Is boiling enough

[Mortblanc]

Also means that even if you do not have a proper container for boiling you can use whatever containment device you can muster (plastic bucket, tarp lined hole in the ground) and drop rocks heated in the fire into the water to get a boil.

And if you are boiling anyway the filter can concentrate on getting the big junk out of the water and does not need to be sophisticated.

Since most of our real purification filters have a specific use life measured in liters there is no need to use up their purification abilities when you are going to boil anyway.