The second most important survival item is water. Most of us can survive for a few days without food. I could go a month. But I digress. You had better have potable water soon after you go into survival mode. Fortunately, most of us have access to bottled water. Each bottle is a mini-canteen. I keep a couple in my truck and I also have some in an emergency bag I keep called my BUG OUT BAG(BOB). This should get me home or at least to some drinkable water.
Each person in your household should have a gallon a day just to drink. If the water supply is interrupted, you will also need some for washing up, cooking, washing clothes, and dishes. If the water is there but contaminated, it could be used to flush toilets, but not much else. A whole house water filter can filter out debris, but not bacteria, viruses or chemicals. Put a second filter in line, which has another filter such as activated charcoal and your water becomes even purer, but bacteria and viruses can still get through. At this point boiling the water will kill those. Also chlorine, such as Clorox will kill all of the live stuff. It takes very little, 8 drops per gallon will do. Keep in mind that clorox deteriorates a little each year(about 20% degradation) so use the freshest you can find. Make sure you use the plain kind, not the fragrant type. The chlorine used in pools can also be used, and since it isn't mixed with water, it should have a long shelf life. It is Calcium Hypochlorite, and it has to be mixed according to the following recipe:
1. One heaping teaspoon of 78% Calcium Hypochlorite mixed with 2 gallons of water.
2. Add this mix to contaminated water at a 1:100 ratio.
For example 0.55 gallons of the mix will treat 55 gallons of contaminated water.
You need to be thinking in terms of 55 gallon drums of water. If you have a family of four, at one gallon/person/day 55 gallons will last almost two weeks for drinking water only. Add the other uses and it could only last 4 or 5 days. There may be other sources of water in your house however. I have a water heater which holds at least 50 gallons, and there is a waterbed in the house. All the toilets are full, but one flush and they're empty. So, flush selectively during survival mode.
If you aren't worried about environmental contamination of it, rainwater is a source. Put a barrel or 5 gallon bucket under a downspout and collect it. It still should be treated and filtered, but it's wet.
For the serious survivor, and come to think of it, you should be serious, water filters are the answer. There are filters which have been around for over 100 years, ceramic filters. They have been used by travellers and missionaries in God-Forsaken countries and work perfectly. The ceramic filters have small pores which filter the water. Inside, activated charcoal filters even more and improves the taste. But what keeps the missionaries on mission is the ceramic part is impregnated with silver. The silver kills all of the bugs that can make you sick or kill you. The purpose of this is to inform you about what is available. An easy way to let you see what I'm talking about is to provide a link to the store where I bought mine. You can buy them anywhere. I'm not selling for the guy, but click here to see them: