Sawyer’s 4-liter dual bag portable ultra purification system physically removes viruses from water. Use this system for one person or a group of people. The convenient bag to bag gravity system doesn’t require any pumping. Simply fill up the grey reservoir with unfiltered water, attach the filter and tube to the clean blue reservoir and gravity does the rest. Once the blue reservoir is full hang by its handle to fill water bottles, containers, and pots and pans from the dispensing cap. Bladders are 20-inches x 10-inches each. Perfect for hunting, camping, fishing, disaster relief and emergency preparedness kits or on adventure travel where potable tap and bottled water are not easily carried or safe to drink.