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Guide to Water Purification

OA Guide to Water Purification

bpfmcover2.jpgThe Backpacker's Field Manual

by Rick Curtis

published by Random House 2005


Available at

This material is taken from Chapter 6 - Wilderness Travel from The Backpackers Field Manual by Rick Curtis. For more details on this exciting book check out The Backpacker's Field Manual Page.

This material is provided by the author for educational use only and is not a substitute for specific training or experience. Princeton University and the author assume no liability for any individual's use of or reliance upon any material contained or referenced herein. When going into outdoors it is your responsibility to have the proper knowledge, experience, and equipment to travel safely. This material may not be reproduced in any form for commercial or Internet publication without express written permission of the author. Copyright © 1999, all rights reserved, Random House Publishing & Rick Curtis, Outdoor Action Program, Princeton University.

Dipping your head into a cold mountain stream and taking a long refreshing drink is an experience that has basically vanished from most of the wilderness areas around the world. The last truly fresh drink of water I had was in Fiordland National Park in New Zealand at a waterfall coming down from a pristine mountain ridge, no Giardia here. With the increased use of the wilderness there has also been an increase in the amount of bacteriological contamination of backcountry water supplies. The U.S. Environmental Protection Agency reports that 90 percent of the world’s water is contaminated in some way. There are a variety of microscopic organisms that can contaminate water supplies and cause potentially serious, even fatal, illnesses among wilderness travelers. The major danger in the backcountry from these infections is fluid loss due to diarrhea and vomiting, which can lead to hypovolemic shock and possibly death (see Diarrhea or Vomiting, page 000; Fluid Electrolyte Replacement, page 000; Shock, page ­ 000). Does this mean that every single stream is contaminated? Of course not, the problem is that it’s very hard to know what is and what isn’t and something that was safe last year might be contaminated now.

To be safe then, unless you are really sure about a particular water source, you should be prepared to treat it. Gastrointestinal (GI) infections like Giardia can be serious and if left untreated can cause long term GI problems (see Gastrointestinal Infections, page 000). Remember, however, that infections can also be spread through poor personal hygiene, something that purifying your water won’t ­prevent. For information on daily fluid requirements see Chapter 3 “Cooking and Nutrition.” For information on emergency water sources and survival see Chapter 8 “Safety and Emergency Procedures.”

There are a number of different approaches to treating water. I’ll go into detail on each method so you can decide what’s best in your particular circumstance.

•  Boiling

•  Chemical Treatment

•  Ultraviolet Radiation

•  Water Filters

•  Water Purifiers

Biologically Contaminated vs. Toxic ­Water

Biologically contaminated water is water that contains microorganisms such as Giardia (a common microorganism that, if not killed, leads to intestinal disorders), bacteria, or viruses that can lead to infections (see Gastrointestinal Infections, page 000). Toxic water sources contain chemical contamination from pesticide runoffs, mine tailings, and so on. Boiling, filtering, or chemically treating water can remove or kill microorganisms, but it will not remove chemical toxins. This is also the case when using a solar still (see page ­ 0000).


Boiling is the most certain way of killing all microorganisms. It won’t remove chemical contaminants but it kills all the little beasties. It was once thought that you had to boil for five minutes. According to the Wilderness Medical Society, water temperatures above 160°F (70°C) kills all pathogens within 30 minutes and water above 185°F (85°C) within a few minutes. So in the time it takes for the water to reach the boiling point (212°F or 100°C) from 160°F (70°C), all pathogens will be killed, even at high altitude. To be extra safe, let the water boil rapidly, covered for one minute, especially at higher altitudes since water boils at a lower temperature (see page 000.) If boiling is your primary method of water purification, make sure you take that into account for the amount of fuel you plan to bring.

Chemical ­Purification

There are two main types of chemical treatment: those using iodine and those using chlorine. There are a variety of products on the market, so follow the directions on the bottle. Be advised that some of these products will have an expiration date and become ineffective after that point. Also, with some tablet products, once the bottle has been opened, the tablets must be used within a certain period. If you have an open bottle and you can’t remember when you last used it, buy a new bottle.  Note: Some chemical treatments are not effective against all microorganisms. According to the Centers for Disease Control chemical treatment using either iodine or chlorine-based products have not been shown to be effective against Cryptosporidium (see page 000).

General Procedures for Chemical Treatment ­

•   The effectiveness of all chemical treatment of water is related to the temperature, pH level, and clarity of the water. Cloudy water often requires higher concentrations of chemical to ­disinfect.

•   If the water is cloudy or filled with large particles, strain it, using a cloth, before treatment. Large particles may be purified only ‘on the ­outside.’

•   Add the chemical to the water and swish it around to aid in dissolving. Splash some of the water with the chemical onto the lid and the threads of the water bottle so that all water areas are ­treated.

•   The water should sit for at least 30 minutes after adding the chemical to allow purification to occur. If using tablets, let the water sit for 30 minutes after the tablet has ­ dissolved. It may take up to 4 hours to treat water contaminated with Cryptosporidium.

•   The colder the water, the less effective the chemical is as a purifying agent. Research has shown that at 50°F (10°C), only 90 percent of ­ Giardia cysts were inactivated after 30 minutes of exposure. If the water temperature is below 40°F (4°C), double the treatment time before drinking. It is best if water is at least 60°F (16°C) before treating. You can place the water in the sun to warm it before ­ treating.

Iodine ­Treatment

Be aware that some people are allergic to iodine and cannot use it as a form of water purification. Persons with thyroid problems, people who are taking lithium, women over fifty, or pregnant women should consult their physician prior to using iodine for purification. Also, some people who are allergic to shellfish are also allergic to iodine. If someone cannot use iodine, use another of the methods of purification. Iodine is light sensitive and must always be stored in a dark bottle. It works best if the water is over 68°F (21°C). The water can be warmed in the sun before treating or hot water can be added. Iodine has been shown to be more effect than chlorine­ -­ based treatments in inactivating Giardia cysts. Note however: iodine-based products are not effective against Cryptospirodium.

Generally, the procedure is for using iodine is as ­ follows:

•   Liquid 2% Tincture of Iodine Add 5 drops per quart/liter when the water is clear. Add 10 drops per quart/liter when the water is ­ cloudy. Wait time is 30 minutes for water over 68°F (21°C).

•   Polar Pure Iodine Crystals Fill the Polar Pure bottle with water and shake. The solution will be ready for use in one hour. Add the number of capfuls (per quart of water treated) listed on the bottle, based on the temperature of the iodine solution. If the water is cloudy, double the number of capfuls. The particle trap prevents crystals from getting into the water being treated. It is important to note that you are using the iodine solution to treat the water, not the iodine crystals. The concentration of iodine in a crystal is poisonous and can burn tissue or eyes so be sure to prevent pouring crystals into the water. Let the treated water stand for 30 minutes before drinking. In order to destroy Giardia cysts, the drinking water must be at least 68°F (20°C). Refill the treatment bottle after use so that the solution will be ready one hour later. Crystals in the bottle make enough solution to treat about 2,000 quarts/liters. Discard the bottle when empty.

•   Potable Aqua This is an iodine tablet product. Follow the manufacturer’s ­ instructions for use. Use two tablets per quart and wait 30 minutes. Potable Aqua Plus includes a second agent that removes the iodine taste. After waiting 30 minutes of treatment, add 2 tablets of PA Plus per quart, stir well, and wait 3 minutes before drinking. Each bottle contains 50 tablets, enough for 25 quarts.

Chlorine ­Treatment

Chlorine can be used for persons with iodine allergies or restrictions. Remember that water temperature, sediment level, and contact time are all elements in killing microorganisms in the water. Note: most chlorine-based products are not effective against Cryptospirodium.

•   McNett Aqua Mira (AquaMira) is a chlorine dioxide based chemical water treatment kit. It is an easy-to-use, two-part system. Place 7 drops Aquamira (Part A) and 7 drops Activator (Part B) in the mixing cap. Wait for 5 minutes until the solution turns yellow and pour the solution into the water to be treated.  Wait for about 15 minutes. If Cryptosporidium is suspected, double the dose of Part A and Part B and wait for at least 30 minutes. Turbid water with high organic load should be pre-filtered with a charcoal filter before using Aqua Mira. After treatment there is no aftertaste. The kit treats up to 30 gallons of water. Aqua Mira has a shelf life of four years. If stored properly after opening, the product will remain effective until the expiration date.

•   Katadyn MP1 Purification Tablets are chlorine dioxide. They are effective against Cryptosporidium, Giardia, bacteria, and viruses. One tablet treats 1 quart (1 liter) of water. Bacteria and viruses are killed in approximately 15 minutes with water at 68 degrees Fahrenheit (20 degrees Celsius). Cryptospodium purification takes about four hours. The tablets do not leave an unpleasant aftertaste.

Mixed-Oxidant Solutions

Another approach to water purification is using mixed-oxidants. This technique, originally developed for municipal water systems, has been miniaturized for backpackers. The MSR Miox pen uses basic salt (Sodium Chloride) in fresh water to create a salt solution. By applying electricity to the solution it is broken up into an oxidant solution of chlorine and hypochlorite. The oxidant solution is then poured into your water to destroy any microorganisms. It takes about 15 minutes for the solution to kill bacteria and viruses. The solution will kill Cryptosporidium (see page 000) but it takes 4 hours, which shows you how difficult this little bug is to kill.

An important feature of using simple salt as the disinfectant source is that salt has an infinite shelf life, so the purifier will still function even if has been stored for a long period of time. The purifier will treat approximately 200 liters of water on one set of batteries. At standard doses the disinfectant solution does not leave an aftertaste, unlike chlorine or iodine tablets. The Miox is about 6 inches (15 centimeters) longand only weighs 3.5 ounces (99 grams). It uses two CR123 Lithium batteries. Like any battery-powered system, if the batteries fail it won’t work.

The Miox can be set to create an oxidant solution for from ½ quart (500 millliters) to 1 gallon (4 liters). Once the oxidant solution has been generated by the Miox unit it is poured into the water to be purified. Shake or stir the water so that the oxidant is thoroughly mixed then test it with one of the included test strips. If the strip gives a positive reading, wait 10 minutes and check again with a test strip. If the second test if positive, splash treated water in the threads of the water container, and wait 20 minutes before drinking. If the first test is negative, create another batch of oxidant, retreat and retest. Note: to treat water with Cryptosporidium you need to wait for 4 hours after the test strip shows positive before drinking.

Tricks of the ­ Trail

Water Treatment Backups What happens if you are planning to boil all your water and your stove breaks? Any of the methods for water purification can fail so always have at least one backup method. This can be any combination of methods (boiling, filter, chemical, etc,). I’m the cautious type, so I usually have two backup methods: water filter and 2% tincture of iodine or Polar Pure iodine crystals, and I can always boil the water. If boiling is your backup method, make sure you have enough ­fuel.

Fix the Taste Chemically treated water can be made to taste better after treatment by aerating it (pouring it back and forth between containers). Adding vitamin C (about 50 milligrams) to iodized water completely eliminates any taste or color of iodine. The vitamin C in drink mixes like Tang has the same ­ effect. Other methods include adding a pinch of salt per quart. You must wait until after the chemical has completely purified the water before adding flavorings.

Ultraviolet Light

Ultraviolet (UV) light has been used for years as a form of sterilization in hospitals and other settings. UV radiation at certain frequencies kills microorganisms so carefully applied frequencies of UV light can be used to purify water. Such techniques do no remove chemical contaminants in the water.

•   Hydro-Photon Steri-pen is a battery operated UV water purifier. Using a calculated dose of UV light it takes less than a minute to purify 16 oz. (0.4 liters) of water. There are a few caveats to using the Steri-Pen. It should not be used in discolored water or water containing particulate matter since the UV light can’t kill what it can’t penetrate and it is only designed to generate enough UV to purify 16 oz. (0.4  liters) at one time. The Steri-Pen kills all microorganisms including protozoa, bacteria and viruses. When immersed in water the UV lamp is basically safe, but UV light can be harmful to eyes and skin so it should never be turned on except when it is immersed in water. The other issue with any electric device is battery or other failure (


There are a number of devices on the market that filter out microorganisms. A water filter pumps water through a microscopic filter that is rated for a certain-size organism. The standard size rating is the micron (the period at the end of this sentence is about 600 microns). Depending on the micron rating of the filter, larger organisms like Giardia are filtered out while smaller organisms (like viruses) can pass through.  

Not all filters are created equal. Filters are rated in three size categories, filter, microfilter and purifier based on the size of microorganisms they will catch. Be cautious when selecting a filter. If the device is listed as a ‘water purifier’ then it removes all microorganisms either through filtration or through a combination of filtration and chemical treatment. You should know what potential organisms you need to treat for. You don’t want to go to an area where a virus like hepatitis A is present in the water (a problem in some developing countries) that would require a purifier with a filter that will handle only large organisms like Giardia. Below are common microorganisms and the filter size ­needed:

Organism          Examples                                                      General Size                  Filter Type              Size ­ Rating

Protozoa           Giardia, Cryptosporidium                             5 microns or larger        Water filter           1.0–4.0 ­ microns

Bacteria            Cholera, E. coli, Salmonella                         0.2–0.5 microns             Microfilter             0.2–1.0 ­ microns

Viruses             Hepatitis A, rotavirus, Norwalk virus          0.004 microns                Water purifier       to 0.004 ­ microns

Common Practices for Using a Water ­ Filter

•  Filter the cleanest water you can find. Dirty water or water with large suspended particles will clog your filter more ­ quickly.

•  Prefilter the water either through a prefilter on the pump or strain it through a ­ bandanna.

•  If you must filter very cloudy water, let it stand for several hours for particles to settle ­out.

There are two basic types of filters.

•   Membrane Filters use thin sheets with precisely sized pores that prevent objects larger than the pore size from passing through. Pro: Relatively easy to clean. Con: Clog more quickly than depth filters. Example: Katadyn Hiker.

•   Depth Filters use thick porous materials such as carbon or ceramic to trap particles as water flows through the material. Pro: Can be partially cleaned by backwashing. Activated carbon filters also remove a range of organic chemicals and heavy metals. Con: Rough treatment can crack the filter, rendering it useless. Examples: MSR WaterWorks II, Katadyn Pocket, Aqua Mira Water Bottle Filter.

•   Combination Methods use both a membrane filter and a chemical treatment. Pro: The filter typically has larger pore sizes and therefore only removes larger organisms (like the hard to kill Cryptosporidium) then the chemical treatment, either iodine or cholorine-based, kills smaller organisms including viruses. This is a good alternative to waiting for hours to get rid of Crypto. Examples: The Katadyn Exstream is a combination filter and iodine cartridge. The Aqua Mira Water Bottle Filter and Aqua Mira chlorine treatment uses an activated charcoal filter and then you would treat the water using the Part A and Part B chemicals described above.

How do you Know if Your Filter Will Work?

There are no current required certifications for water filters and purifiers. The Centers for Disease Control provide basic scientific information about the different microorganisms and what is takes to remove/kill them ( NSF International is an independent, not-for-profit company that tests and certifies some water filters and purifiers. Since this is a voluntary certification process which costs the manufacturer significant money, not all manufacturers submit their product for certification testing so lack of certification does not mean that the product is not effective. You can search to see if a particular product is NSF certified on their Web site (


What Type of Filter?

The type of filter depends a lot on the amount of water you need to purify. Solo hikers and small groups can use any of the options described. Larger groups need to look more closely at factors like the amount of chemicals needed or the pumping capacity of the filter as compared to the cost. The other thing is to look at specific features. I carry different purification solutions depending on where I am going and what I am doing.

For example, when I travel in the developing world I want a full purification system to remove all microorganisms including viruses. When I’m kayaking I use a Katadyn Exstream water bottle filter. I can scoop water out of the river and drink it straight from the bottle. Since this filter uses iodine resin it can’t be used by people with iodine allergies.

Tricks of the ­ Trail

Filter & Purifier Contamination If the filter takes a serious fall, it could crack internally. If the filter inside cracks, unfiltered water can flow through the crack. Some water filters come as sealed cartridges, making it impossible to inspect the actual filter cartridge for cracks. Treat your filter with care, and if it takes a significant impact, replace it.

The intake hose from a water filter/purifier has been submerged in unfiltered water. Treat this hose as “contaminated” and keep it in a separate plastic bag.

Any filter or purifier serves a collection point for nasty microorganisms, so when cleaning or changing your filter/purifier you need to be careful, especially individuals with impaired immune systems. Handle your filter or purifier cartridge with gloves, dispose of the used cartridge and gloves carefully, and wash your hands scrupulously afterwards. This is one advantage of sealed filters and may be an issue for people with weakened immune systems.