Abstract: The water treatment cartridge is a cartridge adapted to fit into a conventional water filter housing to release a sanitizing dose of a water-treating chemical, and in particular chlorine dioxide, into a water treatment system or unit process. The cartridge is filled with a water treating media, comprised preferably of a mixture of an inert filler material and a chemical composition that releases chlorine dioxide on contact with water. The concentration of the chlorine dioxide released into the water, as well as the duration of the release, may be varied by the selection of the filler material and amount of the filler material in the water treating media.