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Water Disinfection
Risk of Water-Borne Infection
Water disinfection is an essential component of the prevention strategy for enteric infections. In developing countries, surface water may be highly contaminated with human waste. The World Health Organization (WHO) reports that 780 million people still lack access to an improved drinking water supply, and 2.4 billion people lack access to improved sanitation. This lack of safe drinking water, sanitation, and hygiene accounts for an estimated 2 million deaths in children under 5 years of age, nearly 90% of diarrhea in all ages, and as much as 4% of all deaths globally. Urban tap water may become contaminated from aged, overwhelmed sanitation plants and deteriorating water distribution systems. Bottled water is a convenient solution, but in some places, it may not be superior to the tap water. Moreover, the plastic bottles create a huge ecological problem, since most developing countries do not recycle plastic bottles. Even in developed countries with low rates of diarrhea illness, wilderness travelers who rely on surface water for drinking and residents in areas affected by a disaster should take steps to ensure microbiologic quality. In the United States, there have been water-borne outbreaks of Giardia , Shigella , Campylobacter , Escherichia coli O157:H7, norovirus, and Cryptosporidium , some from untreated surface water and others from community water systems.
The list of potential water-borne pathogens is extensive and includes bacteria, viruses, protozoa, and parasitic helminths. More than 120 different enteric viruses alone can be transmitted by fecal-contaminated water. Most of the organisms that can cause traveler's diarrhea can be water-borne; however, the majority of travelers' intestinal infections are probably transmitted by food. Cholera and Salmonella typhosa are well known to cause extensive water-borne outbreaks. Water is considered the main route of transmission for hepatitis E and is one of the potential routes for hepatitis A and salmonellosis.
Risk of illness depends on the number of organisms ingested, which in turn depends on the degree of water contamination from human and animal waste, immune status and individual susceptibility, and virulence of the organism ( Table 7.1 ). Microorganisms with small infectious dose can even cause illness through recreational water exposure such as swimming as a result of inadvertent water ingestion. Organisms that have been implicated recently in outbreaks resulting from recreational water exposure (including several in the United States) include Giardia , Cryptosporidium , Shigella , E. coli O157:H7, norovirus, and gastroenteritis from other unidentified enteric viruses.
TABLE 7.1
Minimal Infectious Dose
| Organism | Minimal Infectious Dose |
|---|---|
| Salmonella | 10 5 |
| Vibrio cholerae | 10 3 |
| Cryptosporidium | 30 |
| Giardia | 10 |
| Enteric viruses | 1-10 |
Persistence of microorganisms in water facilitates the potential for transmission; cold water greatly prolongs survival ( Table 7.2 ). Some enteric bacteria can also survive and even multiply in organic-rich tropical waters. There is a common misconception that streams “purify” themselves over a short distance. Natural die-off of organisms and the disinfection effects of ultraviolet light decrease the number of viable microorganisms, but these are not reliable enough to ensure potable water in a stream. Microorganisms also clump to particles and settle to the bottom in still water but are easily stirred up and redistributed. This does suggest that when taking surface water from a lake, one should try to obtain the water from underneath the surface, where particles float from surface tension, while not disturbing bottom sediment.
TABLE 7.2
Survival of Microorganisms in Water
| Organism | Survival |
|---|---|
| Vibrio cholerae | 4-5 weeks in cold water |
| Giardia lamblia | 2-3 months at 5-10° C |
| 10-28 days at 15° C | |
| Cryptosporidium | 12 months in cold water |
| Enteric viruses | 6-10 days at 15-25° C |
| 30 days at 4° C | |
| Norovirus | 61 days in ground water at 25° C |
| Hepatitis A | 12 weeks in temperate water |
| 6-12 months in cold water | |
| Salmonella , Shigella | Half-life 16-24 h in temperate stream |
Many organisms, such as Giardia , Salmonella , and Cryptosporidium , can be zoonotic and have animal reservoirs, but most surface water contamination probably comes from human fecal contamination. It is important to properly dispose of personal waste. Wilderness hikers and backpackers should bury feces 6-10 inches in the soil, at least 100 feet from any water source and any natural drainage.
Accurate information concerning water quality is difficult to obtain in any country. Where sanitation systems are lacking, which is still the case in many rural areas of developing countries, all surface water should be considered highly contaminated, and tap water should be highly suspect. Any water that receives partially treated wastes, including in North America, is likely to contain pathogenic microorganisms, especially protozoa. Unfortunately, natural metals such as arsenic or chemical and nuclear wastes from industrial dumping and agricultural and mining run-off may be unrecognized or unacknowledged pollutants of water supplies. Expatriates and long-term travelers staying in a given area should try to obtain information from the Consulate or other expatriates about the safety of the local municipal water supplies.
Field Techniques for Water Treatment
Fortunately, there are reliable field methods for ensuring the microbiologic safety of drinking water. A large body of recent research confirms the beneficial effect of all standard techniques at the household or individual point of use level to improve water quality and reduce diarrheal illness ( Table 7.3 ). The main methods to eliminate microorganisms from water are heat, filtration, chemical disinfection, and ultraviolet treatment. Other techniques may be needed to improve the esthetic quality of the water or to remove chemical contamination. Each technique is discussed along with its respective advantages and disadvantages. Understanding the principles of water disinfection helps in choosing a method appropriate for the risk, location, and size of the group.
TABLE 7.3
Efficacy and Effectiveness of Point-of-Use Technologies for Household Use in the Developing World Households
Data from multiple studies analyzed and summarized by Sobsey 2008; with data from Bielefeldt 2009; Sobsey 2002; WHO 2011( table 7.8 ).
| Treatment Process | Pathogen | Optimal Log Reduction a | Expected Log Reduction b | Diarrheal Disease Reduction c |
|---|---|---|---|---|
| Ceramic filters | Bacteria | 6 | 2 | 63% (51-72%) for candle filters 46% (29-59%) for bowl filters |
| Viruses | 4 | 0.5 | ||
| Protozoa | 6 | 4 | ||
| Free chlorine | Bacteria | 6 | 3 | 37% (25-48%) |
| Viruses | 6 | 3 | ||
| Protozoa | 5 | 3 | ||
| Coagulation/chlorination | Bacteria | 9 | 7 | 31% (18-42%) |
| Viruses | 6 | 2-4.5 | ||
| Protozoa | 5 | 3 | ||
| Biosand filtration | Bacteria | 3 | 1 | 47% (21-64%) |
| Viruses | 3 | 0.5 | ||
| Protozoa | 4 | 2 | ||
| SODIS | Bacteria | 5.5 | 3 | 31% (26-37%) |
| Viruses | 4 | 2 | ||
| Protozoa | 3 | 1 |
SODIS, Solar disinfection.
a Skilled operators using optimal conditions and practices (efficacy); log reduction: pretreatment minus post-treatment concentration of organisms (e.g., 6 log = 99.999% removal).
b Actual field practice by unskilled persons (effectiveness). Depends on water quality, quality and age of filter or materials, following proper procedure, and other factors.
c Summary estimates from published data. Depends on consistency and correct use of technique, integrity of techniques (e.g., cracked filter), and other household sanitation measures.
Definitions
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 can be applied to heat and filtration.
Pasteurization is similar to disinfection but specifically refers to the use of heat, usually at temperatures below 212° F (100° C), to kill most enteric pathogenic organisms. Disinfection and pasteurization should not be confused with sterilization , which is the destruction or removal of all life forms. The goal of disinfection is to achieve potable water, indicating that a water source, on average over a period of time, contains a “minimal microbial hazard,” so the statistical likelihood of illness is minimized.
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 (e.g., Environmental Protection Agency [EPA] classification of water purifier), but purification, as used here, may not remove or kill enough microorganisms to ensure microbiologic safety.
Heat
The advantages of heat for water disinfection are the following:
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It is widely available.
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It imparts no additional taste to the water.
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It inactivates all enteric pathogens.
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Efficacy of heat treatment is not compromised by contaminants or particles in the water, as in the case of halogenation and filtration.
The major disadvantages of heat are the following:
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Heat does not improve the taste, smell, or appearance of poor-quality water.
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In many areas natural fuel is scarce or unavailable; 1 kg of wood is required to boil 1 L of water.
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Liquid fuels are expensive for developing countries and heavy to carry for the wilderness traveler.
Heat inactivation of microorganisms is exponential and follows first-order kinetics. Thermal death point is reached in a shorter time at higher temperatures, whereas temperatures as low as 140° F (60° C) are effective with a longer contact time. Pasteurization uses this principle to kill food-borne enteric food pathogens and spoiling organisms at temperatures between 140 and 158° F (60-70° C), well below boiling.
Heat resistance varies with different microorganisms, but common enteric pathogens are readily inactivated by heat ( Table 7.4 ). Bacterial spores (e.g., Clostridium spp.) are the most resistant; some can survive 212° F (100° C) for long periods. Clostridium spores are wound pathogens that are ubiquitous in soil, lake sediment, tropical water sources, and the stool of animals and humans. Water sterilization is not necessary for drinking, since these most resistant organisms are not water-borne enteric human pathogens.
TABLE 7.4
Data on Heat Inactivation of Microorganisms a
| Organism | Lethal Temperature/Time |
|---|---|
| Giardia | 131° F (55° C) for 5 min |
| 212° F (100° C) immediately | |
| Entamoeba histolytica | Similar to Giardia |
| Nematode cysts Helminth eggs, larvae, cercariae |
122-131° F (50-55° C) (time not specified but should be similar to Cryptosporidium ) |
| Cryptosporidium | 113-131° F (45-55° C) for 20 min |
| 148° F (64.2° C) for 2 min | |
| 162° F (72° C) heated up over 1 min | |
| Escherichia coli | 131° F (55° C) for 30 min |
| 140-144° F (60-62° C) for 10 min | |
| Salmonella and Shigella | 149° F (65° C) for <1 min |
| Vibrio cholerae | 212° F (100° C) for 30 s |
| Enteric viruses | 131-140° F (55-60° C) for 20-40 min |
| 158° F (70° C) for <1 min | |
| 167° F (75° C) for <0.5 min | |
| Hepatitis A | 185° F (85° C) for 1 min |
| 140° F (60° C) for 10 min | |
| Hepatitis E | 140° F (60° C) for 30 min |
| Bacterial spores | >212° F (100° C) |
a Endpoint in most studies is death of 100% of organisms. Some studies use 99.9% inactivation, but longer time in contact with heat will rapidly result in inactivation of all micro-organisms.
Protozoan cysts, including Giardia , Entamoeba histolytica , and Cryptosporidium are sensitive to heat, killed rapidly at 131-140° F (55-60° C). Parasitic helminth eggs and larvae, and cercariae of schistosomiasis, are equally susceptible to heat.
Vegetative bacteria and most enteric viruses are killed rapidly at temperatures above 140° F (60° C) and within seconds by boiling water. Typical pasteurization processes include heating to 145-149° F (63-65° C) for up to 30 min or flash pasteurization using high temperature-short time at 160-162° F (71-72° C) for 15-30 seconds. Recent data confirm that common water-borne enteric viruses, including hepatitis A virus (HAV), are readily inactivated at these temperatures.
In recognition of the difference between pasteurizing water for drinking purposes and sterilizing for surgical purposes, most sources now agree that boiling for 10 min is not necessary. Heating water on a stove or fire takes time, which counts toward disinfection while the temperature rises from 131° F (55° C) to the boiling temperature. Although attaining boiling temperature is not necessary, it is the only easily recognizable endpoint without using a thermometer. Therefore, any water brought to a boil should be adequately disinfected. For an extra margin of safety, the water should be brought to a boil, the stove turned off, and the pot covered for a few minutes before using the water. The WHO concurs with this conclusion, but the Centers for Disease Control and Prevention (CDC) and the EPA still recommend boiling for 1 min to allow for an extra margin of safety. The boiling point decreases with increasing altitude, but this is not significant with regard to the time and temperature required for thermal death ( Table 7.5 ).
TABLE 7.5
Boiling Temperature and Altitude
| Elevation | Boiling Point of Water |
|---|---|
| 10,000 ft (3048 m) | 194° F (90° C) |
| 14,000 ft (4267 m) | 187° F (86° C) |
| 19,000 ft (5791 m) | 178° F (81° C) |
The use of hot tap water, “too hot to touch,” has been suggested to prevent traveler's diarrhea in developing countries. However, testing shows considerable variation in the temperature of hot tap water (most between 131 and 140° F (55-60° C), but some lower) and in maximum tolerated temperature-to-touch (below 131° F (55° C) for some people). If no other means of water treatment is available, using hot tap water that has been kept hot in a tank for some time is a reasonable alternative. Travelers staying in hotels or other accommodations with electricity can conveniently bring water to a boil with a small electric heating coil or with a lightweight electric beverage warmer brought from home.
In hot, sunny climates, temperatures adequate for pasteurization can be achieved by solar heating using a solar oven or simple reflectors.
Filtration
Many commercial products are available for individuals and groups to filter water in the field. Advantages of filtration include:
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Filters are simple to operate.
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Filters add no unpleasant taste, and many will improve the taste and appearance of water.
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Most filters require no holding time, and water can be consumed as it comes out of the filter.
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It may be rationally combined with halogens if the filter is not rated for viral removal.
Disadvantages of filtration include:
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It is expensive compared with halogen treatment.
-
Filters can be heavy and bulky, which may be significant if carrying the gear oneself.
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Many filters will not remove viruses sufficiently and may require a second step with halogens to assure viral inactivation.
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A cracked filter element or a leaking seal can allow channeling of water around the filter and will let contaminated water pass through the device.
-
Filters clog quickly if the water is dirty or has a lot of suspended particles and eventually will clog from filtering even “clear” surface water. Most micropore filters include a prefilter for larger particulates. Laboratory paper filters with a pore size of about 20-30 µm or even coffee filters can be used to prefilter the larger particulate debris from dirty water and may also retain parasitic eggs and larvae (see the section on Clarification below for other methods). The user should know how to clean or replace the filter elements to re-establish flow.
The effectiveness of filters depends on pore size ( Tables 7.6a and 7.6b ), but other variables influence filter efficiency, including the characteristics of the filter media and the water, as well as flow rate. 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. Ultrafiltration membranes are required for complete removal of viruses, colloids, and some dissolved solids. Nanofilters can remove other dissolved substances, including salts (sodium chloride) and endotoxins from water. Reverse osmosis removes monovalent ions (desalination) and nearly all organic molecules.
TABLE 7.6a
Levels of Filtration
| Filtration Level | Minimal Pore Size (µm) | Particles Removed |
|---|---|---|
| Microfiltration | 0.1 | Protozoa, bacteria, algae, particles, sediment |
| Ultrafiltration | 0.001 | Viruses, colloids, some dissolved solids |
| Nanofiltration | 0.0001 | All microorganisms, dissolved substances, salts, endotoxins |
| Reverse osmosis | 0.00001 | All of above plus monovalent ions and nearly all organic molecules |
TABLE 7.6b
Susceptibility of Microorganisms to Filtration
| Organism | Approximate Size (µm) | Maximum Filter Pore Size (µm) |
|---|---|---|
| Nematode eggs | 30 × 60 | 20 |
| Giardia | 6-10 × 8-15 | 3-5 |
| Entamoeba histolytica | 5-30 (average 10) | 3-5 |
| Cyclospora | 8-9 | 3-5 |
| Cryptosporidium oocysts | 2-6 | 1 |
| Enteric bacteria | 0.5 × 3-8 | 0.2-0.4 |
| Viruses | 0.03 | 0.01 |
Filtration is usually a two-step process: physical (separation of particles from liquid) and chemical (attachment of microorganisms to the medium), which may allow increased efficacy of virus removal. Most field filters are not membranes but rather depth filters, with maze-like passageways that trap particles and organisms smaller than the average passage diameter.
Most field devices are microfilters that are adequate for cysts and bacteria but may not sufficiently remove viruses, which are a major concern in water with high levels of fecal contamination. It is important for point-of-use filters to achieve the EPA standard of 4-log reduction (99.99%) of viruses, given the small infectious dose. Most viruses adhere to larger particles or clump together into aggregates that may be removed by the filter, in addition to any electrochemical adherence to the filter media. Reverse osmosis filters that desalinate will also remove viruses; however, these are currently too expensive and slow for use in a hand pump for land travel. Iodine resin filters will kill bacteria and viruses by contact with the iodine, not by mechanical filtration. Several portable filters have test results that meet the EPA standards for removal of viruses: General Ecology First-Need mechanical microfilter claims electrochemical removal of viruses; Sawyer Water Purifier, Lifestraw, MSR and other products that use hollow fiber membrane filtration remove viruses with ultrafiltration capability.
Some filters can be readily and inexpensively built in developing areas. One is a ceramic filter shaped like a flower pot or rounded cone made from local substances like porous fired clay (diatomaceous earth). The other is a biosand filter that uses successive layers of progressively more coarse sand and gravel. The uppermost layer of fine sand further builds a biologic layer that contributes to the filtration effectiveness.
If the water supply is suspected of being heavily contaminated with biologic wastes and additional assurance is needed, then a second step with chemical treatment of the water before or after filtration can kill viruses. Many filters contain a charcoal stage that will remove the halogen, if applied prior to filtration. Alternately, prior filtration allows lower halogen doses to be used for the chemical inactivation step.
Filters for international and wilderness travelers are listed in Table 7.7 .
TABLE 7.7
Portable Field Water Filters and Purification Devices
| Manufacturer, Product/Manufacturer's Website a | Microbial Claims b | Operation | Primary Filter, Additional Elements, Stages, Comments c | Capacity | Retail Price (US$) d |
|---|---|---|---|---|---|
| Aquamira www.mcnett.com Water filter bottle and Frontier |
P, B, V | In line, drink-through in sport bottle, water bag, or gravity drip | Prefilter, porous plastic microfilter, new Redline filter for virus removal, carbon shell. May be used in conjunction with Aquamira water treatment—chlorine dioxide stabilized solution | 1-2 people | 20-50 |
| Aquarain www.aquarain.com | |||||
| Aquarain 200 and 400 | P, B | Gravity drip | Stacked bucket filter with one to four candle filters with ceramic elements. Carbon core, stainless steel housing | Small group | 240-320 |
| British Berkfeld www.jamesfilter.com | |||||
| Big Berkey and multiple other models | P, B | Gravity drip | Bucket filter, one to four ceramic elements with carbon matrix or candle filters of compressed carbon. Available in stainless steel or Lexan housing with 1.5-6 gallon lower reservoir. | Household or moderate-sized group | 240 (100-320) |
| General ecology www.generalecology.com | Claims for viral removal are based on electrostatic attraction in structured matrix compressed carbon block filter. Variety of sizes and configurations also available for in-line use and electric-powered units | ||||
| First Need XLE | P, B, V | Hand pump | Compressed charcoal | Small group | 120 |
| Base camp | P, B, V | Hand pump or electric | Compressed charcoal element similar to First Need. High flow, high capacity. Stainless steel housing. Prefilter. Electric models also available | Large group | 700 |
| Trav-L-Pure | P, B | Hand pump | Same compressed charcoal filter element as First Need in plastic housing | Small group | 230 |
| Hydro-photon www.hydro-photon.com | Hand-held purifier | Ultraviolet purifier uses batteries with timer. Active end of unit is held in bottle or other small container of water. | 1-2 people | ||
| Multiple Steripen models: Classic Opti Adventurer Ultra Freedom Aqua |
P, B, V | Units differ in size, battery (AA, CR 123, rechargeable), LED display, and other features. | 50-100 | ||
| Katadyn www.katadyn.com | Unless otherwise specified, filter elements are 0.2 µm ceramic depth filter. | ||||
| My Bottle | P, B, V | Sport bottle | Iodine resin with filter for protozoan cysts, and granular activated charcoal | 1-2 people | 40-60 |
| Hiker and Hiker Pro | P | Hand pump | Pleated glass-fiber 0.3-µm filter with granular activated charcoal core and prefilter; for high-quality source water, removes “most” bacteria | 1-2 people | 60-85 |
| Gravity Camp and Basecamp | P, B | Gravity drip | Pleated glass fiber 0.3 µm with activated carbon; reservoir bag with in-line filter | Small group | 60 |
| Mini | P, B | Hand pump | Ceramic filter with prefilter | 1-2 people | 90 |
| P, B | Hand pump | Ceramic filter with prefilter | Small group | 300 | |
| Ceradyn and Gravidyn | P, B | Gravity drip | Bucket filter, three ceramic candles; optional activated carbon core filters with Gravidyn | Small-large group | 160-190 |
| Combi | P, B | Hand pump | Ceramic filter and activated carbon cartridge; can be converted for in-line faucet use | Small group | 180 |
| Expedition | P, B | Hand pump | Ceramic filter with intake pre-filter; stainless steel housing | Large group | 1250 |
| Survivor 06 Survivor 35 (power units also available) |
P, B, V | Hand pump | Reverse osmosis filter; desalinates as well as disinfects, for ocean survival; very low flow rate; power units available | 1-2 people | 1000-2200 |
| Cascade Designs www.cascadedesigns.com | |||||
| MSR Sweetwater microfilter | P, B | Hand pump | 0.2-µm depth filter with granular activated carbon and pre-filter; purifier solution (chlorine) as pre-treatment to kill viruses | Small group | 90 |
| Miniworks EX | P, B | Hand pump | Ceramic filter with activated carbon core and pre-filter | Small group | 90 |
| HyperFlow and AutoFlow | P, B | Pump or gravity drip | Microfilter (0.2 µm) with hollow-fiber technology | Small group | 100-120 |
| Guardian | P,B,V | Hand pump | Hollow fiber technology with 0.02 micron pore size to remove viruses. | Small group | 350 |
| Sawyer www.sawyerproducts.com | |||||
| Water filter Point One Biologic (available in wide array of packaged products) |
P, B | Multiple applications including sport bottle and in-line cartridge, gravity drip, bucket filter, or faucet attachment | Hollow-fiber technology, 0.1 µm in versatile filter cartridge | 1-2 people or small group | 40-90 |
| Water purifier Zero Point Two (available in wide array of packaged products) |
P, B, V | Multiple applications, same as Point One filter | Hollow-fiber technology, 0.02 µm, in versatile filter cartridge | 1-2 people or small group | 130-220 |
a This is not a comprehensive list. Models change frequently. A manufacturer's website is provided if it contains product information; otherwise, search manufacturer and brand with any major search engine to find large retail sites that provide detailed product information.
b B, Bacteria; P, protozoa; V, viruses.
c Consider additional features, such as flow rate, filter capacity, size, and filter weight.
Clarification
The appearance of cloudy water can be improved by several other means. Large particles will settle out over a period of several hours by sedimentation. The supernatant can then be filtered and/or chemically treated. Smaller suspended particles can be removed by coagulation-flocculation. A pinch of alum, an aluminum salt, is added to a gallon of water, mixed well, and then stirred occasionally for 30-60 min. The quantity added does not need to be precise, and more can be used as needed. The small particles clump (flocculate) and then settle out over minutes to hours. The supernatant is then decanted, or the mixture is poured through a paper filter before proceeding with microfiltration and/or chemical treatment. Coagulation-flocculation removes many microorganisms as well as other impurities from the water, greatly improving taste, smell, and microbiologic safety of cloudy water. However, coagulation-flocculation should not be used as a sole step for disinfection; it should be followed by chemical treatment, filtration, or ultraviolet treatment.
Granular Activated Charcoal
Granular activated charcoal (GAC) improves water quality by removing organic pollutants and chemicals by adsorption. GAC can remove objectionable color, taste, and smell from water. Although some microorganisms will adhere to GAC or become trapped in charcoal filters, GAC does not remove all microorganisms; thus, it does not disinfect. In fact, charcoal beds become colonized rapidly with nonpathogenic bacteria. One rational use of GAC is to remove the color and taste of iodine or chlorine after disinfection. If used to remove halogen, one must wait until after the required contact time before running water through charcoal or adding charcoal to the water. Granular activated charcoal is commonly incorporated into point-of-use water filters ( Table 7.7 ).
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