When you need to rehydrate yourself, it's often a bad idea to simply pull water from any indiscriminate source and begin drinking. Instead, turn to one of the many methods outlined below. They'll tell you how to filter and purify your water and how to store it.

Does All Water Need to Be Treated?
In the wild, rainwater you’ve collected in clean containers is generally safe, as is snow that you’ve melted. Water in the wilderness is also almost always safe if you’ve collected it via transpiration or a still (if the plant itself isn’t poisonous, of course). If you collect the water by any other means, though — from a stream or lake (flowing water is better than stagnant, but still not foolproof), dew, etc. — it should be filtered and/or purified; you never know what might be lurking in the ground or upstream from your collection spot.
In urban areas, rainwater may not be safe to drink, as it traveled through polluted air. And if you’re traveling in a developing country where the safety of the tap/well water is questionable, you’ll want to stick to drinking bottled water (not always an option in rural areas), or consistently purify your water.
Water Filtration
Using a water filter, especially a commercially tested one (versus just the DIY backwoods variety), can indeed eliminate some bacteria. But not all. Filters can take care of protozoa and bacteria, but they can’t get rid of any viruses present in the water — those are simply too small for the mesh to catch.
Generally, for backpacking and survival purposes, water in the U.S. and Canada is rated as safe for filtration-only methods and devices; this is especially true for mountainous areas. When folks get sick while backpacking or camping and blame it on the water, it’s often found to actually be sanitation-related (not washing hands, not disposing of waste properly or far enough from campsite, etc.).
Water Purification
Water purification makes H2O safe to drink by deactivating all harmful pathogens, including viruses. Purification doesn’t eliminate contaminants though. Dirty water that’s been purified is still dirty water, and probably needs filtering (that should happen first, actually).
Purification happens primarily through boiling, chemical agents, or UV light. It’s especially important when traveling outside first world countries, where viral infections are more common.
DIY Filters
In general, you only want to use DIY filters when no other option is available. You certainly don’t want to go camping and only plan on using rocks, sand, and dirt to filter your water. These are for survival purposes only. If at all possible, it’s recommended that you still purify water (by either boiling or adding tablets/chemicals) after filtering it with these methods.
Commercial Filters
Survival Straws. Survival straws have burst onto the market in the last 5 years or so. The idea is that you can drink water right through the straw (or water bottle with straw attached) and it’ll be safe because of the various filters contained within. The majority of straws on the market can eliminate bacteria and protozoa, but not viruses. They generally have no purifying element. Most do, however, have a carbon filter, which will eliminate off tastes and odors. Just be sure to check the specs, based on your needs, before purchasing.
Pump/Gravity Filter. The name here is misleading, as many of these commercial filters also act to purify the water. Many utilize a ceramic filter to eliminate larger pathogens, as well as silver to destroy viruses. These pump filters operate quickly, filtering and purifying up to a quart per minute, but require a power source (either your arm, or in some cases, a battery).
Gravity filters operate more like IV bags; they’re slower, but require no batteries or human operation. With any commercial pump or other filter, again just check the specs. If it mentions a purifying element, you’re good to go for any situation. If not, just know it won’t eliminate all risk.
These commercial filters are often bigger and bulkier than other options, so will take up more space in a backpack or survival kit. While they can be an expensive up-front purchase, they’ll last a very long time.
Boiling
Boiling water is the standard backwoods purification treatment for water. According to the EPA, one minute of a rolling boil will kill all of the bad stuff, including viruses. (If at high altitude — above 5,000 feet — increase the time to 3 minutes.) Any metal or glass container will do, but if you don’t have that available, you can heat rocks in a fire and place them into your receptacle.
One downside of boiling your water is that it obviously requires a fire and therefore fuel, which in some cases isn’t possible (although it’s more possible if you read our article on how to start a fire without matches). It also leads to evaporation and the loss of some water volume, which should be taken into account in situations where every drop may be precious.
Chemical (Iodine/Chlorine/Bleach)
Iodine. Use a 2% tincture, and apply 5 drops per quart of water. If the water is cloudy, add 10 drops. Let sit for 30 minutes before drinking. Iodine comes in a small, very portable bottle, and also has other uses like treating cuts and warts. This is definitely an item to have in your emergency kit or bug-out bag.
Iodine will lead to an off taste, and isn’t safe for pregnant women or those with shellfish allergies. Children are also often averse to iodine because of the taste; know that ahead of time if backpacking or camping. Iodine is generally the cheaper of the chemical methods, and works quicker.
Chlorine. Generally comes in the form of tablets, which you just drop into a liter of water, and let the chemicals do their work. After about 4 hours, your water will be safe to drink and free of all harmful pathogens.
Chlorine’s cons are that it has a longer waiting period, and is a little more expensive per use than other methods. On the pro side, most of chlorine dissipates in that 4-hour timeframe, meaning the water’s taste isn’t as affected. It also has a longer shelf life.
Bleach. In urban emergency scenarios, bleach can be used to purify water. Most bleaches use a form of liquid chlorine called sodium hypochlorite. As just mentioned above, chlorine is a water purification agent, so it makes sense that bleach could be used.
Household varieties will generally contain 5-8% sodium hypochlorite — be sure to check the label before using, and if it’s higher than that don’t use it. Using a dropper, add 2 drops per quart, and let stand 30 minutes before drinking.
UV/Solar
SODIS. Solar water disinfection (SODIS) relies on the power of the sun to make your water safe. Believe it or not, if you have a clear water bottle, you can simply leave it in direct sunlight for ~12 hours (24-48 on cloudy days), and celestial UV rays will kill most (not all) bacteria and microorganisms. Some experts say to leave water exposed for a full day no matter what, just to be extra safe. It all depends on your need. Since this particular UV method doesn’t necessarily kill all bacteria/viruses, it’s best used for survival purposes only, or in areas known to have safe(ish) water.

While you can always turn to a DIY water filter in the worst of survivalist disasters, it's far better to instead use one of the methods above. As for which method is best, well, that all lies in interpretation. If you are going to rely on purification, you should also couple it with some form of filtration to clean out the larger chunks.

If you fail to do this, you could leave some viruses or bacteria intact because the purification methods will be unable to clean all the debris. Overall, though, particularly in North America, a pump filter certainly seems to be the favored option among many preppers and outdoorsmen.

For more information on how you can purify or filter water for safe consumption, check out Art of Manliness.

Featured Image via U.S. Army Field Manual