If you want to make bugging in a viable and safe option then you have to be able to filter and purify safe, clean water. Without water you will very soon be in a very bad and quickly worsening situation. My gravity filter has the Doulton ceramic filters in it, I intend to replace these with the Berkey Black Filters and to get the reverse osmosis filters at some point in the future also. The Black Berkey filters can by themselves filter a lot more harmful things out of the water than the ceramic filters and the reverse osmosis will then reduce the amounts of Sodium Fluoride and other nasty poisons.
Obviously you would filter water before putting it into the Berkey and in SHTF you would need to store a clean supply of water specifically for cleaning the gravity filter. I have a video on my VidMe channel showing how to clean a gravity filter system here - https://vid.me/rJDk
Be safe and be prepared
Geordie Prepper
all photography © Geordie Prepper Productions