You can go several days without food (even weeks if you had to), but having drinkable water on hand during a survival situation is an absolute must. You’ll only be able to survive a few days without it.
This is probably the simplest method, providing you have the right supplies with you. Water treatment tablets are easy enough to buy at any outdoor store, and can be carried in a small bottle whenever you are out camping or hiking. You just add a tablet or two (read the instructions for the particulars) to the right volume of questionable water, give it a good shake, wait half an hour and drink.
Now here we have the reliable old-school method: boiling. You just need a heat-proof container and a way to make fire. Once you get the water boiling, keep it going for at least 5 minutes. That’s really all you need to kill off anything dangerous. A lid will help conserve the water as well as the heat for a quicker boil. Let it cool and you’re ready for a drink.
If you are purifying water for cooking, you don’t necessarily have to boil it first and then start cooking. As long as the water is boiling for the required time, it’s going to be clean in the end.
A small pocket water filter can be a worthwhile investment for your gear, and it allows you to clean up water as long as the filter holds out. That means you don’t have to worry about rationing water treatment tablets. Katadyn makes an excellent one that uses a ceramic filter, but there are several other makes and models you can check out. Some really small ones are built into a straw so you can filter as you drink. They’re great for portability but not too good to produce clean water for washing or drinking. Something to keep in mind when choosing your equipment.
No matter which of these techniques you’re using, you want to start off with water that is as clean as possible in the first place. Strain it through a few layers of fabric to remove dirt, debris or insects.
If the weather does you a favor, you may be able to collect rainwater in a container. This is the only natural source of water you can immediately trust to drink without doing anything to it. A spread out sheet of plastic or tarp can really boost your collecting surface, rather than just leaving a cup to sit in the rain.
You’ll notice that for any of these techniques, you’ll need to have some supplies with you. Tablets, containers, fire-starting materials, filters and more. Make sure you have these in your kit when you head out into the wilderness, just to be on the safe side. Having the right equipment for at least 2 water purifying methods is a good idea.
One thing you need to keep in mind with any water treatment, the main purpose and goal is to remove dangerous parasites, bacteria and other living things that will make you sick. Chemical contamination won’t be fixed by these methods. So water that has heavy metal contamination will still have those heavy metals in it after you boil it, for example. Unfortunately, there really isn’t anything you can do about that. These are more long-term health concerns and really shouldn’t concern you in a short-term emergency situation. It’s the parasites you need to focus on. Kill them and you’re good to go.