Field Water Disinfection
Risk and Etiology
Infectious agents in contaminated drinking water most commonly associated with the potential for causing illness in a wilderness setting include bacteria, viruses, and protozoa. The main reason for treating drinking water is to prevent gastrointestinal illness from fecal pollution with enteric pathogens. Appearance, odor, and taste do not reliably estimate water safety. 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 (Boxes 45-1 and 45-2).
Specific Etiologic Agents
1. The infectious dose of enteric viruses is only a few infectious units in the most susceptible people.
2. Hepatitis A virus, norovirus, and rotavirus are the main viruses of concern for potable water supplies.
3. 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.
Protozoa
1. 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. The first two are the most important for wilderness travelers.
2. Giardia cysts have been found as frequently in pristine water and protected sources as in unprotected waters.
3. Many of the species seemingly capable of passing Giardia cysts to humans, including dogs, cattle, ungulates (deer), and beavers, are present in wilderness areas.
4. Cryptosporidium is an emerging enteric pathogen that has overtaken Giardia as the most common waterborne protozoa. Many aspects of the epidemiology and transmission appear similar to those related to Giardia.
Definitions
1. Disinfection, the desired result of field water treatment, means the removal or destruction of harmful microorganisms.
2. 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.
3. Disinfection and pasteurization should not be confused with sterilization, which is the destruction or removal of all life forms.
4. 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.
5. Water sterilization is not necessary because not all organisms are enteric human pathogens.
6. Purification is the removal of organic or inorganic chemicals and particulate matter to remove offensive color, taste, and odor. The term is frequently used interchangeably with “disinfection,” but purification may not remove or kill enough microorganisms to ensure microbiologic safety.
Heat
1. The boiling time required is important when fuel is limited.
2. Enteric pathogens, including cysts, bacteria, viruses, and parasites, can be killed at a temperature well below boiling (Table 45-1).
3. Thermal death is a function of both time and temperature; therefore lower temperatures are effective with longer contact times.
4. The boiling point decreases with the lower atmospheric pressure present at high elevations (Table 45-2).
Table 45-2
Boiling Temperatures at Various Altitudes
| ALTITUDE (ft) | ALTITUDE (m) | BOILING POINT |
| 5,000 | 1524 | 95° C (203° F) |
| 10,000 | 3048 | 90° C (194° F) |
| 14,000 | 4267 | 86° C (186.8° F) |
| 19,000 | 5791 | 81° C (177.8° F) |
5. The majority of the time required to raise the temperature of water to its boiling point works toward disinfection, so water is safe to drink by the time it has reached a full rolling boil. For an extra margin of safety (e.g., to kill hepatitis A virus), keep the water covered and hot for several minutes after boiling.
6. Pasteurization (at subboiling temperatures with extended contact times) has been successfully achieved using solar heating. A solar cooker constructed from a foil-lined cardboard box with a glass window in the lid can be used for disinfecting large amounts of water by pasteurization. This could be a low-cost method for improving water quality, especially in refugee camps and disaster areas.
7. When no other means are available, using hot tap water as drinking water may prevent traveler’s diarrhea in developing countries. As a rule of thumb, water too hot to touch is within the pasteurization range. However, lukewarm tap water can contain pathogenic microorganisms.
Filtration, Adsorption, and Clarification (Fig. 45-1)
1. Field filters that rely solely on the mechanical removal of microorganisms may be adequate for cysts and bacteria but may not reliably remove viruses, which are a major concern in water where high levels of fecal contamination are present (e.g., in developing countries).
2. They have the advantages of being simple and requiring no holding time.
3. Most viruses adhere to larger particles or clump together into larger aggregates that may be removed by a filter. However, filtration is not an adequate method to eliminate viruses because the infectious dose of an enteric virus may be quite small. Filters are often expensive and can add considerable weight and bulk to a backpack.
4. Some devices are designed as purely mechanical filters, whereas others combine filtration with granular activated carbon (GAC).
5. The size of a microorganism is the primary determinant of its susceptibility to filtration. Filters are rated by their ability to retain particles of a certain size.
6. All filters eventually clog from suspended particulate matter, present even in clear streams, requiring cleaning or replacement of the filter. The ability to easily service a unit in the field is an advantage. Flow can be partially restored to a clogged filter by back flushing or surface cleaning, which removes the larger particles trapped near the surface.
Microfiltration, Ultrafiltration, and Nanofiltration
1. In general, portable filters for water treatment can be divided into microfiltration with pores down to 0.1 µm, ultrafiltration that can remove particles as small as 0.01 µm, and nanofiltration with pore sizes as small as 0.001 µm or less.
2. Microfilters are effective for removing protozoa and bacteria, algae, most particles, and sediment but allow dissolved material, small colloids, and some viruses to pass through.
3. Ultrafiltration membranes are required for complete removal of viruses, colloids, and some dissolved solids.
