When out in the wilderness, lost or otherwise, there are three basic necessities you need to tend to--Fire, Shelter and Water. Hikers and campers usually do not carry enough water to help them survive being lost or stranded in the wilderness for more than a day. Whether the climate is hot or cold, people need about two quarts of liquid per day and that is without strenuous exercise or extreme heat.
Even in cold climates, without sweating, you can become dehydrated by just the moisture being expelled by your breath. Having the knowledge on how to purify water can mean the difference between life and death out in the wilderness.
Boiling With a Fire Safe Container
The easiest way to make water safe to drink is by boiling it. As long as you have a fire safe metal container--a pot, pan, metal tin, soda can or the like--you can easily purify your water. If your water source is a stagnant pond with algae or is particularly muddy, use a cloth like a t-shirt to strain the water as best you can before boiling. The best water source is moving water from a running stream or even a waterfall. The more motion the water has, the less bacteria and parasites will be able to grow. You should always treat your water before drinking it, but if you have no other options, try to find a water source with the most movement.
Do note that this method does kill 99.9% of parasites and bacteria in the water, but it still will most likely smell and taste a bit like sewage. Boiling it does not improve the taste, but it does make it safe to drink.
When boiling water to make it safe to drink, you only have to let the water reach a rolling boil. However, if the water is very sketchy, you can boil it for an additional 1-3 minutes. Any longer and you are just wasting your water to evaporation. It may not taste fantastic, but it will be clean.
Boiling Without a Fire Safe Container
Say you are absent anything remotely metal and you find yourself in a place with very little litter. Maybe you have a container that holds liquid, but it's plastic or glass and cannot be placed by the fire to boil it.
Another way of boiling water is the hot stone method. All you have to do is find some smooth stones, preferably washed of all mud or moss and toss them in the fire. After heating the stones for around 15 to 20 minutes, fish them out with a piece of wood or two and drop them into your water. After about three stones, your water should have completely reached a rolling boil and will be safe to drink. Though, because of the rocks, it will taste a bit ashy, but a little ash never hurt anyone.
You will most likely need to find new stones each time you do this. Most stones do not fare well against rapidly being heated and cooling. They tend to crack.
Treating With Bleach
I know what you are thinking, your mother always told you to never drink bleach. However, if you have been drinking city water, you have been drinking a little bleach for awhile. Do not get me wrong though, like your mother said, bleach will kill you. However in proper amounts, it will kill everything else in your drink but you.
The rule of thumb for using bleach for purification is to use 8 drops of bleach per gallon of questionable water and 16 drops for cloudy water. Eight drops measures out to about 1/4 of a teaspoon if you do not have a liquid dropper. You should mix well then let the water sit for 30 minutes.
After 30 minutes, if the water does not smell just faintly of bleach, add more and repeat the process. Do not exceed 16 drops of bleach, though.
Treating With Iodine
Iodine is a popular purification method among campers and hikers. Not only can it treat your water, but it is also really helpful for cleaning wounds, which is why it is present in almost all wilderness first aid kits. Purifying water with iodine is a lot like purifying water with bleach. Per gallon you need ten drops of iodine if the water is clear, and double that if it is not. Then mix well and let sit for 30 minutes. The water will taste terrible, but it will be clean.
This method should be avoided if you are pregnant or have a thyroid problem. It is also recommended that you warm your water via sunlight or fire before using iodine as it is less effective if the water is below 68 degrees. With all the regulations and terrible taste, it is easier just to boil water.
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Purification and Desalination With a Solar Still
The solar still is my favorite method of purifying water, sometimes I make one just for fun (because I think watching science is fun). However, the downside of a solar still is it does not yield much water. However, if you have not located a steady water source it is a good back up. The best part of a solar still is that if you only have sea water to drink (which will kill you via over-salting), it desalinates it and makes it a little salty but safe to drink.
To make a solar still all you have to do is fill a container with dirty water, or something like a wet cloth or even seaweed/kelp. Though do not fill it to the brim, you need to place a cup in the center of your water to catch the clean water. Cover the container with plastic wrap or a plastic bag and place a rock in the middle.
Put the container in the sun for a few hours and the clean condensed water will stick to the top, droop down to the middle where you placed the rock on top, and drip into your container! You don't necessarily even need to do this in a container, you can do it by digging a hole in the ground and sealing the plastic with dirt or sand. This is definitely not an efficient way, but it is a nice back up.
Drinkable Water Sources That Do Not Need Purification
Nature has a nice way of purifying its own water. Here are a few natural sources of water that do not need purifying.
Snow - Freshly fallen snow, providing there is no visible dirt in it, is safe to melt and drink. If you are stranded in the snow, you should not eat snow without melting it first. This will cool your body and have hypothermia set in faster. Though if you are doing a lot of hiking, snow is okay to eat without melting if you are starting to sweat. You never want to excessively sweat when stranded in the cold.
Rain - Rain water, no matter where you are (as long as it is not acid rain), is alright to drink. Just make sure you have a clean way to catch it. Drinking from rain puddles off the ground is not safe.
Morning Dew - If you set ways out to catch morning dew, it is safe to drink. You just have to make sure you get up before the sun dries it all up. Do not expect to get a lot of water from the morning dew though.