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Saw a video on youtube where some guy made a water filter out of a bottle, charcoal, sand and little rocks. He commented that since the charcoal is very porous it's great for filtering. Then, after constructing the filter he continued to filter some dirty water and commented that the first little bit is going to be nasty since its getting the dirt off the sand and rocks in the filter.

This doesn't make sense to me, if the charcoal is so great, and all that the rocks and sand bring to the party is dirt, why not just fill the bottle with all charcoal? Not to mention that charcoal is much easier to come by than various grains of sand in a forest or a jungle or whatever.

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    The idea is kind of silly, because the need for purifying backcountry water is basically a myth. If you're in an emergency survival situation like this, the last thing you should be worrying about is improvising a way to perform unnecessary purification of your water.
    – user2169
    Commented Jul 8, 2014 at 23:17
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    @BenCrowell In survival situation there are many scenarios where you may not have access to a pristine water source in which case you might have to deal with whatever is at hand. The author of the video also mentions that this is just for water filtration, and not purification.
    – ppl
    Commented Jul 9, 2014 at 0:22
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    Who's to say the sand isn't full of bacteria? What if an animal had died on the sand and had been dragged away recently. Filtering though sand could actually introduce contaminates. Your better off simply drinking from a fast flowing stream. If it's stagnant, filtering isn't going to do anything, simply don't touch it.
    – user2766
    Commented Jul 9, 2014 at 8:44
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    possible duplicate of What is the safest way to purify water?
    – user2766
    Commented Jul 9, 2014 at 13:15
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    @BenCrowell. Purifying any water not coming from a spring is common sense, not a myth. Ever heard of giardiasis for example?
    – Jani Hyytiäinen
    Commented May 3, 2017 at 7:27

3 Answers 3

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According to wildwoodsurvival, the sand helps remove suspended particles. Charcoal helps to remove chemical impurities. The charcoal plays the role of active carbon.

The water filtration process use multiple materials, from coarse to fine, to prevent clogging. As a last stage, the charcoal can also help remove some bacterias.

Note that the author of the video recommendeds this method solely for water filtration and not water purification.

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  • Doesn't charcoal filtration also get rid of protozoan cysts such as giardia? Can you clarify "is not recommended?" Who doesn't recommend it and why not? Personally, my concerns would be the following. (1) Backcountry water is normally fine anyway. Using this technique takes water that's probably clean and healthy, and mixes it with a bunch of dirty rocks and sand, which are probably covered with dirt, which contains a lot of bacteria. (2) In a hypothetical emergency situation, one should focus on the main issues, such as getting found or finding one's way to safety. [...]
    – user2169
    Commented Jul 9, 2014 at 0:44
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    [...] Time spent on silliness like water purification is increasing one's risk by causing delay. (3) If you hypothetically get giardia or bacterial diarrhea, that will show up after a time lag. E.g., giardia has an incubation period of about a week. By that time, presumably you're safe anyway. (4) Scientific research shows that when people get backcountry diarrhea, it's typically from hand-to-mouth contamination from their hiking partners. In the hypothetical emergency situation, an easier and much more effective method of prevention would be just to avoid hand-to-mouth contamination.
    – user2169
    Commented Jul 9, 2014 at 0:48
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    I actually agree with you on backcountry water but this is not a backcountry camping question. The video referred by the OP is about filtering water and not purification. i.e. You have brown water, it is the only available water, what can you do with it? Yes, it is a hypothetical scenario fitting, in my humble opinion, with the topic of survival. I think you are confusing survival and backcountry camping tags.
    – ppl
    Commented Jul 9, 2014 at 0:57
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    @BenCrowell Some people would recommend to NOT try to find your way to safety and instead stay put. Also, keeping yourself busy can help you stay busy and calm. Even if you built the water filtration device and never use it; it may have served a different purpose. I don't see how this is relevant to the OP question. The question is not "please debate the usefulness of creating such a contraption" but rather the purpose of one of its components it. Feel free to create a question where it states "Should I create such a contraption? this would be much more appropriate.
    – ppl
    Commented Jul 9, 2014 at 1:05
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    @BenCrowell Btw, did you even watch the video? The water used is definitely not what you would call "probably clean".
    – ppl
    Commented Jul 9, 2014 at 1:07
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His filter has basically two stages:

  1. A series of mechanical filters (coarse rocks, fine rocks, coarse sand, fine sand) to remove macroscopic contaminants (eg. dirt) from the water. By using several different sizes of material, he's extending the life of the filter: each layer will tend to block material that would clog the next layer down, with the fine sand acting to protect the charcoal.
  2. A chemical filter (charcoal) to remove chemical contaminants from the water. This doesn't work by physically blocking them, but by absorption: essentially, the chemicals "stick" to the charcoal as the water moves past it.

It's important to note that this will not deal with microbes. Some protozoa will be stopped by the fine sand, and some viruses will stick to the charcoal, but most will get through. If you want to be safe, you need to use this in combination with boiling or another method of dealing with microbial contamination.

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Any little bit a person can do that can increase their chances of staying healthy is not only important, but potential lifesaving.

I have lived in the bush and was in the military and in an emergency situation, you definitely want to think about water cleanliness. No, you may not be able to guarantee you will not get sick, but getting lost in the bush plus intestinal issues equal dehydration, which is, without a doubt, one of the most dangerous positions one can find themselves in when in a survival situation.

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  • I've edited the question, I think you answered the question in the last sentence and the rest was background, so I've edited to move the important bit first. Please try and make a point to actually answer the question concisely as possible. Your answers will be received better that way :)
    – user2766
    Commented Sep 2, 2015 at 13:14

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