FYI, I ran across this UV light purification system. Great for hiking and the like where there is locally available water, but of unknown purity.
www.steripen.com
some advantages over the Katadyn type; certain disadvantages too (batteries, 4 AA, but can be NiMH for 180 x 16 oz. treatments) $100 Oh, and the water has to be clear before use so you still may need a filter to remove visible dirt.
Yep. Bacteria have to be hit by the light to get zapped.