Some filters are purifiers and some aren't. I've found if your in the high country and clear water, you don't need a purifier. I use the Sawyer filter. $20 at Walmart. Most importantly, it will fit in your shirt pocket at night so it won't freeze. Freezing will wreck a filter. For 4 people, use a gravity system. Aquamira WaterBasics has a very inexpensive gravity system with different filters for different water quality. I purchased it, but haven't used it yet. The larger the bags on both end, the more convenient it is. With 4 people, we always carry a spare filter. A Sawyer is so light you won't notice it in your pack. I made my own gravity system years ago (should have patented it). A wine bag on top with a punched hole through the cap and for a tube. The old clear swamp cooler 1/4" tubing is light and works. Push the air out of the bottom bag, and let water run through the line befor hooking to the filter. With about 5 ft. of tubing, you can filter a quart in 5 minutes. I love Platypus, but they are pricey.