If you're a backpacker you need a reliable yet lightweight method for purifying and filtering water in the wild. Most commercial methods are either bulky and heavy or need supplies such as salt and/or batteries. Instead you can make a gravity water filtration system using two water bladders, a ~$20-35 Aquamira Frontier Pro filter or likewise and a pack of chlorine dioxide purification tablets.
YouTube user and veteran backpacker Jason Klass demonstrates the light weight, versatility, and low cost (around $US40 for everything) of this setup. Basically you fill your "dirty" water bottle directly from a river, lake, or stream, add a purification tablet and connect your clean water bottle to the Aquamira filter. The other side of the filter is connected to the water resevoir you drink from. Hang the dirty water from a tree branch and keep the clean water on the ground and in half an hour you'll have a couple of litres of clean water to drink.
Why does this system use both a chemical and a mechanical filter? The aquamira filter is good at keeping out dirt and objects that would change the taste of your water but does not filter closely enough to remove most bacteria and viruses. A chlorine dioxide chemical treatment (same as what most municipal water supplies use) will kill 99.999 per cent of all viruses and bacteria and shouldn't change the taste of the water. If you run out of purification tablets while on the trail you can always gravity filter the water into a pot and boil it.