The Platypus GravityWorks 4. 0L Filter System has earned a legion of fans since coming on the scene, and with a full complement of backcountry-ready features, it's not hard to tell why. The simple setup comes with two four-liter Platy Zip reservoirs, one for our dirty water and one for our clean water, and all we have to do is place the bag full of cool, refreshing stream water above the empty bag and let gravity do the rest. After 2. 5 minutes, we'll have four liters of clean, filtered water ready to drink, and if it's a trek back to the campsite, we can re-fill our dirty bag, shut the hose clamp, and we'll have a refill ready to go so our whole group can stay hydrated. To clean the filter, we simply need to reverse the bags for four seconds, which flushes away built up sediment, and our filter is ready to go once again.