Most portable filters are microfilters that can readily remove protozoan cysts and bacteria, but may not remove all viruses, which are much smaller. Ultrafiltration membranes are required for complete microbial removal, including viruses.
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 (e.g., hand pump or gravity); and (4) cost. All filters eventually clog from suspended particulate matter, present even in clear streams, requiring cleaning or replacement of the filter.
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 a filter material will reduce microbiologic contamination. Ceramic clay is widely available and very inexpensive to locally manufacture ceramic filters in the shape of a sink or flower pot that is set into a larger container to collect the filtered water.
Sand filters can be highly effective at removing turbidity and improving microbiologic quality. 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. 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 two layers of wire mesh, separates the sand and gravel layers. A sand filter also can be improvised with stacked buckets of successive filter layers with holes in the bottom to allow water passage.
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. Disinfection effectiveness is determined by the microorganism, the disinfectant, contact time, and environmental factors. Both chlorine and iodine are widely available worldwide in multiple formulations. Hypochlorite, the major chlorine disinfectant, is currently the preferred means of municipal water disinfection worldwide. The most commonly available form of chlorine is hypochlorite (household bleach 5-8%, or concentrated “swimming pool” granules or tablets 70%). Chlorine is still advocated by the WHO and the CDC as a mainstay of large-scale community, individual household, and emergency use. Another advantage of hypochlorite is the ease of adjusting the dose for large volumes of water.
Iodine is also effective in low concentrations for killing bacteria, viruses, and some protozoan cysts. Because of its effect on the thyroid, which uses iodine, the World Health Organization (WHO) recommends iodine only for short-term emergency use.
Common bacteria are very sensitive to halogens. Viruses, including hepatitis A, have intermediate sensitivity, requiring higher concentrations or longer contact times. Protozoan cysts are more resistant than enteric bacteria and enteric viruses but Giardia and amebic cysts can be inactivated by field doses of halogens. 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.
Understanding factors that influence chemical disinfection, chiefly the inverse relation between concentration and time allows flexibility to minimize chemical dose and improve taste or, conversely, to minimize the required contact time. Cold water slows chemical reactions and 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 often associated with turbidity (cloudiness). 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. Refer to tables in the Guideline for recommended doses and time for disinfection.
Taste of chlorine or iodine in water can be improved by several means. One method is to use the minimum necessary dose with a longer contact time. Another method is to use higher doses and remove the taste through a chemical reaction by adding a small pinch ascorbic acid (vitamin C), available in crystalline or powder form.
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. Since 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 two-step process of (1) filtration or coagulation–flocculation (clarification technique), followed by (2) chlorine. Heat (boiling) is effective as a one-step process in all situations but will not improve the look and taste of the water. Cloudy water should first be clarified before using hypochlorite or filtration.
On long-distance ocean-going boats where water must be desalinated as well as disinfected during the voyage, only reverse osmosis membrane filters are adequate. Water storage also requires consideration. Iodine will work for short periods only (i.e., weeks) because it is a poor algaecide. For prolonged storage, water should be chlorinated and kept in a tightly sealed container to reduce the risk of contamination. For daily use, narrow-mouthed jars or containers with water spigots prevent contamination from repeated contact with hands or utensils.
Sanitation and water treatment are inextricably linked. Personal hygiene, particularly handwashing, prevents spread of infection from food contamination during preparation of meals. Disinfection of dishes and utensils is accomplished by rinsing in water containing enough household bleach to achieve a distinct chlorine odor. Use of halogen solutions or potassium permanganate solutions to soak vegetables and fruits can reduce microbial contamination, especially if the surface is scrubbed to remove dirt or other particulates, but neither method reaches organisms that are embedded in surface crevices or protected by other particulate matter. 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. Groups of 3 persons or more should dig a common latrine to avoid numerous individual potholes and inadequate disposal.
Backer HD, Derlet RW, Hill VR. Wilderness Medical Society Clinical Practice Guidelines for water disinfection for wilderness, international travel, and austere situations. Wilderness Environ Med. 2019;30(4S):S100–20.