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:
- 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)
- 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.
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?