Systems, methods, and devices for preparation of water for various uses including blood treatment are described. In embodiments, fluid is passed through a first filtration step which is effective for creating purified water and a pair of ultrafilters placed at the outlet. The ultrafilters are separated by an intervening flow path to prevent grow-through from the outlet end upstream. In embodiments, a recirculation path with an air removing filter helps to eliminate air from the ultimate product water.