When is improvised water filtration warranted for survival?

Question

In the comments of the accepted answer to this longstanding question: Water filters - why sand and rocks? the makings of a debate started, but no definitive consensus was reached. I'm going to do my best to make an attempt to summarize each side:

For

• in a survival situation with only murky water available and whether help is coming or not remains uncertain, an improvised filtration system of rocks, sand and charcoal can make the water safer to drink (conceding it doesn't purify water)

Against

• in a survival situation, it may be more constructive to devote time and energy to getting home.
• Additionally, there is no way to know that there was not a recently decomposing animal (or other bacteria source) on the very sand you are using which would introduce even more harmful contaminants into the water.

Question

I see the logic in both sides of the argument, and I think there is an implicit distinction between filtering and purifying. With that in mind, when is it warranted to filter water in a survival situation?

For example, is the only use case to filter debris before boiling it over a fire to make it truly safe to drink? Or is improvised water filtration with sand, rocks and charcoal a waste of time and energy altogether?

Answer 1

It depends on the timeframe,

• Dehydration will kill you within days, giardia takes much longer to develop and can be treated once you are out of the backcountry. People have died because they didn't want to drink muddy water.

• If you are going to be a long time out from getting back, keeping healthy is important because getting stomach bugs/diarrhea will really hurt your energy levels.

In normal situations though, you want to have something to purify the water to avoid getting sick.

Answer 2

A sand filter won't take out bacteria, but will take out larger critters like giardia.

Anything that makes the water clear will increase the penetration distance of sunlight. UV is tough on bacteria.

Mud is not particularly harmful. You can boil muddy water and drink it.

(On the Athabasca and Slave rivers the current is strong enough to suspend coarse sand in the water. At rest stops you can hear the sand hitting your canoe. Our routine on these rivers was to dip a cup of water and set it on the bottom of the canoe to settle. Blow the floaties (bugs, sticks, natural detergents...) off the top, drink the top 3/4 of what was left, swirl and discard the sand and silt, and start a cup for your next drink.)

Answer 3

Always...Yes ALWAYS boil the water after filtration, Assuming that you have the materials to do so. Does not matter how "clear" the water appears...Boiling is still necessary in circumstances where chemical purification is not avail. As many before me have said. you do not want a stomach problem while you are trying to survive. Filters can be created in many ways with little materials needed...A simple empty plastic bottle or can will suffice, small gravel, charcoal from your fire and sand,(Do a Google search/youtube search,There are many videos on making such a filtration system using only those 3 items, small gravel charcoal and sand). Even if there is no heat source/fire avail, putting the filtered water in a clear plastic or glass container and leaving it in the sun for a day will help in removing any bacterial contaminates,(but possibly not ALL contaminates). I suggest that if you use this method, that you not seal the container, instead using a piece of cloth or wadded grass to seal the container will allow the water to "breathe". I am not making this comment simply out of any need or desire to look knowledgeable, I am making this comment because I have BEEN in the survival position many times...mostly due to circumstances of my own choosing. I make periodic 6 to 9 month sojourns into the wild because I enjoy the solitude and the challenge. My longest experience was on the Mn Canadian border for a period of just over 9 months...through a very cold and bitter winter. These trips are always made with only the supplies and material that I carry on my back...so I am well enough versed in the art to have not had anything happen that I was not able to handle.