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Common Methods of Water Disinfection

Table of Contents
  1. Boiling
  2. Distillation
  3. Chemical Water Disinfection
  4. UV Water Purification
  5. Filtration

Drinking water disinfection is routine in developed countries with adequate sanitation and safe hygiene practices. Depending on the source of the water, municipal water treatment plants use a variety of physical and chemical water disinfecting techniques before the water reaches your sink at home. The most common methods of water purification include heat, distillation, chemical treatment, UV light and filtration. In developing countries, where residents don't have access to safe sanitation and hygiene, alternative methods are often used. In emergency situations, it is good to familiarize yourself with the various techniques used so that you can disinfect the water yourself, should the need arise.

Boiling


Waterborne illness is caused by bacteria, viruses and parasitic cysts - Giardia and Cryptosporidium - often found in rivers, lakes and other dirty water sources. Heating the water to boiling point kills these disease-causing microorganisms and is the surest way to make contaminated water safe for drinking. The Centers for Disease Control (CDC) and the Environmental Protection Agency (EPA) both recommend that you boil water vigorously for one minute, to ensure that all microorganisms are killed. At altitudes higher than one mile, boiling point decreases, so you should boil the water for at least three minutes. Boiling will not rid the water of any unpleasant taste. However, aeration of the water may improve bad taste. To aerate, pour the water back and forth from one container to another and allow it to sit for a few hours, or add a pinch of salt for each liter or quart of water boiled. In developed countries, boiling is usually the recommended solution in emergency situations where accidental water contamination has been reported. However, if boiling is not an option, residents often resort to drinking bottled water.

Distillation


Distillation is the process of boiling water to produce water vapor which then condenses onto a cool surface. The solutes do not normally vaporize and are left in the boiling solution leaving the condensed water vapor almost completely pure. However, it is possible to get water that is not completely pure at the end of this process, due to contaminants with similar boiling points and unvaporized droplets being carried with the steam. Distillation is a controversial water disinfecting method, because it de-mineralizes the water, leaving it void of essential minerals like magnesium and calcium, which prevent nutritional deficiency. It also increases the risk of toxic poisoning, as it more readily leaches metals like lead from household piping.

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Chemical Water Disinfection


Two of the most common chemical water disinfecting agents, which may be used in an emergency situations, are chlorine and iodine (halogens). Chloramines and Ozone are also used by water treatment plants. In order for chemical disinfection to be effective, the water must be filtered and settled first. Chemical disinfection often leaves an undesirable taste in water, which an activated carbon filter can remove post-treatment. Otherwise, you can minimize bad taste and odor by reducing the disinfectant concentration and increasing contact time before drinking. Another option to remove the taste of chlorine and iodine from water is to add a pinch of ascorbic acid, which is available in crystal or power form. This converts chlorine into chloride and iodine into iodide, both of which have no taste or odor.
  • Chlorine may be used in gas, liquid or solid form to disinfect water. Because chlorine gas is highly toxic and can be dangerous if released into the atmosphere, this form of disinfection must be done in a very controlled environment. Otherwise, the danger is avoided by the use of chlorine in liquid form (sodium hypochlorite) or solid form (calcium hypochlorite). Household bleach is made up of 3-6 percent sodium hypochlorite and the EPA recommends using it to disinfect water in emergency situations. Waste water treatment plants typically use a 12 percent solution. Calcium hypochlorite water purification tablets are readily available for emergency situations as well. While chlorine is a highly effective and widely used method of water disinfection, it reacts with organic compounds in water, forming trihalomethanes (THMs) and haloacetic acids, which are carcinogenic in large quantities. The best way to avoid this is to remove as many organics as possible, prior to disinfection. If necessary, some commercial water filters also remove chlorine and its byproduct,s post-treatment.
  • Iodine, like chlorine, is available in liquid and tablet form. Iodine is used by the thyroid and is recommended for emergency use only, in limited quantities. Iodine water purification is not recommended for people with who are pregnant, have thyroid disease, or an iodine allergy.
  • Chloramine does not form THMs or haloacetic acid as chlorine does, and, as a result, is becoming more commonly used as a water disinfectant. However, nitrates, which impart a bad taste and odor and may be harmful to humans, are sometimes formed as a byproduct instead. Chloramines are made when ammonia is added to water containing chlorine, or when water containing ammonia is chlorinated. They are less effective than chlorine at killing viruses or protozoa. 
  • Ozone is a powerful oxidizing agent that is toxic to most waterborne organisms. It is widely used in water treatment plants in other parts of the world, especially in Europe but is fairly new to the U.S. Ozone is often accompanied by a secondary disinfectant, such as chlorine, because although it effectively kills existing microorganisms, it leaves few residuals to prevent the future growth of microorganisms in the water, post-treatment. The danger of ozone is the production of carcinogenic bromate as a byproduct. However, bromide ions are required for this to occur, and there should be few, if any, in treated water.

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UV Water Purification


Ultraviolet radiation penetrates the cell walls of microorganisms and disrupts their DNA, making them unable to reproduce. Though they remain present in the water, they are inactive, and thus, leave no risk of waterborne illness. UV light is effective against viruses and bacteria but may not inactivate parasitic cysts. As with ozone, a secondary method is often necessary to prevent the regrowth of organisms, post-treatment.

Filtration


Filtration is often the first and last step taken in these common methods of water disinfection. It is recommended prior to the disinfection of water to remove particles that hinder the disinfecting process. Following disinfection, filtration may also be used as a secondary method to remove cysts, along with the bad taste and odor caused by chemical byproducts.


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