When it comes to having water in an emergency, the best defense is a good offense (or is it vice versa? To be honest the metaphor is pretty convoluted here), and the best thing to do is to have an emergency water supply prepared BEFORE the stuff hits the fan. For more info in that, check out our article on Emergency Water Supply and Storage.

Seriously, do it.

But let’s say you are caught unprepared, or your initial supply has run out. What then?

The answer, my friend, is to find a natural water source to filter and purify. We do that by going out, and trying to find a well, stream, river, lake, whatever. Start with the cleanest salt-free water available, and go with cold and running over warm and stagnant if possible.

But wait- you don’t want to drink it just yet! It is very likely that that water is contaminated with things that could make you really sick, and we need to remove those if we’re planning on drinking it.


There are 2 types of possible contaminants we need to worry about: pathogens, and pollutants.

The largest type of pathogen is protozoa (the largest of microorganisms) which range from 1-16 microns (a micron is 1/1000 millimeters). Some of the protozoa we need to worry about are parasites such as Giaridia Lamblia and cryptosporidium.

Bacteria are medium-sized pathogens, which range from 0.2-10 microns. Some of the bacteria we need to worry about are E coli, cholera, and salmonella.

Viruses are the smallest pathogens, ranging from 0.02-0.1 micros. Some of the viruses we need to worry about are Hepatitis A, Norwalk, and Polio.

Pollution is pretty straight forward- any man made substance that is harmful to the human body. Chemicals, fuels and sewage all fall into this category.

Drinking contaminated water can lead to fever, fatigue, cramps, dehydration, nausea, diarrhea, and can even prove fatal to those with weakened immune systems. Not something you want to have to deal with in an already stressful survival situation.

To make sure we eliminate these contaminants, we need to engage in a 2 step process- We’ll want to filter the water, and then purify it.


This step is about filtering impurities from a source of water. The larger the particles are, the easier it is to filter, and the smaller they are the harder to filter. Filtering methods will remove free floating particles and some microorganisms, but won’t remove some chemicals, odours or pollutants.

For larger particles and debris, strain with a paper towel, clean cloth or coffee filter. That’s the easy part. For the smaller contaminants, we need to get more creative.

Some common substances used for filters:

Ceramic: the best, but most expensive filtering method. It has the smallest pore size (0.1-0.5 microns). Used by the Red Cross and World Health Organization.

Glass fibre or compressed surgical paper: next best (.2-1.0 micron pore size), much cheaper.

Hard-block carbon: larger pores (0.4-2 microns), not as good at filtering particles or microorganisms, but has the added benefit of absorbing some chemicals, odours and pollutants. Not effective on its own, but great as a 2nd or 3rd stage filter.

After filtering, we’ll want to purify the water to get rid of the really tiny contaminants.


These methods will kill most, if not all microorganisms. They generally won’t remove pollutants and odours.

Boiling: boiling water will kill all pathogens, and it’s cheap and easy. 2-3 minutes of rolling boil is generally enough, although if water and power aren’t scarce you could go up to 10 minutes to ensure you kill any and all possible pathogens. There are downsides to it, though- it requires fuel and electricity that may be unavailable, some of the water will be lost as steam, it requires a long cool down time and it’s impractical for purifying anything other than small amounts of water.

Iodine: A disinfectant, very good at killing most pathogens, and inexpensive. Dosages will vary, and pregnant women should avoid water purified with iodine. Also it’s not recommended for long term use. Use the common 2% iodine solution; use 3 drops per quart of clear water and 6 drops per quart of cloudy water. Stir and allow to sit for 30 minutes before using/filtering again.

Chlorine: Another disinfectant option. Chlorine is very poisonous, so make sure to use correct amounts should you choose to use it. Use regular household bleach with 5.25% sodium hypochlorite. Avoid anything scented or with added cleaners. Add 8 drops of bleach for every gallon of water, stir and let sit for 30 minutes. The water should have a slight bleach odour. The water will clear, and dirt and particles will settle on the bottom.

Ultraviolet: There are UV lamps available that can kill pathogens pretty effectively. The UV sterilizes the microorganisms, and if they can’t reproduce they can’t make you sick. It won’t  eliminate debris, chemicals or pollutants, so it makes a great 2nd or 3rd stage of purification but isn’t effective on its own. And it requires electricity to run, which may not be available in an emergency situation.

Solar Still: the idea is to wrap a clear, plastic sheet or some kind over a source (sea water, plants, the ground). Solar energy passes through the sheet, and warms the source. Water evaporates and collects on the sheet, and runs down the plastic into a container for storage. Solar stills are cheap to make, and are very effective at distilling pure water. The process is very slow though, and only small amounts of water can be collected per day.

Distillation: a great final step in the process, which will remove microorganisms, heavy metals, salts and most chemicals. To distill water, you need to boil it, and collect the steam water that condenses afterwards. It will be pure water, as all the impurities will remain in the pot. For information, read this article on how to distill water at home.

After filtering and purifying the water, it should be safe to drink.

Good luck and stay prepared!

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  1. Good comprehensive post. Here’s one little thing I’ve read and tried with no ill effects so far – you don’t really have to boil water for a full ten minutes… bringing it to a boil is enough to kill pathogens and whatever other bugs might be in there. As you noted ten minutes will consume a lot of fuel. No big deal around a campfire, but if it’s your alcohol stove or something it’ll get expensive real quick.

    I’ve used iodine as well (nasty tasting, but better than getting sick out in the field) and one thing I found out is that some people are allergic to it. I hiked a portion of the Appalachian Trail with a girl who was allergic to it and we wound up using filters instead of iodine because of it.

  2. Yeah most people say 2-3 minutes will most likely kill any pathogens. I have heard of rare cases where it could take up to 10 minutes though, and felt I should be safe and say 10 lest one day someone message me upset that they got sick from water they boiled.

    I think I’ll make an edit to make that clearer, though, as if energy and water are in short supply 2-3 minutes of boiling are totally fine.

    • That’s a good safe route. This past weekend I went out in the woods with just my knife and a firesteel (post coming soon) and got some water out of a stream. I boiled it for about five minutes just to be sure. I’d rather be safe than sorry that’s for sure.

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