The most effective way to clean found water is to boil it. But, how do you do that if you don't have a metal container? Or any container at all? With a rock of course.
Tools: You're going to need a fire and obviously the ability to start one. Bonus points if you carry a method to reliably process wood and start a fire, regardless of weather conditions. You'll also need something that can hold a decent amount of water, but the beauty of this method is that you can use literally anything capable of holding water or even sterilize a puddle. You'll need a rock with a good amount of surface area, in proportion to the amount of water you're trying to boil. And, you also need some way to move said rock to your container of water; two sticks will get that job done in a pinch.
Step One: Find water. You're obviously doing this to try and clean dirty water, but starting with the cleanest, clearest water possible is going to make your life easer and your end product less gross. Filter any nastiness out if possible, even if that's just by passing the water through your t-shirt or similar.
Step Two: Start a fire. Do this close to the water source you're drawing water from in case you need to boil more than one container of water or your container's not all that portable.
Step Three: Add rocks. Shove a few good size rocks into the heart of the fire, making sure they're surrounded by hot coals and burning wood. Using more than one rock at once will give you the ability to add more to the water should the first fail to achieve boil.
Step Four: Drop rock(s) into water. After 15 minutes or so, the rocks should be good and hot. Lift them out of the fire and drop them in the water. The water should immediately reach a roiling boil, killing any viruses or bacteria. Your water will be safe to drink once it's cooled down. If you've pulled water out of a mud puddle, ditch or similar gross place, don't expect it to taste great, but it will be safe to consume. Note that boiling does not remove chemicals, pollution, spilt fuel and similar contaminants.
Safety: Don't pull rocks out of water and put them into the fire, that'll cause them to explode, sending shards and sparks flying. For the same reason, don't use sandstone or rocks with visible cracks or layers through them. Smooth, solid rocks are best. Only start a fire where one is permitted, unless it's a genuine emergency. Be careful lifting the rocks out of the fire, don't lean over the fire while you're doing so, exposing your torso and head to heat and flame. Always fully extinguish any fire until it's cool to the touch.
Video: Chris Brinlee Jr.
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