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Homemade water purifier for a bug out bag

Does anybody know how you can make a homemade water purifier small enough to fit in a bug out bag?  I am looking for something that I can be sure that will make water safe for me to drink without using chemicals.

You can buy a Lifestraw or similar device at gun shows for about $15. If you decide to make something yourself, I wouldn't trust anything less than boiling.
You will need to get something that will filter out up to 3 microns (I think thats right but could be wrong) And use activated carbon in line of the water flow. If your looking to do it the best way for a BOB just have in your bag a water container that you can boil in and fire makeing equipment. Boiling is the only to purify water 100% without chemicals.