Use the Sawyer lightweight 3-way inline filter water treatment system to filter water directly from a river, lake or stream along the trail or from other non potable sources. Works great as a hydration pack reservoir filter for long hikes where the water source is in quesion. You can even just attach it to a household faucet filter. For Hydration packs, insert the filter inline with the drinking tube, then simply fill your reservoir with unfiltered water and drink away. As long as the filter is inline on your hydration pack you don’t need to worry about cleaning the reservoir. You can also attach a drink tube of your choice (sold separately) to the filter, then submerge the filter into dirty water and drink directly from the source. This is a great alternative to carrying potable water long distances or while out canoeing, kayaking, backpacking, or travelling abroad. Filter cartridge is 1.75 inches x 3.5 inches.