Water is absolutely necessary for human survival. At the same time water can contain many dangerous pathogens.
- Boiling - will sterilize the water by killing bacteria in the water. If you collect the steam it will have less of a chance of carrying chemicals. By using a lower temperature less chemicals will rise with the steam. The water doesn't even have to boil if you are collecting the evaporating water.
- Distilling - See the library's version
- Rain water, melted snow, and new dew are usually safe. Rain water can be collected from run off from tents and or homes, although for drinking I would recommend purifying the water first.
- Water purification tablets.
- Water purification pumps. Try to use water that is already clear or do a pre-filtering with a cloth to remove dirt for better taste and a lower likelihood of clogging your filters.
- Solar stills can be used either built into the ground or using a container in a sunny location.
Built into the ground all that is required is a piece of clean plastic to cover the hole, a catch basin and a long tube if possible. The sunnier the location the more water will accumulate on the bottom side of the plastic. You would place you catch basin in the hole cover it with the plastic and place a small stone directly above the catch basin. You then place the tube in the catch basin and drink through the tube. If the soil is somewhat dry or you want to increase the production of water you can add moist plant matter or water that you would not want to drink directly including salt water and urine. Be extremely careful to keep your catch basin clean on the inside. Salt water should never be ingested and urine can be, I don't recommend it but you can get away with it for a while in an emergency.
If you use a container as a solar still it works almost the same as above with the exception that you must put liquids in the outer container to create the drinking water. After that it is the same. If you use a clearer outer container it can speed up production of drinking water. Mirrors can also be used to increase the amount of light getting to the water that you are trying to evaporate. Keep in mind that you are not trying to boil the water as that can increase the chemicals that evaporate with the water.