Sand won't filter out bacteria. Filtering through sand and charcoal will remove some things - large particles, some toxins, but not bacteria etc.
There's been some conceptual work done with solar ovens for poor nations - its' possible to use a solar oven to heat water beyond boiling, but for
that you'd need at minimum a pot with lid, plastic bag, and some sort of highly reflective stuff.. But it is an alternative to having to make a fire,
and while you might not have supplies, there's a lot of garbage in the world just kind of floating out there..
If at minimum, you can find a plastic bag, you can tie the bag around a tree limb and wait - water will evaporate from the leaves etc, leaving you at
least a few mouthfuls of clean water (hopefully anyway). I think that'd be preferable to drinking the water from a stream if you had no way of
boiling it, though you wont' get much water from this method, just a few tablespoons probably..
For the safest water, you need some sort of filtering system, and some way to boil it. No supplies at all is a tough one, because whatever you do,
you're going to have to find something as a "supply" and how you take care of your water is all based on that.. Eg, can you tear up your cellphone
and use the battery + wire to create a closed loop? If you can, your closed circuit will heat up hot enough to be able to light something flammable..
(In grade school I found this out by using a regular battery and some wire and basically skipping the lightbulb in my circuit to turn it off..
Apparently I was supposed to design a switch, instead of just a loop of wire. All they told me was to make the bulb go off.. I did, but accidentally
melted the plastic battery holder too)..
Obviously there's the stick and grass way to make fire.. there's using glass to create fire.. flint, etc.. But all results depend on where you are,
what you have on you and around you, what you can find. A survival situation with no supplies all depends on the knowledge in your head, which is why
it's especially important to know as many different ways of obtaining water as possible.. Shelter and food (the two other basic requirements of
immediate survival) can be winged by most people.. water, and by extension fire, less so.
also, found this one.. I'm not entirely sure this will work, and one of the comments mentions that it doesn't.. Could possibly depend on the type of
plastic, also I'm a bit dubious about heating plastic at all, since you could have chemicals leaching into the water from the bottle, but..
[edit on 5-8-2009 by Inannamute]