Purifying Water in the outdoors is very important and can keep you from getting a case of the trail trots. To purify water there are several options: Buy a water filter, buy iodine tablets, or boiling.
Buying a water filter is expensive and can be rather heavy, iodine tablets taste bad and iodine is not really all that good for you and boiling water requires a stove and fuel. There is a fourth option which few people know about but is both cheap and simple. Chlorine bleach, yes that is right. Many public water systems actually put simple store bought bleach into the water to purify it for drinking.
Here is the trick. First select water that is clear, the best water is fast moving and running across gravel and sand. Second put about two drops of chlorine bleach into your two liter bottle and let sit with the lid off for about 30 minutes. The water should have a slight chlorinated smell, like a swimming pool, if not add a few more drops. The water is now safe to drink.
As a side note; do not use bleach that is scented or has softeners. Clorox bleach is a good choice. Again water is your most important commodity and clean water is more precious than gold.