Warning: California Proposition 65
If the GravityWorks 4.0L Filter System were any easier, it'd have a trail of amorous followers that would never leave it alone. As it is, there are plenty of devotees (purely platonic, of course) who can't get enough of the world's simplest water filtration system. Just fill one of the Platy Zip bags with water, place it above the empty bag, open the hose clamp, and let gravity do the rest. In 2.5 minutes, you'll have four liters of clean water, and if camp is far away, fill the other bag to carry back with you and clean when you need it. Open the hose clamp to dispense water, and shut it to stop the flow. To clean, simply reverse the bags for four seconds, running water through the filter in reverse. That's seriously it.