Platypus Gravity Works 4L
No more pumping "time-outs" - this water filter lets you sit back and enjoy the day while it filters 4 litres of water in only 2.5 minutes. Great for groups, the bags can store and transport 8L of water. It's easy to use - simply fill the "Dirty" bag with water and hang it in a tree, open the valve, and let gravity pull the water through the filter and into the "Clean" bag. (Make sure you drink out of the right bag.) Just before the dirty bag empties, backflush some clean water through the system by hanging the clean bag higher for a few seconds. This keeps your filter clean.
The 0.2 micron hollow fiber filter removes bacteria and protozoa including giarda, cryptosporidium, E. coli, salmonella and cholera.