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Water Filtering and Purification
by Sean Keener
Wilderness EMT


A.  Giardia Lamblia & Cryptosporidium You can call them giardia and crypto. These two protozoa are the guys you hear about all the time. When found in water, both are oocysts, a dormant phase of some microbes where they take on a hard outer shell. Once ingested, they come to life and attach themselves to your intestine, causing diarrhea and other gastrointestinal problems.

Giardia is spread through contaminated feces. An interesting thing about this little guy is that it can be vectored from other animals. So, if you're drinking water from a river where an infected beaver lives, the river is thus contaminated. Symptoms of giardia infection (giardiasis) include loss of appetite, lethargy, fever, explosive diarrhea, loose or watery stool, stomach cramps, upset stomach, bloating, and flatulence. With the flatulence comes a foul and sulfurous-tasting burp, that I'm told is so nasty that it causes additional nausea and vomiting. Symptoms typically begin 1–2 weeks after infection and may wane and reappear cyclically. If gone untreated, symptoms may last 6 weeks. Common treatment is Flagyl, re-hydration, and an occasional Imodium tablet. The good news is only about 40% of people infected actually exhibit symptoms. Recent archaeological data has shown that giardia was a leading cause of death among Crusaders at the Siege of Acre during the Third Crusade.

Cryptosporidiosis starts and ends faster. Symptoms begin 2-10 days after ingestion and last for about two weeks. Symptoms include watery diarrhea, cramps, and a low fever. The only available treatment at this time is supportive care with re-hydration and an occasional imodium tablet. I know that some people are asymptomatic when they contract cryptosporidiosis, but I don't have any hard numbers on this.

Both of these little buggers exist in water sources EVERYWHERE. As I said, both are spread through contaminated feces, so wash your hands. One of the recent crypto outbreaks is believed to have started at a daycare center when an employee changed a diaper and didn't cleanup properly.

B.  Bacteria

Common harmful bacteria include Salmonella, Camplylobacter jejuni, and E. coli. In some parts of the world, water may contain bacteria that can cause cholera, dysentery, and typhoid. Some harmful bacteria is small enough that it will pass through a 0.2 micrometre filter. This is mostly a concern in undeveloped countries.

C.  Viruses (virii)

The most common viruses that live in water that I can think of are hepatitis A, norovirus, and rotavirus, though there are many others. Most virii that humans can get from contaminated water will be vectored from other humans, so out-of-the way lakes and streams are statistically better places to find virus-free water.

D.  Chemical contaminants

Pesticides, fertilizers and other agricultural chemicals can contaminate water sources, and God knows what other stuff might be intentionally put in there. Chemicals can be difficult to remove, so if you suspect contamination, you should find a different water source.

An easy (but not all-encompassing) test is to check for life in the water source. If there are fish, frogs, and whatnot living in the water, chances are good it is free from chemical (and toxic metal) contamination.

E.  Toxic metals

Iron, manganese, aluminum, beryllium, chromium, lead, etc. These aren't so much an immediate health concern as a long term one. Generally the water will look and smell funny if there is a seriously toxic dose of metal in it. For example, when Quantico Creek was contaminated with sulfur and iron in the mid 90s, you could smell a hint of rotten eggs from a mile away. Like chemicals, if you suspect toxic metal contamination in your water source your best bet is to find a different one.

F.  Algae, fungi, particulate matter and other "nasties"

These can impart an odd or foul taste to your water or cause other health issues. Another problem with particulate matter is that it impedes chemical and UV purification.


Methods of filtering, purifying, disinfecting or decontaminating your drinking water:

A.  Heat

Heat kills microorganisms and virtually all enteropathogens are readily killed at temperatures well below the boiling point. The process of heating water to a boil makes it hot enough long enough to disinfect it, even at higher elevations. There is no need to boil water for 5, 10, 20 minutes as some say. Bringing water to a good rolling boil is adequate for disinfection.

Heat only kills living things, thus chemicals and all that stuff will still be there.

B.  Iodine

Iodine has been used for almost a century to disinfect drinking water and many of the expensive store bought disinfection products are just iodine in tablet form. Iodine will kill or inactivate virii, bacteria, and most protozoa. It is considered "OK" at killing giardia and somewhat ineffective at killing crypto, due to their hard shells. Iodine comes in many different forms and disinfection is temperature dependent, so I'll give you some guidelines.

Preparation/Iodine Concentration/Amount per liter:

Iodine Topical Solution/ 2% / 8 drops
Iodine Tincture / 2% / 8 drops
Lugol's Solution / 5% / 4 drops
Povidone-Iodine (Betadine(R)) / 10% / 4 drops
Tetraglycine hydroperiodide (Globaline, Portable Aqua, EDWGT) / 8mg / 1 tablet

     If your water is clear, use these disinfection times:

60F/15C or higher: 15 minutes
40F/5C: 30 minutes

     If the water is cloudy, double the time.

**If you're using a solution that isn't listed above, the concentration you're looking for is 8mg of iodine per liter of water.**

In general, if you are in a hurry double the chemical dose and halve the contact time; if you want better flavor halve the dose and double the contact time.

One-half of an iodine prep pad from a first aid kit has enough iodine to treat 1 liter of water. Just tear the pad in half, drop it in your container, and fish the pad out after waiting for the contact time above. A whole pad creates an iodine concentration suitable for a wound wash, especially good for kids because it does not sting much. This concentration is safe to drink if required, but it will not taste very good. While iodine may not kill all giardia or crypto, they are only harmful if introduced into the G.I. tract, not the bloodstream.

If you pass your water through a 0.4 µm filter or smaller, you can use an iodine concentration of 1mg per liter of water to kill any remaining virii or small bacteria. This equals 1 drop for most liquid formulations.

Povidone is a binding solution that allows higher concentrations of iodine in water. Although the overall concentration of Betadine is 10% per bottle, the povidone content makes it difficult to gauge the concentration per drop. That's why you add the same number of drops for a bottle of 10% Povidone-Iodine as you do for a 5% bottle of Lugol's.

IMPORTANT NOTE: Iodine should not be used by persons with an allergy to iodine, persons with active thyroid disease, or pregnant women.

C.  Household Bleach (Sodium Hypochlorite)

Chlorine bleach contains about 5 or 6 percent sodium hypochlorate. It is effective in killing most microbes, EXCEPT crypto.

The calculated solution is much easier than iodine: 3 drops per liter at normal temperature, clear water, 30 minutes contact time and 5 drops per liter if cloudy or cold, 60 minutes contact time. There should be a slight hint of a chlorine smell. If there isn't, treat again.

The use of chlorine bleach to disinfect water is very dependent on the pH of the water. Water that has not been filtered may have organic particles that will absorb chlorine, either nullifying its ability to disinfect or releasing it very slowly. Without a chemical test kit, the only viable way to make sure the chlorine hasn't been absorbed is to add it until you can smell it.

Of course, don't use household bleach that has fragrance, soap, or other additives.

D.  Chlorine Dioxide (also called Stabilized Oxygen)

Commercially available with MSR's Miox purifier, AquaMira tablets, and Katadyn Micropur MP-1 tablets. I don't know of any other commercial preparations. Effective at killing microbes including giardia and crypto. Follow the manufacturer's instructions.

E.  UV light and oxygenation (SODIS Method and Steri-Pen)

Giardia and crypto, as well as several other microbial nasties, have been shown to be somewhat sensitive to UV light. The process is simple, but only works with clear water. If the water is cloudy, it must be filtered until clear. The SODIS method uses a combination of UV-A/UV-C exposure and heat to disinfect.

Place water in a clear plastic or glass bottle (label removed), cap, and shake for 1 minute. Remove cap and place in on sheet of corrugated iron or on a rooftop in direct sunlight for 6 hours on a clear or up to 50% cloudy day. Greater than 50% cloudiness requires 2 full days of exposure. Precipitation diffuses UV light, so no disinfection will take place if it's raining. In very hot areas, if the water temp rises to 120F, only 1 hour in the sun is needed.

A few notes on the bottles. PET (PolyEthylene Terephtalate) diffuses UV-A light less than PVC, so PET is preferred. To tell the difference, PVC bottles usually have a bluish gleam. If burnt, PVC smells strongly like smoke, while PET smells sweet. Also, since water diffuses UV, bottles greater than 5 inches in diameter should not be used. Most 12oz soda bottles should be fine.

Also, due to ambient temperatures, this process is best viable if you're located between 35N and 35S latitudes.

The Steri-Pen produces UV-C light to deactivate giardia, and crypto. Your water container must not be larger than one liter and it must be wide-mouth. Camelbak bladders and reused soda bottles will not work. You dip the Steri-Pen in your water, activate and stir for two minutes. The stirring oxygenates the water and moves the contents close enough to the lamp for the UV to kill any nasties. As with the SODIS method, the water must have very low turbidity or it will diffuse the UV and fail to disinfect.

F.  Filtration

This is not going to be an all-encompassing study on various filtration methods. We're going to stick to compact pump systems and improvised filtration systems.

Backpacking style pump systems vary across the board. Most will remove protozoa (giardia and crypto, specifically), bacteria, particulate matter and have SOME effect on the levels of toxic metals and chemicals. Pump filters have no effect on virii, the little buggers are MUCH smaller than the filter material. Most manufacturers have good info on their websites regarding what their filters will, will not, and might do. No manufacturer claims 100% reliability. You want a filter that has an absolute pore size of 0.4 µm (micrometre) or smaller. Some manufacturers claim smaller pore sizes although it is only an average.

All filters will eventually clog, and will clog more frequently if you're filtering cloudy water. To avoid this, wrap the source hose with a coffee filter or a bandanna. This will pre-filter the large particulate matter which causes major clogs.

To make somewhat of an improvised filter, you can punch holes in the bottom of a coffee can, line it with a coffee filter, and then place layers of sand, crushed charcoal, sheets of paper, and more coffee filters. Use your imagination. This is not a substitute for a real filter, but in many cases something is better than nothing. If you're using just a chemical treatment option, you can cover your container with a bandanna or coffee filter to reduce cloudiness. It doesn't seem like it would work, but simple cloth filters have been used in India to noticeably reduce Vibrio cholerae, the bacteria that causes cholera. As I said before, something is better than nothing.

For reference, here are some size examples of stuff you might find in your water:

Filter pore size: 0.2-0.3 µm
Giardia cyst: 6.0 µm
Crypto cyst: 4.0 µm
Bacteria: 1.5-3.0 µm
Viruses: 0.004-0.03 µm

µm = micrometre = 1 millionth of a meter

Some more expensive filters come with an iodine element to kill microorganisms that make it through the filter, and then a charcoal filter to remove the iodine. DO NOT WASTE YOUR MONEY. Temperature variations in the water combined with the short contact time with the iodine element make it unreliable at killing "nasties". Also, none of these systems have an indicator to notify you when the iodine has expired.

G.  Potassium Permanganate

Potassium Permanganate (herein referred to as KMnO4) has a lot of mixed data associated with water purification. There is not a lot of hard, modern scientific data readily available to me because it has not been used to treat water in developed countries for some time, partly due to it's high cost, relative weakness compared to other chemical alternatives, and the fact that it turns treated water pink. It can be found at chemistry shops and some pool supply stores. Shipping will incur a HAZMAT fee.

KMnO4 takes the form of deep purple crystals. The treatment method is to add individual crystals until the water turns a light pink color, and let stand for a 30 minute contact time. It should not require more than 3-4 crystals per liter. This formulation has been proven effective against bacteria and virii, but I can find no studies or hard evidence of it's effectiveness on protozoa.

Adding 6-10 crystals to one liter of water (or until the water turns purple) will create a combination wound disinfectant, hand sanitizer, and a soak treatment for canker sores, dermatitis, pompholyx, and fungal infections. At this concentration the water will also make a good dye for snow or water to create a survival signal. It is not advisable to drink this concentration.

That all sounds pretty scary, huh? It's not.

If you're out in the woods and you come across a stream, you have a halfway decent chance of not drinking anything nasty. On top of that, you have the chance that your immune system will take care of any "nasties" that you ingest. On top of THAT, following the 72hr rescue theory, you have a VERY GOOD chance of being rescued and under the care of a physician before any symptoms occur. So if you *need* to drink, drink. As a survival instructor once said, "I don't wanna hear about any of my students dying next to a stream because they didn't have any iodine."

In a disaster, however, all bets are off. Do everything you can to disinfect and purify your water.


     The best way to assure clean and disinfected water is to filter first, then disinfect by use of chemicals or heat.

     Add flavoring AFTER you treat the water. Flavoring may contain ascorbic acid (vitamin C), which deactivates iodine. It also makes the water turbid, which provides a place for microbes to hide from other methods of filtration. If you don't have any flavoring, add a 50mg tablet of Vitamin C to remove the taste of iodine or improve the taste of other chemicals.

     Carbonated water is acidic enough to kill bacteria that may contaminate it.

     Like food traps, more is better. There's nothing wrong with filtering what you need now, chemical treating what you need later, and UV treating what you need tomorrow.

     There are 4 billion cases of diarrhea each year, and 2.5 million cases end in death, most among children under 5. This is equivalent to one child dying every 15 seconds or 20 747s crashing every day. Water purification is not something to be taken lightly.

     If you have to dunk your bottle in a water source to fill it, don't forget that whatever is in the water is also ON YOUR HANDS. WASH THEM.

     Pet peeves: It's a micrometre (µm). A micron is a measure of the thickness of wool, not a measurement of pore size. A micrometer is a measuring device, not a unit of measure.

     As an option for a filter system to work on virii and bacteria as well as protozoa, use this method: Assuming your water is fairly clear, bottle it and add the chemical of your choice. Give it a dwell time of 5 minutes, then filter into another bottle. The chemical will quickly kill the little guys, and the filter will remove the hard-shelled protozoa that require the longer contact times. The filter will also remove most of the taste of the chemical treatment.

     Be advised, the bottle you use for initial gathering should be considered "dirty" and should only be used for initial gathering until you chemically treat it with a full dwell time.

Sean Keener
Former USAF SERE Specialist
Active Wilderness EMT

About the Author

Sean Keener is a former Boy Scout where he spent several summer camps teaching survival skills. He is a former USAF SERE Specialist and considers himself a student of all survival concepts. A Wilderness EMT, Sean also studies conventional and military survival, bushcraft and primitive living. This article on Water Filtering and Purification was reproduced here with the permission of the author. It is another survival preparedness article offered by Best Glide ASE in an effort to make persons safer and more prepared in a survival situation. The views expressed above are the view of the author and are not verified or substantiated by Best Glide ASE. As always, nothing is a direct replacement for personal research, study and survival training.

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