What would be a good portable water filter to invest in? I want something that will still be usable many years from now (in case I don't need to use it for a long time). I like the X-Pack: DGR Industrial Products, Inc. :: Cleaners, Waxes and Polishes :: Water Purification & Disinfection :: X-Pack Water Treatment Unit but it only has a shelf life of three years. So if I don't use it by then, I'll have to buy another one every three years. I'd rather buy several good water filters that will last for a long time.