Before we get to priority #4 here is the summary of your top 5 Priorities of Emergency Preparedness. They are:
1. Make a Plan. Review, Rotate, Replace. Make adjustments for family size, age and local community needs. Consider evacuation, communication and staying put.
2. Gather Grab and Go Supplies. 72 hour kits, MREs, food bars, water pouches, water purification, first-aid, storage.
3. Gather Shelter in Place Supplies. Radios, flashlights, first-aid, sanitation, hygiene, warmth, shelter, storage.
4. Clean Water. Water storage, barrels, filters, storage.
5. Shelf Stable Food. Items in pantry, short-term and long-term food storage items. 
Priority 4 – Clean water Having access to clean water in an emergency is critical and almost always overlooked until it is too late. Here are a few obvious points to understand about water:
• Without water you die very quickly Depending on your health and age, you will live for 5-7 days without clean water. You can live much longer without food.
• Water is heavy and takes up a lot of space A gallon of water weighs about 8.3 pounds. Water is not easy to transport because it is so heavy. That means your emergency water plan should account for that.
Keep these items in mind when you are planning. For example, you may want to put a gallon of water per day in your 72 hour kit. But, that will weigh about 25 lbs. Are you strong enough to carry that water around? Are your children strong enough? Will it even fit?
The recommended amount of water from a variety of sources (like the American Red Cross) is 1 gallon of water per person per day. That is water for drinking, cleaning, cooking and sanitation. It is not for just drinking. You don’t drink a gallon of water on a normal day, but to be safe and to take of other needs, 1 gallon of water is the recommendation. If you start doing the math on how much storage space that would require for you and your family for 1 year, you will start to understand the recommendations below.
Here is my approach to water and what I recommend. In an emergency, you need to have three ways of getting to clean water:
1. Store water. Every shelter-in-place supply should have water storage. A small word of caution: Don’t try to go cheap on your water storage. Buy a food grade water storage container like a 55, 30 or 5 gallon container and use that. Don’t fill up your old juice bottles or your old milk jugs. They'll crack more easily and they're not designed for long term storage. The barrels and water containers are not to terribly expensive and when the time comes that you need the water, you will be glad you stored water you can actually use. Also, have a variety of sizes of storage containers. Don’t just have a 55 gallon barrel that you never move and never clean out or fill up. Use smaller, easier to transport containers like a 5 gallon stackable. Remember, water is heavy! Also, you won’t rotate your water as much as you think you will, so make sure you use some sort of water stabilizer that allows the water to be stored safely for 5 years without rotating. We have a Water Preserver that works well.
2. Have a portable water filter. If you do have the water stored but you are not sure if it is safe or you come across water in an emergency and you don’t know if it is safe, that leads me to my second point. These portable water filters are extremely handy and will allow you to clean suspect water that you come into contact with and make it drinkable. These portable micro filters will remove bacteria and protozoa from the water, but won’t kill viruses. If you have a stream, lake, pond, river or well by your house; you will be able to clean the water from those sources using these filters. Each filter will clean up to 500 gallons of water. That is nearly ten 55 gallon drums worth of water. An impossible amount of water for most people to store. I really like or MSR MiniWorks water filter and the Katadyn Combi. They are also great for backpacking and camping.
3. Have water purification tablets. These are very handy to have around and a small bottle will chemically treat up to 25 quarts of water. They work fairly quickly (typically in less than a half an hour) and will kill bacteria, protozoa and viruses. Remember that if you don’t have any of these three things, you can still clean water by boiling it. As a matter of fact, boiling is most effective way to clean water except it won’t take out the floaties like a filter will. The problem with boiling water to drink is it uses a lot of fuel that you may not want to use for that purpose. Also, don’t forget to have portable water pouches in your emergency supplies. They have a 5 year shelf life and are very handy to have around.