Water purification

City Prepping – 55 gallon barrels

How to store water for emergencies: Waterfull Barrel review

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CDC PDF

EPA PDF

Water Treatment Guide

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From the CDC

  • Potential health effects from ingestion of water contaminated with Cryptosporidium are:
    • Gastrointestinal illness (for example, diarrhea, vomiting, cramps).
  • Sources of Cryptosporidium in drinking water are:
    • Human and animal fecal waste.
  • Methods that may remove some or all of Cryptosporidium from drinking water are:
    • Boiling (Rolling boil for 1 minute) has a very high effectiveness in killing Cryptosporidium;
    • Filtration has a high effectiveness in removing Cryptosporidium when using an absolute less than or equal to 1 micron filter (NSF Standard 53 or 58 rated “cyst reduction / removal” filter);
    • Disinfection with iodine or chlorine is not effective in killing Cryptosporidium;
    • Disinfection with chlorine dioxide has a low to moderate effectiveness in killing Cryptosporidium;
    • Combination filtration and disinfection has a very high effectiveness in removing and killing Cryptosporidium when used with chlorine dioxide and an absolute less than or equal to 1 micron filter (NSF Standard 53 or 58 rated “cyst reduction / removal” filter).
  • Potential health effects from ingestion of water contaminated with Giardia are:
    • Gastrointestinal illness (for example, diarrhea, vomiting, cramps).
  • Sources of Giardia in drinking water are:
    • Human and animal fecal waste.
  • Methods that may remove some or all of Giardia from drinking water are:
    • Boiling (Rolling boil for 1 minute) has a very high effectiveness in killing Giardia;
    • Filtration has a high effectiveness in removing Giardia when using an absolute less than or equal to 1 micron filter (NSF Standard 53 or 58 rated “cyst reduction / removal” filter);
    • Disinfection with iodine or chlorine has a low to moderate effectiveness in killing Giardia;
    • Disinfection with chlorine dioxide has a high effectiveness in killing Giardia;
    • Combination filtration and disinfection has a very high effectiveness in removing and killing Giardia when used with chlorine dioxide and an absolute less than or equal to 1 micron filter (NSF Standard 53 or 58 rated “cyst reduction / removal” filter).
  • Potential health effects from ingestion of water contaminated with bacteria are:
    • Gastrointestinal illness (for example, diarrhea, vomiting, cramps).
  • Sources of bacteria in drinking water are:
    • Human and animal fecal waste.
  • Methods that may remove some or all of bacteria from drinking water are:
    • Boiling (Rolling boil for 1 minute) has a very high effectiveness in killing bacteria;
    • Filtration has a moderate effectiveness in removing bacteria when using an absolute less than or equal to 0.3 micron filter;
    • Disinfection with iodine or chlorine has a high effectiveness in killing bacteria;
    • Disinfection with chlorine dioxide has a high effectiveness in killing bacteria;
    • Combination filtration and disinfection has a very high effectiveness in removing and killing bacteria when used with iodine, chlorine, or chlorine dioxide and an absolute less than or equal to 0.3 micron filter (NSF Standard 53 or 58 rated “cyst reduction / removal” filter)
  • Potential health effects from ingestion of water contaminated with viruses are:
    • Gastrointestinal illness (for example, diarrhea, vomiting, cramps), hepatitis, meningitis.
  • Sources of viruses in drinking water are:
    • Human and animal fecal waste.
  • Methods that may remove some or all of viruses from drinking water are:
    • Boiling (Rolling boil for 1 minute minimum) has a very high effectiveness in killing viruses;
    • Filtration is not effective in removing viruses;
    • Disinfection with iodine or chlorine has a high effectiveness in killing viruses;
    • Disinfection with chlorine dioxide has a high effectiveness in killing viruses;
    • Disinfection has a high effectiveness in killing viruses when used with iodine, chlorine, or chlorine dioxide.

Please remember that:

  • Boiling can be used as a pathogen reduction method that should kill all pathogens. Water should be brought to a rolling boil for 1 minute. At altitudes greater than 6,562 feet (greater than 2000 meters), you should boil water for 3 minutes.
  • Filtration can be used as a pathogen reduction method against most microorganisms, depending on the pore size of the filter, amount of the contaminant, particle size of the contaminant, and charge of the contaminant particle. Manufacturer’s instructions must be followed. More information on selecting an appropriate water filter can be found at http://www.cdc.gov/parasites/crypto/gen_info/filters.html. Only filters that contain a chemical disinfectant matrix will be effective against some viruses.
  • 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 effectiveness of pathogen reduction depends on the product. Depending on these factors, 100% effectiveness may not be achieved. Manufacturer’s instructions must be followed.
  • If boiling water is not possible, a combination of filtration and chemical disinfection is the most effective pathogen reduction method in drinking water for backcountry or travel use. Manufacturer’s instructions must be followed.

Other treatment methods can be effective against some of the above pathogens:

  • Ultraviolet Light (UV Light) can be used as a pathogen reduction method against some microorganisms. The technology requires effective prefiltering due to its dependence on low water turbidity (cloudiness), the correct power delivery, and correct contact times to achieve maximum pathogen reduction. UV might be an effective method in pathogen reduction in backcountry water; there is a lack of independent testing data available on specific systems. Manufacturer’s instructions must be followed.
  • MIOX® systems use a salt solution to create mixed oxidants, primarily chlorine. Chlorine has a low to moderate effectiveness in killing Giardia, and a high effectiveness in killing bacteria and viruses. Manufacturer’s instructions must be followed.

Important: Water that has been disinfected with iodine is NOT recommended for pregnant women, people with thyroid problems, those with known hypersensitivity to iodine, or continuous use for more than a few weeks at a time.

Sanitation

In addition to using the appropriate drinking water treatment methods listed above, you can also protect yourself and others from waterborne illness in the backcountry or while traveling by paying attention to good sanitation practices:

  • Burying human waste 8 inches deep and at least 200 feet away from natural waters.
  • Practicing good personal hygiene. Wash hands before handling food, eating, and after using the toilet.

From The EPA

ONLY USE WATER THAT HAS BEEN PROPERLY DISINFECTED FOR DRINKING, COOKING, MAKING ANY PREPARED DRINK, WASHING DISHES AND FOR BRUSHING TEETH.

  • Use bottled water or water you have properly prepared and stored as an emergency water supply.
  • Boil water, if you do not have bottled water. Boiling is sufficient to kill pathogenic bacteria, viruses and protozoa (WHO, 2015).
    • If water is cloudy, let it settle and filter it through a clean cloth, paper boiling water towel, or coffee filter.
    • Bring water to a rolling boil for at least one minute. At altitudes above 5,000 feet (1,000 meters), boil water for three minutes.
      • Let water cool naturally and store it in clean containers with covers. 
      • To improve the flat taste of boiled water, add one pinch of salt to each quart or liter of water, or pour the water from one clean container to another several times.
  • Disinfect water using household bleach, if you can’t boil water. Only use regular, unscented chlorine bleach products that are suitable for disinfection and sanitation as indicated on the label. The label may say that the active ingredient contains 6 or 8.25% of sodium hypochlorite. Do not use scented, color safe, or bleaches with added cleaners.If water is cloudy, let it settle and filter it through a clean cloth, paper towel, or coffee filter.
    • Locate a clean dropper from your medicine cabinet or emergency supply kit.
    • Locate a fresh liquid chlorine bleach or liquid chlorine bleach that is stored at room temperatures for less than one year.
    • Use the table below as a guide to decide the amount of bleach you should add to the water, for example, 8 drops of 6% bleach, or 6 drops of 8.25% bleach, to each gallon of water. Double the amount of bleach if the water is cloudy, colored, or very cold.
    • Stir and let stand for 30 minutes. The water should have a slight chlorine odor. If it doesn’t, repeat the dosage and let stand for another 15 minutes before use.
    • If the chlorine taste is too strong, pour the water from one clean container to another and let it stand for a few hours before use.
Volume of Water Amount of 6% Bleach to Add* Amount of 8.25% Bleach to Add*
1 quart/liter 2 drops 2 drops
1 gallon 8 drops 6 drops
2 gallons 16 drops (1/4 tsp) 12 drops (1/8 teaspoon)
4 gallons 1/3 teaspoon 1/4 teaspoon
8 gallons 2/3 teaspoon 1/2 teaspoon

*Bleach may contain 6 or 8.25% sodium hypochlorite.

If you don’t have liquid bleach, you can use one of the other disinfection methods described below.

  • Granular calcium hypochlorite. The first step is to make a chlorine solution that you will use to disinfect your water. For your safety, do it in a ventilated area and wear eye protection. Add one heaping teaspoon (approximately ¼ ounce) of high-test granular calcium hypochlorite (HTH) to two gallons of water and stir until the particles have dissolved. The mixture will produce a chlorine solution of approximately 500 milligrams per liter. 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. If the chlorine taste is too strong, pour the water from one clean container to another and let it stand for a few hours before use. CAUTION:HTH is a very powerful oxidant. Follow the instructions on the label for safe handling and storage of this chemical.
  • Common household iodine (or “tincture of iodine”). You may have iodine in your medicine cabinet or first aid kit. Add five drops of 2% tincture of iodine to each quart or liter of water that you are disinfecting. If the water is cloudy or colored, add 10 drops of iodine. Stir and let the water stand for at least 30 minutes before use.
  • Water disinfection tablets. You can disinfect water with tablets that contain chlorine, iodine, chlorine dioxide, or other disinfecting agents. These tablets are available online or at pharmacies and sporting goods stores. Follow the instructions on the product label as each product may have a different strength.

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From FEMA

Determining Water Needs

Store at least one gallon of water per person for three days, for drinking and sanitation. A normally active person needs about three quarters of a gallon of fluid daily, from water and other beverages. However, individual needs vary, depending on age, health, physical condition, activity, diet and climate.

Take the following into account:

  • Children, nursing mothers and sick people may need more water.
  • A medical emergency might require additional water.
  • If you live in a warm weather climate more water may be necessary. In very hot temperatures, water needs can double.

Water Tips

  • Never ration drinking water unless ordered to do so by authorities. Drink the amount you need today and try to find more for tomorrow. Minimize the amount of water your body needs by reducing activity and staying cool.
  • Drink water that you know is not contaminated first. If necessary, suspicious water, such as cloudy water from regular faucets or water from streams or ponds, can be used after it has been treated. If water treatment is not possible, put off drinking suspicious water as long as possible, but do not become dehydrated.
  • Do not drink carbonated or caffeinated beverages instead of drinking water. Caffeinated drinks and alcohol dehydrate the body, which increases the need for drinking water.

Water Storage

Buy commercially bottled water and store it in the sealed original container in cool, dark place.

If you must prepare your own containers of water, purchase food grade water storage containers. Before filling with chlorinated water, thoroughly clean the containers with dishwashing soap and sanitize the bottles by cleaning with a solution of 1 teaspoon of non-scented liquid household chlorine bleach to a quart of water. Water that has not been commercially bottled should be replaced every six months.

Water Treatment

If you have used all of your stored water and there are no other reliable clean water sources, it may become necessary to treat suspicious water. Treat all water of uncertain quality before using it for drinking, food washing or preparation, washing dishes, brushing teeth or making ice. In addition to having a bad odor and taste, contaminated water can contain microorganisms (germs) that cause diseases such as dysentery, cholera, typhoid and hepatitis.

There are many ways to treat water. 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.

Boiling

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.

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.

Chlorination

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.

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 another 15 minutes. If it still does not smell of chlorine, discard it and find another source of water.

Other chemicals, such as iodine or water treatment products sold in camping or surplus stores that do not contain 5.25 or 6.0 percent sodium hypochlorite as the only active ingredient, are not recommended and should not be used.

Distillation

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 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.

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From Princeton University

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.

Boiling

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.

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 (www.hydro-photon.com).

Filtration

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 Giardiaare 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:

                                                                                                                                                                     Particle
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 (www.cdc.gov/travel/food-drink-risks.htm). 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 (www.nsf.org).

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.

Water Purification 1943 – The War Department