Hopefully none of us will be reduced to this,but just in case.A water purifier can be nade out of a 5 gallon bucket,by layering sand and crushed charcoal from a fire.Layer several inches of sand in the bottom of a clean bucket with a layer of about an inch of charcoal on top.you will need several layers of both,ending up with sand on top until about 2/3 of the bucket is full of layering.Then you would fill the bucket to the top with water 2 to 3 times and allow to drain through hole you have punched in the bottom.Your bucket purifier at this point should be ready to use.I have tried this with lake water and it worked.Of course it did'nt taste like bottled water,but I believe it would work in a survival situation,and can be obtained with easily to get items in a survival situation.