A reverse osmosis filtration device which comprises a flexible elongated membrane bladder spirally wrapped around a purified water duct in a series of spaced-apart coils supported about their edges by perforated spacers. The device is installed in a water supply conduit to extract a volume of purified water under typical household line pressure while allowing the brine-carrying feed water for other normal household usage such as clothes and dishwashing, toilet flushing, showering and bathing.