campfire that will catalyze C-F. The process allows solids to settle to the bottom or float to the surface of the water container where they are easily removed by carefully decanting clear water off the top or by coarse filtration using a coffee filter or tight-weave cloth. C-F removes many, but not all, microorganisms, so should be followed by a second technique for disinfection. A C-F agent combined with a chlorine disinfectant is commercially available in tablets or powder packets.

Adsorption

Granular activated charcoal (GAC) improves odor and taste and reduces contaminants through adsorbing organic and inorganic chemicals—including chlorine and iodine disinfectants, pesticides, and most heavy metals. For this reason, GAC is incorporated into many household and portable field filters. GAC filters may trap but do not kill microorganisms, so another method should be used to remove microbial contamination.  

Filtration

Filtration is primarily a physical process that depends primarily on the size of the organism and filter pore size (Figure 1). Commercial products with a wide variety of designs and types of filter media are sold widely. Manufacturers claiming a US Environmental Protection Agency (EPA) designation of water purifier for their products must conduct their own testing to demonstrate their filters can remove at least 99.9999% of bacteria, 99.99% of viruses, and 99.9% of Cryptosporidium oocysts or Giardia cysts.

Filter pore size

Microfilters with a pore size 0.1-1 µm, should readily remove bacteria and protozoan parasites like Cryptosporidium and Giardia, but they do not reliably remove enteric viruses (e.g., norovirus) that have an average size of 0.03 µm. Enteric viruses are a concern wherever there is human or animal activity in the watershed and poor sanitation. Progressively finer levels of filtration, such as ultrafiltration, nanofiltration, and reverse osmosis, can remove viruses (Figure 1).

In resource-limited settings, effective filters can be made from ceramic clay or layers of sand and gravel (ie, slow sand or biosand). 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. Their effectiveness for decreasing turbidity (cloudiness) may be used as an indicator that the filter material at least will reduce microbiologic contamination.

Source: Auerbach PS, Cushing TA, & Harris NS. Field Water Disinfection. In Auerbach’s wilderness medicine (7th ed., p. 1996). 2017, Elsevier.

Chemical disinfection

Chlorine compounds and iodine

Chemical disinfectants are strong oxidizers that inactivate microorganisms by denaturation of proteins and disruption of cell membranes. Chlorination is the most widely used method worldwide to improve and maintain the microbiologic quality of drinking water. Sodium hypochlorite, the active ingredient in common household bleach, remains the primary water disinfectant promoted by CDC and the World Health Organization (WHO) for most settings. Other chlorine-containing compounds, available in granular or tablet formulations (e.g., calcium hypochlorite and sodium dichloroisocyanurate), can also be effective for water disinfection.

Iodine is also effective in low concentrations for killing bacteria, viruses, and some protozoan cysts; however, because of its effect on the thyroid, which uses iodine, the World Health Organization (WHO) recommends iodine only for short-term emergency use. People with unstable thyroid disease, known iodine allergy, or pregnant women should not use iodine for chemical disinfection.

Since the dose can be modified according to the volume and clarity of water, chemical disinfection methods can be used by individual travelers, small or large groups, and communities for either emergency or long-term use.

The main factors for chemical disinfection are concentration of disinfectant and exposure (contact) time. Temperature and turbidity are secondary factors. Bacteria and viruses are more sensitive to chemical disinfection that protozoal cysts. Concentrations of chlorine and iodine used in the field will inactivate Giardia cysts when higher concentrations or longer contact times are used. However, Cryptosporidium oocysts are poorly inactivated by chlorine- or iodine-based disinfection at practical concentrations, even with extended contact times. Refer to tables in the Guidelines for recommended doses and time for disinfection, as well as other disinfectants such as chlorine dioxide, silver, hydrogen peroxide, and citrus juice.

To improve taste as well as effectiveness of chemical disinfection, clarify cloudy water using settling, C-F, or filtration before adding the disinfectant. Additional means of removing the taste or chlorine or iodine can be used after disinfection contact time, including a pinch of citric acid (vitamin C) powder. 

Ultraviolet Radiation (UVR)

UVR kills bacteria, viruses, and both Giardia and Cryptosporidium oocysts in water with sufficient dose and exposure time. Small portable battery-operated units that deliver a metered, timed dose of UVR are sold, as well as larger power-dependent units where water flows through fixed UVR sources. UVR is blocked by particles, so highly turbid water should first be clarified.

Solar irradiation

Natural UVR from sunlight (solar disinfection or SODIS) also can improve the microbiologic quality of water. Water in clear plastic beverage bottles is exposed to sunlight on a reflective surface for a minimum of 6 hours--two days in cloudy conditions. This technique is especially useful in austere situations such as refugee camps or disaster zones.

Choosing a technique

Table 1 summarizes advantages and disadvantages of field water disinfection techniques. The decision of which method to utilize requires an assessment of the likelihood of water contamination from human and animal activity, resources available, the number of people served, and other factors. The optimal treatment in some environments may require a two-step process of (1) filtration or coagulation–flocculation (clarification technique), followed by (2) chlorine. Where water must be desalinated as well as disinfected, only reverse osmosis membrane filters are adequate.

Table 1: Comparison of water treatment and disinfection techniques

Source: Backer H, Hill V. Water Disinfection. In: CDC Yellow Book: Health Information for International Travel. 2024

Water, Sanitation, and Health

Safe drinking water is critical but not sufficient to prevent enteric illness. The combined roles of safe water, hygiene, and adequate sanitation (WASH) in reducing diarrhea and other diseases are well documented.  Handwashing reduces spread of infection from food contamination during preparation of meals. For water storage with daily use, narrow-mouthed jars or containers with water spigots prevent contamination from repeated contact with hands or utensils. Travelers to remote villages, wilderness areas, and disaster situations should assure proper waste disposal to prevent additional contamination of water supplies. Human waste should be buried 8–12 in deep, at least 100 ft from any water, and at a location from which water run-off is not likely to wash organisms into nearby water sources.

For additional information, recommendations, tables and references, please refer to the official guidelines as published in the Journal of Wilderness and Environmental Medicine.

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