Water
You can survive only about three days without water. From a strictly survival point of view, water is one of the most important element for your body's survival. A person can lose all reserve carbohydrate and fat, and about half the body's protein without being in real danger. A loss of only 10 percent to 22 percent body weight as water is fatal. The amount of water lost from the body through urine, water vapor from the lungs, and through perspiration averages 2.5 liters per day.
Under serious disaster conditions, no water can be presumed safe for consumption. Typhoid fever, dysentery, and infectious hepatitis are diseases associated with unsafe water. Water purification techniques may be effective in removing some, but not all contaminants from water. The only way to guarantee a safe water supply is to store it away yourself--before a disaster. The simple truth is that water supplies across the USA are rapidly deteriorating in both quality and safety.
Emergency Water Purification
We need to discuss the concepts of water purification. Water contains three things which must be revoved to become potable (drinkable). Water contains chemicals, debris and bacteria.
Chemicals such as oil, grease, gas, pesticides, acids, and fertalizers get into the water. The only way I know of removing chemicals is by using a carbon filter. When purifying water, it is best to start with chemical free water.
Debris can be removed using filtering techniques. If water contains debris, strain through paper towels, paper coffee filters, or several layers of clean cloth into a container to remove any sediment or flaking material. Keep filtering until the water is clear. You can make a lot of these filters which remove debris from the water. Be creative.
Bacteria is usually the hardest because you cannot see when this is done, however, it is very important. During a disaster you don't want to compound your problems by being sick or having to care for a lot of sick individuals. These individuals will not be able to help in survival. Fever, cramps, fatigue, diarrhea, nausea, dehydration, and general malaise can either sneak up on you slowly, or slam you all at once. During the manifestation period, being more than 10 seconds away from the nearest bathroom is often a bad idea. (Functional bathrooms may be in short supply during water or utility emergencies). It's easy to see that the best idea is not to take risks where you might be exposed to bad drinking or cooking water.
Filter
Filters which purify water in all three areas need to contain carbon and 2-3 microns in filter size. The best filters are combination charcoal and ceramic. There are many places that sell camping filters in various sizes. In homes and RVs a combination of filters is usually used, i.e., you have a sediment filter, a carbon filter, and a UV bacterial killer.
Boiling
Bring water to a rolling boil and keep it simmering for at least several minutes. Add one minute of boiling to the initial 10 minutes for every 1,000 feet above sea level. Cover the pot to shorten boiling time and conserve fuel. To improve the flat taste of boiled water, aerate it by pouring it back and forth from one container to another and allow it to stand for a few hours, or add a pinch of salt for each quart or liter of water boiled.
Bleach
Use two drops of bleach or three drops of tincture of iodine per liter. Mix thoroughly by stirring or shaking water in a container. Let the water stand for 30 minutes. A slight chlorine odor should be detectable. If not then the dosage should be repeated and the water allowed to stand for an additional 15 minutes. Liquid bleach loses strength over time. If the bleach is one-year-old, double the amount. Two-year-old bleach should not be used.
Do not use granular forms of household bleach as they are poisonous.
SODIS
"Solar Water Disinfection" is an interesting method bacteria disinfection using Solar UV-A radiation and temperature (pasturaziation) .
Distillation
Distillation is an effective process and, what's more important, it can be done with a lot of improvisation. You can heat water with whatever is at hand: fire, electricity, mirrors, or whatever. You can use almost anything that holds water for a boiler, as long as you can direct the steam into a cooler. All you need is something that will just cool the steam down.
It does not remove substances that have boiling points at a lower temperature than water. Some of these substances are oils, petroleum, alcohol and similar substances, which in most cases don't mix with water.
Some Interesting Water Purification links: