Making Water Safe to Drink

By Peter Kummerfeldt

I often hear two statements made regarding drinking water from outdoor sources. Some claim “I never treat the water I drink in the outdoors” while others say “I never drink the water because it’s got bugs in it.” In the first instance not treating water increases the risk of gastrointestinal illness and in the second instance not knowing how to make the water safe to drink dramatically increases the risk of dehydration and the many other problems associated with becoming dehydrated in the field. 

While the risk of water borne disease is much lower in North America than internationally, particularly in developing countries, it does exist and wherever possible water should always be treated to remove or kill harmful pathogens. The primary reason to treat drinking water is to prevent gastrointestinal illness from fecal pollution. 

To be safe water must be disinfected, which is defined as “the removal or destruction of harmful microorganisms.” To do this water must be boiled, treated with chemicals or filtered. “Disinfection” of water should not be confused with “purification” of water which may or may not remove or kill enough of the pathogens to ensure a person’s safety, so, to be certain that the water you drink is free from illness causing agents, it must be disinfected.

The first step in disinfecting water is to select the cleanest, clearest source of water available to you. Inorganic and organic material such as clay, silt, plankton, plant debris and other microscopic organisms reduce the effectiveness of using either chemical or filtration disinfection. Chemicals used to disinfect water will clump to any particulate in the water reducing its ability to disinfect the water. Water containing a lot of material will also quickly clog a filter. When using murky water allow it to settle and then filter it through your shirt-tail, bandanna or other piece of cloth. Better still take a few coffee filters along with you to strain your water. 

Boiling.Bringing water to a boil kills any organisms in it. Contrary to the advice given in many other sources, the water does not have to be boiled for “ten minutes plus a minute for every thousand feet above sea level!” The time it takes to bring water to a boil, and the temperature of the water when it boils, regardless of the altitude, is sufficient to kill Giardia, Cryptosporidium and any other water borne pathogens. Continuing to heat the water after it boils wastes fuel, evaporates the water and delays consumption. Keep in mind that being able to boil water to disinfect it is dependent on having fuel available to burn; a metal container to heat the water in, and the time needed to boil the water.

Chemicals.Chemicals that have the ability to effectively disinfect water are known as halogens and include iodine and chlorine. The effectiveness of halogens is directly related to its concentration, the amount of time the halogen is left in contact with the water and the temperature of the water – the colder the water the longer the contact time!.

· Iodine in tablet form and liquid is available. Choose the tablets. Liquid iodine is messy, the containers are prone to leak, and you are never quite sure whether or not you have used too much or too little liquid. Potable Aqua tablets, ( are widely available, and still used as one of the primary ways of making water safe to drink. Iodine kills harmful bacteria, viruses and most protozoan cysts often found in untreated water. Iodine is NOT an effective halogen when Cryptosporidium is present in the water. Fifty gray tablets are contained in a small, dark bottle. The recommend dosage of two tablets per quart or liter of water is sufficient to kill organisms such as Giardia. Two tablets should definitely be used if the quality of water is suspect, i.e. you are using water from river sources along which people live. Once the tablets are placed in the water it should be allowed to sit for at least thirty minutes, longer if the water is very cold or very dirty, and then shaken to ensure that the iodine and the water is thoroughly mixed. The dissolved tablets do leave a slight iodine taste in the water which some find disagreeable, in which case ascorbic acid (lemon juice or lemonade powder) can be added to neutralize the iodine flavor. Iodine tablets are commonly packaged with a second, similar sized bottle of ascorbic acid (PA Plus) tablets, that deactivate the iodine making the water pleasant to drink. One tablet is usually enough to reduce the iodine taste to tolerable levels. Do not add PA Plus or other sources of ascorbic acid to your water until after thirty minutes contact time has elapsed.
   Iodine tablets deteriorate on exposure to heat, humidity, moisture, reducing the effectiveness of the tablets. Over time, opening and closing the cap to remove tablets results in the normally gray colored tablets changing to a green or yellow color – they should not be used! Military iodine tablets, sometimes found in military surplus stores, should also not be used – the military got rid of them because their shelf life (four years) had expired! Always carry the tablets in the original container. Decanting a few into other inappropriate containers results in a rapid deterioration of the tablets upon exposure to light and humidity. People who are allergic to iodine should use a chlorine based disinfectant or a filter. People with known thyroid problems should not use iodine to disinfect their water. Pregnant women should check with their doctor before using iodine to disinfect their water. Potable Aqua or other sources of iodine should not be used as a long term (more than six weeks) method of disinfecting water

· Chlorine. Several products are on the market that release chlorine when placed in water. Chlorine is an effective agent against bacteria, viruses and, unlike iodine, chlorine is also an effective agent against cysts such as Cryptosporidium. Another advantage of using chlorine is that it leaves little or no aftertaste. Potable Aqua Chlorine Dioxide water purification tablets ( or Micropur MP-1 tablets made by Katadyn (www.katadyn) are two examples of readily available disinfecting tablets 

Filtration.A lot of confusion exists regarding the usefulness of filters to effectively disinfect water. Some filters remove only the “big stuff” such as Giardia and Cryptosporidium while others also remove viruses. Some devices are pumps; some are bottles that require you to suck the water through a filter matrix, while others rely on gravity. Prices vary tremendously depending on the type of device you buy.

Generally filters that remove giardia and cryptosporidium are sufficient for ninety percent of your needs in North America. Where viruses are a known or suspected medical threat to your safety a filter with a finer pore size or a pump that incorporates an iodine resin to chemically kill the viruses must be used. Devices that remove only the larger organisms are usually referred to as “filters” while those that remove both the larger organisms and viruses are commonly known as “purifiers,” but don’t rely on these terms to guide your purchase – read the fine print!

There are many bottle filters on the market and as is often the case, you get what you pay for. Inexpensive filters commonly sold at sports shows and Saturday morning flea markets will not stand up to the rigors of back country activities. Purchasing a filter from a specialty backpacking or emergency preparedness store will cost you a bit more but the filters are reliable, tough, and then when you need to replace the filter cartridge, they are available. I particularly like the Sawyer ( or Katadyn ( when I need a bottle filter or purifier. While there are many to choose from my personal favorite is a Pre-Mac “Trekker” pump sold exclusively by Emergency Response International ( These devices are small, light-weight, straightforward to use, and have easily replaceable components.

A walk through any of the better sporting goods retail stores will quickly reveal that there are many products for sale that can be used to treat water. There are also other techniques used to treat water discussed in outdoor safety and survival literature. My experience is limited to equipment and techniques described in this article – equipment and techniques that have served me well for over fifty years in the outdoors in many parts of the world. 

[Editor’s note: The author does not represent any of the companies identified in the article.]