Thanks, Izzy. Pre-filter is a must for me too. We have lots of fresh water around but also lots of nasties from industrial pollution.  Both sounds like a great idea to me. I got a Frontier Filter today and I carry MicroPUR tabs in my PSK so both it is. Thanks!! 
Neither filtration nor chemical treatment will deal with industrial pollution.  A charcoal filter will remove some, but not all chemical pollutants.  The easiest solution is to carry in your water in that case.  
I use micropure for psk type work (small and light).  For my day pack I use filtration or sterilization (Steripen) in remote areas.  Overseas or below civilization I use BOTH.  
There was a good discussion of water treatment recently.
Respectfully,
Jerry