AXEON REF–Series Refillable Cartridges utilize our unique 4.0" configurations that fit most standard size reverse osmosis membrane housing which can be made of fiberglass, stainless steel, or PVC. These cartridges can be filled by the customer with any filtration media of choice for suitability in a wide range of water treatment applications. AXEON REF–Series Refillable Cartridges can be used both before and after the reverse osmosis membrane elements to meet pre-treatment and post-treatment needs, or act as a standalone point of filtration. Features and Benefits: Fits Standard 4.0" RO Membrane Housings Flip Proof Seal Body Materials FDA Approved Made in the U.S.A.