We took this gravity assisted water filtration system for a test run a few years ago on a staff trip. The verdict: it's the cat's meow.
To be honest, the main reason we love this thing is the fact that we are basically lazy. After years of manually hand-pumping drinking water it is refreshing to simply fill a bag with water, hang it in a tree, and walk away. Three minutes later you have a bag-full of clean water - and no sweat lost.
The entire thing weighs just over 300 grams and packs up smaller than the size of a one litre water bottle. It consists of two bags (a "dirty bag" and a "clean bag", which are clearly labeled). Each bag holds 4 litres of water so if you've got a big group you can hold a total of 8 litres in the system at one time.
The bags are connected by a hose where the hollow fibre microfilter is attached. When filtering, hang the dirty bag higher than the clean bag and simply open the clamp to start the flow. To backflush (which you should do after each dirty bag), hang the clean bag above the dirty bag so a small amount of water flows backwards, clearing the filter of any debris.
You can scoop clean water out of the top of the clean bag or detach the hose from the filter and use it to fill water bottles, controlling the flow with the clamp. Simple!
This system is all you need for most Saskatchewan areas. It filters out bacteria and protozoa, including giardia (the dreaded "Beaver Fever") and cryptosporidium. It does not filter out viruses however, so bring Aquatabs, Pristine or a SteriPen if traveling in areas where this may be a problem.
You can find Platypus Gravity Works 4.0 on our website here.