Filter Cartridges WHAT IS A FILTER CARTRIDGE? Doulton Ceramic Water Filter Cartridge A filter cartridge is a ceramic tube, which is cast open at both ends (OBE). Both ends are then closed off with food grade plastic mounts, which enable the cartridge to fit into a filter housing. The mount at the base of the tube is completely closed. The mount at the top of the tube has an opening through which filtered water flows after it has passed through the ceramic. Particles of dirt, bacteria etc. are trapped on the outside of the cartridge.