We all need water to survive. A healthy adult can only live about 3 days in ideal conditions without water. Water should be at the center of your preparedness plans.
A site called Monolithic Marketplace is selling a very simple and inexpensive water filter that really works HERE. This filter can be purchased as an entire kit, ready to assemble, or you can purchase just the filter and fit it to your own containers. With proper maintenance a ceramic gravity filter like this will last a long time.