A portable water filtration system that interconnects two, otherwise separate, water bottles arranged vertically such that water flows under gravitational pressure from a first water bottle through a filter configured to capture small particle impurities to a second water bottle and removes small particle impurities before they enter the second water bottle. The filtration system is implemented within a cap that can attach to a variety of standard water bottles, such as NALGENE™ water bottles. A removable connector ring attaches to the cap to connect a second water bottle to the system. When vertically aligned, water flows from a first water bottle through the filter element in the cap to the second water bottle to remove small and large particles.