It is said you can survive up to 3 weeks with out food but will be dead in about 3 days with out water. Remember the pictures from Katrina? People sat on the roof of their homes for days, surrounded by water, none of it drinkable. The public water supply was useless. The American Red Cross advises you have one gallon of water per-person, per-day. How can we store water for such a cataclysmic event? I don't have all the answers; however, since i work in the waste water treatment business i know a thing or two about water.
First, if you have warning you should fill your bath tub with water. Doing so will give you several days of clean water. Second fill all the $.05 return bottles with water and stuff as many of them as you can in the freezer. Not only will you keep the food frozen longer, but you will have more clean water when it thaws. Third you can always tap the water in the tank of your toilet if you do not use a cleaner. Finally, get a few gallons of water at your grocery store. These steps should get your through a long period of time with public water.
What if you could not stay in your home and you had to leave? Glad you asked. Carrying a gallon of water for everyone in your family for each day becomes impossible. Water weighs in at 8.34lbs per gallon, so you will quickly overburden yourself. With some tools and knowledge you can use a lot of water you find along the way. I do want to point out that YOU DRINK AT YOUR OWN RISK. This is simply a guide, and you have to use your own judgement. First a water filter is a must. The hiker type filters are pretty good. Katadyn and MSR make good models. These will filter out most of the microbes and solids. If you do not have the money to buy a filter you can use a good old coffee filter. A coffee filter may not grab the microbes, but it will get most of the solids out. Coffee filters are surprisingly a great filter. I have filtered samples of "mixed liquor" from our aeration tanks at work through coffee filters. Even though this partially treated sewage looks horrible it is 97-98 percent water! A pass through a second coffee filter will yield great looking water. Now that you have filtered the water you need to disinfect! The better job you did at filtering the water the easier and better the disinfection step will be.
Disinfection is very important and there are several options you can employ. Boiling is the easiest. Just put the water in a pot and let it boil, cool and drink. Chemical disinfection is another option. Here you have to use your head. Chemical disinfects by penetrating the cell walls of the microbe, thus killing it. If your water is murky you will need to add more chemical. The down side is the "contact time". Disinfection takes about 30 minutes. There are several products on the market that you can buy. The most prevalent is house hold unscented bleach! Yup Bleach. Go on-line and research how much bleach to add per 1 US quart. I believe it is 4 drops per quart and wait 30 minutes. Remember when storing bleach place it in an glass amber eye dropper bottle to keep the sun from destroying the chemical.
The other option, UV. Ultra Violet rays are extremely destructive! Just look at what they do to the paint on your car, plastic lawn furniture, and best of all your skin. UV works so good that many of the country's sewage treatment plants use it to disinfect their final effluent. I have personally seen the lab results from work, and can with the up most confidence tell you UV kills. One product on the market is the Steri-Pen. It is simply a UV lamp the size of a pen. Turn it on and stir in you canteen and your good. The upside to UV is it requires very little contact time. The Rays penetrate the cell walls instantly. The microbe does not die; however, the DNA of the Cell is destroyed. With broken DNA chains the microbe is sterile and can not reproduce. This takes about 1 minute.
Remember, when it comes to water, use your head. You will come across water that others may have passed by thinking it was dirty. With the right tools and techniques you will be able to salvage the water you find. I, for one, will be throwing a few coffee filters and a steri-pen in my bug out bag.