Water Disinfection - Chapter 2 - 2020 Yellow Book | Travelers' Health (2022)

Howard D. Backer, Vincent Hill


Waterborne disease is a risk for international travelers who visit countries that have poor hygiene and inadequate sanitation, and for wilderness visitors who rely on surface water in any country, including the United States. The list of potential waterborne pathogens is extensive and includes bacteria, viruses, protozoa, and parasitic helminths. Most of the organisms that can cause travelers’ diarrhea can be waterborne. Many types of bacteria and viruses can cause intestinal (enteric) infection through drinking water. Protozoa that are commonly waterborne include Cryptosporidium, Giardia, and Entameba histolytica (the cause of amebic dysentery). Parasitic worms are not commonly transmitted through drinking water, but it is a potential means of transmission for some.

Where treated tap water is available, aging or inadequate water treatment infrastructure may not effectively disinfect water or maintain water quality during distribution. Some larger hotels and resorts may provide additional onsite water treatment to provide potable water. Travelers can ask the facility manager about safety of their water; however, if there is concern, it may be easiest for travelers to treat the water themselves. Where untreated surface or well water is used and there is no sanitation infrastructure, the risk of waterborne infection is high.

Bottled water has become the convenient solution for most travelers, but in some places it may not be superior to tap water. Moreover, the plastic bottles create an ecological problem, since most developing countries do not recycle plastic bottles. All international travelers, especially long-term travelers or expatriates, should become familiar with and use simple methods to ensure safe drinking water. Several methods are scalable and some can be improvised from local resources, allowing adaptation to disaster relief and refugee situations. Table 2-07 compares benefits and limitations of different methods. Additional information on water treatment and disinfections methods can be found at www.cdc.gov/healthywater/drinking/travel.

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Common intestinal pathogens are readily inactivated by heat. Microorganisms are killed 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 foodborne enteric pathogens and spoilage-causing organisms at temperatures between 140°F (60°C) and 158°F (70°C), well below the boiling point of water (212°F [100°C]).

Although boiling is not necessary to kill common intestinal pathogens, it is the only easily recognizable end point that does not require a thermometer. All organisms except bacterial spores, which are rarely waterborne enteric pathogens, are killed in seconds at boiling temperature. In addition, the time required to heat the water from 60°C to boiling works toward heat disinfection. Any water that is brought to a boil should be adequately disinfected; however, if fuel supplies are adequate, travelers should consider boiling for 1 minute to allow for a margin of safety. Although the boiling point decreases with altitude, at common terrestrial travel elevations it is still well above the temperature required to inactivate enteric pathogens (for example, at 16,000 ft [4,877 m] the boiling temperature of water is 182°F [84°C]). In hot climates with sunshine, a water container placed in a simple reflective solar oven can reach pasteurization temperature of 65°C. Travelers with access to electricity can bring a small electric heating coil or a lightweight beverage warmer to boil water.

Filtration and Clarification

Portable hand-pump or gravity-drip filters with various designs and types of filter media are commercially available to international travelers. Filter pore size is the primary determinant of a filter’s effectiveness, unless the filter is designed to remove microbes by electrochemical attachment to filter media. Filter pore size will be described as being “absolute” or “nominal”: absolute pore size filters will remove all microbes of the identified pore size or larger, whereas nominal pore size filters allow 20%–30% of particles or microorganisms of the pore size to pass through. Progressively smaller pore size filters require higher pressure to push water through the filter, often at a slower rate and higher cost. Filters that claim Environmental Protection Agency (EPA) designation of water “purifier” undergo company-sponsored testing to demonstrate removal of at least 106 bacteria (99.9999%), 104 viruses (99.99%), and 103 Cryptosporidium oocysts or Giardia cysts (99.9%). (EPA does not independently test the validity of these claims.)

Filters with absolute pore size of 1 µm or smaller should effectively remove protozoan parasites like Cryptosporidium and Giardia. Microfilters with “absolute” pore sizes of 0.1–0.4 µm are usually effective at removing bacteria as well as cysts but may not adequately remove enteric viruses, like norovirus (Table 2-08). Water in remote alpine areas with little human and animal activity generally has little contamination with enteric pathogens, so microfilters with ceramic, synthetic fiber, compressed carbon, or large-pore hollow-fiber filter elements are sufficient to remove bacteria and protozoan cysts, the primary pathogens.

For areas with high levels of human and animal activity in the watershed or developing areas with poor sanitation, higher levels of filtration discussed below or other techniques to remove viruses are preferred. If using a microfilter, one option to remove viruses is pretreatment with chlorine. Progressively finer levels of filtration known as ultrafiltration, nanofiltration, and reverse osmosis can remove particles of 0.01, 0.001, and 0.0001 µm, respectively. All of these filters can remove viruses. Portable ultrafilters are the most commonly available “purifying” filters and may operate by gravity, hand-pump, or drink-through. Ultrafilter-based filters will have a rated pore size of 0.01 µm, and should be effective for removing viruses, bacteria, and parasites. All are effective, although drink-through is least practical because of the negative pressure required to draw water through the filter.

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Nanofilters will have rated pore sizes of 0.001 µm and thus will remove chemicals and organic molecules. Reverse osmosis filters (having pore sizes of 0.0001 µm [0.1 nm] and smaller) will remove monovalent salts and dissolved metals, thus achieving desalination. The high price and slow output of small hand-pump reverse osmosis units prohibit use by land-based travelers; however, they are survival aids for ocean voyagers, and larger powered devices are used for military and refugee situations.

In resource-limited international settings, filters may be used in the communities and households that are made from ceramic clay or simple sand and gravel (slow sand or biosand). Gravel and sand filters can be improvised in remote or austere situations when no other means of disinfection is available.

Water can be clarified by using chemical products that coagulate and flocculate (clump together) suspended particles that cause a cloudy appearance and bad taste and do not settle by gravity. This process removes many but not all microorganisms, unless the product also contains a disinfectant. Alum, an aluminum salt that is widely used in food, cosmetic, and medical applications, is the principal agent for coagulation/flocculation, but many other natural substances are used throughout the world. When using alum, a one-fourth teaspoon of alum powder can be added to a quart of cloudy water and the water stirred frequently for a few minutes. The process can be repeated, if necessary, until clumps form. The clumped material is allowed to settle, and then the water is poured through a coffee filter or clean, fine cloth to remove the sediment. Most microbes are removed, but not all, so a second disinfection step is necessary. Tablets or packets of powder that combine flocculant and a chemical disinfectant are available commercially (for example, Chlor-floc and P&G Purifier of Water).

Granular-activated carbon (GAC) treats water by adsorbing organic and inorganic chemicals (including chlorine or iodine compounds) and most heavy metals, thereby improving odor, taste, and safety. GAC is a common component of household and field filters. It may trap microorganisms, but GAC filters are generally not designed or rated for microbe removal and do not kill microorganisms.

Chemical Disinfection


Chemical disinfectants for drinking water treatment, including chlorine compounds, iodine, and chlorine dioxide, are commonly available as commercial products. Sodium hypochlorite, the active ingredient in common household bleach, is the primary disinfectant promoted by CDC and the World Health Organization. Other chlorine-containing compounds such as calcium hypochlorite and sodium dichloroisocyanurate, available in granular or tablet formulation, are equally effective for water treatment. An advantage of chemical water disinfection products is flexible dosing that allows use by individual travelers, small or large groups, or communities. In emergency situations, or when other commercial chemical disinfection water treatment products are not available, household bleach can be used for flexible dosing based on water volume and clarity. Refer to CDC guide­lines at www.cdc.gov/healthywater/emergency/drinking/making-water-safe.html.

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Given adequate concentrations and length of exposure (contact time), chlorine and iodine have similar activity and are effective against bacteria and viruses (www.cdc.gov/safewater/effectiveness-on-pathogens.html). Giardia cysts are more resistant to chemical disinfection; however, field-level concentrations are effective with longer contact times. For this reason, dosing and concentrations of chemical disinfection products are generally targeted to the cysts. Some common waterborne parasites, such as Cryptosporidium and possibly Cyclospora, are poorly inactivated by chlorine- and iodine-based disinfection at practical concentrations, even with extended contact times.

Chemical disinfection may be supplemented with filtration to remove resistant oocysts from drinking water. Cloudy water contains substances that will neutralize disinfectant, so it will require higher concentrations or contact times or, preferably, clarification through settling, coagulation/flocculation, or filtration before disinfectant is added.

Because iodine has physiologic activity, WHO recommends limiting iodine water disinfection to a few weeks. Iodine use is not recommended for people with unstable thyroid disease or known iodine allergy. In addition, pregnant women should not use iodine to disinfect water over the long term because of the potential effect on the fetal thyroid. Pregnant travelers who have other options should use an alternative means such as heat, chlorine, or filtration.

Some prefer the taste of iodine to chlorine, but neither is appealing in doses often recommended for field use. The taste of halogens in water can be improved by running water through a filter containing activated carbon or adding a 25-mg tablet of vitamin C, a tiny pinch of powdered ascorbic acid, or a small amount of hydrogen peroxide (5–10 drops of 3% peroxide per quart), then stir or shake. Repeat until taste of chlorine or iodine is gone.


Chlorine dioxide (ClO2) can kill most waterborne pathogens, including Cryptosporidium oocysts, at practical doses and contact times. Tablets and liquid formulations are commercially available to generate chlorine dioxide in the field for personal use.

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Electrolytic water purifiers generate a mixture of oxidants, including hypochlorite, by passing an electrical current through a simple brine salt solution. Purifier products sold for personal and group travel use produce an oxidant solution that can be added to water to kill microorganisms. This technique has been engineered into portable, battery-powered products that are commercially available.

Ultraviolet (UV) Light

UV light kills bacteria, viruses, and Cryptosporidium oocysts in water. The effect depends on UV dose and exposure time. Portable battery-operated units that deliver a metered, timed dose of UV are an effective way to disinfect small quantities of clear water in the field. Larger units with higher output are available where a power source is available. These units have limited effectiveness in water with high levels of suspended solids and turbidity, because suspended particles can shield microorganisms from UV light.

Solar Irradiation and Heating

UV irradiation of water using sunlight (solar disinfection or SODIS) can improve the microbiologic quality of water and may be used in austere emergency situations. Solar disinfection is not effective on turbid water. If the headlines in a newspaper cannot be read through the bottle of water, then the water must be clarified before solar irradiation is used. Under cloudy weather conditions, water must be placed in the sun for 2 consecutive days. (See www.sodis.ch/index_EN for further information.)

Silver and Other Products

Silver ion has bactericidal effects in low doses, and some attractive features include lack of color, taste, and odor, and the ability of a thin coating on the container to maintain a steady, low concentration in water. Silver is widely used by European travelers as a primary drinking water disinfectant. In the United States, silver is approved only for maintaining microbiologic quality of stored water because its concentration can be strongly affected by adsorption onto the surface of the container, and there has been limited testing on viruses and cysts. Silver is available alone or in combination with chlorine in tablet formulation.

Several other common products, including hydrogen peroxide, citrus juice, and potassium permanganate, have antibacterial effects in water and are marketed in commercial products for travelers. None have sufficient data to recommend them for primary water disinfection at low doses in the field.

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Photocatalytic Disinfection

Advanced oxidation processes use UV light or natural sunlight to catalyze the production of potent disinfectants for microorganisms and can break down complex organic contami­nants and even most heavy metals into nontoxic forms. Titanium dioxide (TiO2) is the most effective substance, but other metal oxides, chitins, and nanoparticles also have oxidative potential. A TiO2-impregnated membrane incorporated into a portable bag is available commercially.


Is a heavy greenish yellow gas used as a disinfectant in water treatment fill in the blank? ›

Chlorine is a strong oxidizing disinfectant that has been used to treat drinking water supplies for more than 60 yr. The gas was named "chlorine" after the Greek word for green, "chloros," because of its characteristic color.

What chemicals list all 3 are used to disinfect the water supply? ›

Most communities use either chlorine or chloramines. Some communities switch back and forth between chlorine and chloramines at different times of the year or for other operational reasons. Less commonly, utilities use other disinfectants, such as chlorine dioxide.

What are the objectives of disinfection of water What are the qualities of a good disinfectant? ›

Water disinfection means the removal, deactivation or killing of pathogenic microorganisms. Microorganisms are destroyed or deactivated, resulting in termination of growth and reproduction. When microorganisms are not removed from drinking water, drinking water usage will cause people to fall ill.

How do you purify tap water? ›

Three Ways To Purify Water
  1. Boiling. Bring the water to a rolling boil for 3-5 minutes. Let cool before drinking.
  2. Disinfect. You can use household liquid bleach (regular household bleach contains 5.25% sodium hypochlorite) to kill microorganisms. ...
  3. Distillation. Fill a pot halfway with water.

What Colour is chlorine water? ›

pale yellow-green

How much chlorine do I need for 1000 Litres? ›

To dose water in a tank with 5 mg/L chlorine use: 40 millilitres of liquid pool chlorine or 170 millilitres of bleach, for every 1000 litres in the tank.

What are the 2 methods of disinfection? ›

Disinfection methods include thermal and chemical processes. Moist heat may be used for items such as crockery, linen and bedpans e.g. automated processes in a machine. Specific chemical disinfectants can be used to decontaminate heat sensitive equipment and the environment.

What are 5 uses of chlorine? ›

Chlorine is used in the preparation of chlorides, chlorinated solvents, pesticides, polymers, synthetic rubbers, and refrigerants.

What bacteria is in water? ›

The principle bacteria pathogens that have been shown to cause human intestinal disease associated with drinking water are: Salmonella typhi, Typhoid fever; Salmonella paratyphi-A, paratyphoid fever; other Salmonella species, salmonellosis, enteric fever; Shigella dysenteriae, S.

Is rain water safe to drink? ›

While useful for many things, rainwater is not as pure as you might think, so you cannot assume it is safe to drink. Rain can wash different types of contaminants into the water you collect (for example, bird poop on your roof could end up in your water barrel or tank).

What are the 6 steps in water treatment? ›

They typically consist of several steps in the treatment process. These include: (1) Collection ; (2) Screening and Straining ; (3) Chemical Addition ; (4) Coagulation and Flocculation ; (5) Sedimentation and Clarification ; (6) Filtration ; (7) Disinfection ; (8) Storage ; (9) and finally Distribution.

What are the 3 types of disinfection? ›

Chlorination, ozone, ultraviolet light, and chloramines are primary methods for disinfection.

What are the 3 levels of disinfection? ›

  • High-level (semicritical items; [except dental] will come in contact with mucous membrane or nonintact skin)
  • Intermediate-level (some semicritical items1 and noncritical items)
  • Low-level (noncritical items; will come in contact with intact skin)

What are the 3 types of disinfection treatment used in freshwater treatment? ›

The different types of chlorine disinfection are batch disinfection, simple chlorination, and superchlorination followed by dechlorination. Chlorination types use various amounts of chlorine.

Is salt a chlorine? ›

Chlorine is a chemical product produced when sodium chloride, sulfuric acid, and manganese dioxide create a reaction. This reaction is used to obtain chlorine. In industries, chlorine is produced by the electrolysis of sodium chloride. Sodium hydroxide and hydrogen gas are released as byproducts.

Is chlorine basic or acidic? ›

Chlorine bleach is a base and is especially good at removing stains and dyes from clothes as well as disinfecting.

Where is chlorine found? ›

Occurrence: Found in nature dissolved in salts in seawater and in the deposits of salt mines. Today, most chlorine is produced through the electrolysis of aqueous sodium chloride. Appearance: Yellowish-green, dense, sharp-smelling gas.

What is the ratio of chlorine to water? ›

To disinfect water, add one part of the chlorine solution to each 100 parts of water you are treating. This is about the same as adding 1 pint (16 ounces) of the chlorine solution to 12.5 gallons of water.

How do you calculate chlorine in water? ›

  1. CHLORINATION. Dosage, mg/l = (Demand, mg/l) + (Residual, mg/l)
  2. HTH Solid (lbs) = (Vol, MG) x (Dosage, mg/l) x (8.34lbs/gal)
  3. Liquid (gal) = (Vol, MG) x (Dosage, mg/l) x (8.34 lbs/gal)

How do you measure chlorine in water? ›

There are three main methods to test free chlorine residual in drinking water in the field in developing countries: 1) Pool test kits, 2) Color-wheel test kits, and 3) Digital colorimeters.

What are the 4 types of disinfectants? ›

Nonoxidizing disinfectants are as follows: quaternary ammonium compounds, amphoterics, biguanides, and acid anionics. Halogens Chlorine and iodine have been used as terminal disinfectants for many years. More recently, bromine and chlorine dioxide have been introduced.

Which chemical is used for disinfection? ›

These include alcohols, chlorine and chlorine compounds, formaldehyde, glutaraldehyde, ortho-phthalaldehyde, hydrogen peroxide, iodophors, peracetic acid, phenolics, and quaternary ammonium compounds.

Why do we disinfect water? ›

Why Do We Need Disinfection? Disinfection of drinking water and wastewater is critical to the protection of public health. All water and wastewater systems should use some form of disinfection process to remove or inactivate microorganisms (pathogens) that can cause disease in humans and animals.

Is chlorine harmful to human health? ›

Exposure to low levels of chlorine can result in nose, throat, and eye irritation. At higher levels, breathing chlorine gas may result in changes in breathing rate and coughing, and damage to the lungs. Additional symptoms of exposure to chlorine can be severe. Workers may be harmed from exposure to chlorine.

What is the chemical name for chlorine? ›

chlorine (Cl), chemical element, the second lightest member of the halogen elements, or Group 17 (Group VIIa) of the periodic table. Chlorine is a toxic, corrosive, greenish yellow gas that is irritating to the eyes and to the respiratory system.

Why is chlorine green? ›

The halogens darken in colour as the group is descended: thus, while fluorine is a pale yellow gas, chlorine is distinctly yellow-green. This trend occurs because the wavelengths of visible light absorbed by the halogens increase down the group.

What are 5 useful bacteria? ›

Below are a few of the probiotics that are taken to treat or prevent disease, and how they're thought to work.
  • Lactobacillus. In the body, lactobacillus bacteria are normally found in the digestive, urinary, and genital systems. ...
  • Bifidobacteria. ...
  • Streptococcus thermophilus. ...
  • Saccharomyces boulardii.

What are 5 examples of bacteria? ›

Examples include Listeria monocytogenes, Pesudomonas maltophilia, Thiobacillus novellus, Staphylococcus aureus, Streptococcus pyrogenes, Streptococcus pneumoniae, Escherichia coli, and Clostridium kluyveri.

Which of the following is used as a disinfectant in water treatment? ›

Potassium permanganate is a weak disinfectant, hence, used in water.

Which of the following chemical is used as a disinfectant for water treatment? ›

Chlorine is the chemical used as a water disinfectant. Water disinfection is done to remove impurities and microbes from water which is obtained from the wastewater treatment plant before it is released into the distribution system.

Which of the following gas is used as disinfectant? ›

Chlorine dioxide gas is used to sterilize medical and laboratory equipment, surfaces, rooms and tools. Chlorine dioxide can be used as oxidizer or disinfectant. It is a very strong oxidizer and it effectively kills pathogenic microorganisms such as fungi, bacteria and viruses.

Which of the following is used as disinfectant? ›

Household bleach (chlorine as sodium hypochlorite) is active against most microorganisms, including bacterial spores and can be used as a disinfectant or sanitizer, depending on its concentration.

What are the 3 types of disinfection? ›

Chlorination, ozone, ultraviolet light, and chloramines are primary methods for disinfection.

What are the 2 methods of disinfection? ›

Disinfection methods include thermal and chemical processes. Moist heat may be used for items such as crockery, linen and bedpans e.g. automated processes in a machine. Specific chemical disinfectants can be used to decontaminate heat sensitive equipment and the environment.

What are the 3 types of disinfection treatment used in freshwater treatment? ›

The different types of chlorine disinfection are batch disinfection, simple chlorination, and superchlorination followed by dechlorination. Chlorination types use various amounts of chlorine.

What type of chlorine is used for drinking water? ›

The three most common types of chlorine used in water treatment are: chlorine gas, sodium hypochlorite, and calcium hypochlorite.

Why 70 alcohol is used for sterilization? ›

70% isopropyl alcohol kills organisms by denaturing their proteins and dissolving their lipids and is effective against most bacteria, fungi and many viruses, but is ineffective against bacterial spores (CDC, 2020).

What is free chlorine in drinking water? ›

The presence of free chlorine (also known as chlorine residual, free chlorine residual, residual chlorine) in drinking water indicates that: 1) a sufficient amount of chlorine was added initially to the water to inactivate the bacteria and some viruses that cause diarrheal disease; and, 2) the water is protected from ...

Is alcohol a disinfectant? ›

Alcohol is often used to disinfect small surfaces (e.g. rubber stoppers of multiple-dose medication vials, and thermometers) and occasionally external surfaces of equipment (e.g. stethoscopes and ventilators).

What is the strongest disinfectant? ›

Sterilants and high-level disinfectants
  1. 1 Formaldehyde. ...
  2. 2 Glutaraldehyde. ...
  3. 3 Ortho-phthalaldehyde. ...
  4. 4 Hydrogen peroxide. ...
  5. 5 Peracetic acid. ...
  6. 6 Hydrogen peroxide/peracetic acid combination.
14 May 2012

Why chlorine is used in water treatment? ›

Besides killing dangerous germs like bacteria, viruses and parasites, chlorine helps reduce disagreeable tastes and odors in water. Chlorine also helps eliminate slime bacteria, molds and algae that commonly grow in water supply reservoirs, on the walls of water mains and in storage tanks.

What are the 4 types of disinfectants? ›

Nonoxidizing disinfectants are as follows: quaternary ammonium compounds, amphoterics, biguanides, and acid anionics. Halogens Chlorine and iodine have been used as terminal disinfectants for many years. More recently, bromine and chlorine dioxide have been introduced.

What are the methods of disinfection? ›

  • Chemical Disinfectants. Alcohol. Chlorine and chlorine compounds. Formaldehyde. Glutaraldehyde. Hydrogen peroxide. ...
  • Miscellaneous Inactivating Agents. Other germicides. Metals as microbicides. Ultraviolet radiation. Pasteurization. Flushing- and washer-disinfectors.
  • Regulatory Framework for Disinfectants and Sterilants.

Why is disinfecting important? ›

Both cleaning and disinfecting are important for reducing the spread of viral illnesses. Some viruses may remain viable (living) for hours to days on surfaces made from a variety of materials. Cleaning surfaces followed by disinfection is a best practice for preventing the spread of viral illnesses in the workplace.


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