Water treatment methods

There are many ways to treat water. None are perfect. Often the best solution is a combination of methods. Before treating, let any suspended particles settle to the bottom or strain them through coffee filters or layers of clean cloth. In this article, we’re going to cover boiling, filtration, chemical disinfection, UV light, solar irradiation and heating, and distillation.


Boiling is the safest method of treating water. In a large pot or kettle, bring water to a rolling boil for one full minute, keeping in mind that some water will evaporate. Let the water cool before drinking. At altitudes greater than 6,562 feet (greater than 2000 meters), you should boil water for 3 minutes.

Boiled water will taste better if you put oxygen back into it by pouring the water back and forth between two clean containers. This also will improve the taste of stored water.

If boiling water is not possible, a combination of filtration and chemical disinfection is an effective pathogen reduction method. Manufacturer’s instructions must be followed.


There’s a wide selection of water filtration products on the market that do not rely on electricity to operate making them a vital element of a disaster preparedness kit. They range from portable filters like straws, bottles, pumps, and gravity bags to bigger countertop systems that are able to process large amounts of water over long periods of time.

Most filter cartridges are made of ceramic, glass fiber, plant fiber, hard-block carbon, or materials similar to compressed surgical paper. Each filter type has its own advantages and drawbacks in regard to pore size, durability, maintenance, and cost. Some products use a multi-stage system that contains more than one media type in the overall cartridge.

Filter pore size is the primary determinant of a filter’s effectiveness, but microorganisms also adhere to filter media by electrochemical reactions. Microfilters with “absolute” pore sizes of 0.1–0.4 µm are usually effective to remove cysts and bacteria but may not adequately remove viruses, which are a major concern in water with high levels of fecal contamination.

Chemical Disinfection

Disinfection can be used as a pathogen reduction method against microorganisms. However, contact time, disinfectant concentration, water temperature, water turbidity (cloudiness), water pH, and many other factors can impact the effectiveness of chemical disinfection. The length of time and concentration of disinfectant varies by manufacturer and the effectiveness of pathogen reduction depends on the product. Depending on these factors, 100% effectiveness may not be achieved. Manufacturer’s instructions must be followed.

The most common chemical water disinfectants are chlorine and iodine (halogens). Given adequate concentrations and length of exposure (contact time), chlorine and iodine have similar activity and are effective against bacteria, viruses, and Giardia cysts (www.cdc.gov/safewater/about_pages/CTfactor-final.pdf). Extending the contact time adds a margin of safety, however, some common waterborne parasites, such as Cryptosporidium, are poorly inactivated by halogen disinfection, even at practical extended contact times. Therefore, chemical disinfection should be supplemented with adequate filtration to remove these microorganisms from drinking water.

Tablets and Drops
Both chlorine and iodine are available in liquid and tablet form. There are also new Chlorine Dioxide products that leave a better taste and smell than chlorine or iodine-based purifiers. Keep in mind that each product has an expiration date and becomes ineffective after that point. Also, once the product has been opened, it usually must be used within a certain period.

You can use household liquid bleach to kill microorganisms. Use only regular household liquid bleach that contains 5.25 to 6.0 percent sodium hypochlorite. Do not use scented bleaches, color-safe bleaches, or bleaches with added cleaners. Because the potency of bleach diminishes with time, use bleach from a newly opened or unopened bottle. Add 16 drops (1/8 teaspoon) of bleach per gallon of water, stir, and let stand for 30 minutes. The water should have a slight bleach odor. If it doesn’t, then repeat the dosage and let stand for another 15 minutes. If it still does not smell of chlorine, discard it and find another source of water.

Because iodine has physiologic activity, it is generally recommended to limit iodine water disinfection to a few weeks of emergency use. Iodine use is not recommended for people with unstable thyroid disease or known iodine allergy. Iodine should not be used by pregnant women because of the potential effect on the fetal thyroid.

Chemical Disinfection Tips

  • If the water is cloudy, strain it with coffee filters or cloth before treatment.
  • If the water is cold, the effectiveness of purifying agent is reduced. Generally, the colder the water the less effective the chemical is as a purifying agent.
  • If the water tastes or smells unpleasant, after proper treatment, try pouring it back and forth between containers. You can also add flavorings like lemonade mixes to improve the taste.

Ultraviolet (UV) Light

Extensive data show that UV light can kill bacteria, viruses, and Cryptosporidium cysts in water. The effect depends on UV dose and exposure time and requires clear water because suspended particles can shield microorganisms from UV rays. These units have limited effectiveness in water with high levels of suspended solids and turbidity. They also have no disinfection residual. Portable battery-operated units that deliver a metered, timed dose of UV may be an effective way to disinfect small quantities of clear water in the field.

Solar Irradiation and Heating

UV irradiation by sunlight in the UVA range can substantially improve the microbiologic quality of water. Recent work has confirmed the efficacy and optimal procedures of the solar disinfection technique. Transparent bottles (such as clear plastic beverage bottles), preferably lying on a dark surface, are exposed to sunlight for a minimum of 4 hours. UV and thermal inactivation are synergistic for solar disinfection of drinking water. The use of a simple reflector or solar cooker can achieve temperatures of 149°F (65°C), which will pasteurize the water after 4 hours. Solar disinfection is not effective on turbid water. If the headlines in a newspaper can’t be read through the bottle of water, then the water must be filtered before solar irradiation is used. If more than half the sky is clouded over, then solar irradiation is not effective and should be repeated before using the water. Solar disinfection of drinking water may be acceptable for austere emergency situations.


While boiling and chlorination will kill most microbes in water, distillation will remove microbes (germs) that resist these methods, as well as heavy metals, salts, and most other chemicals. Distillation involves boiling water and then the collection of only the vapor that condenses. The condensed vapor will not include salt or most other impurities. To distill, fill a pot halfway with water.

Tie a cup to the handle on the pot’s lid so that the cup will hang right-side-up when the lid is upside-down (make sure the cup is not dangling into the water) and boil the water for 20 minutes. The water that drips from the lid into the cup is distilled.

Silver and Other Products

Silver ion has bactericidal effects in low doses and some attractive features, including the absence of color, taste, and odor. The use of silver as a drinking water disinfectant is popular in Europe, but it is not approved for this purpose in the United States, because the silver concentration in water is strongly affected by adsorption onto the surface of the container, and there has been limited testing on viruses and cysts. In the United States, silver is approved for maintaining the microbiologic quality of stored water.

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

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