Many of the filter cartridges look very similar. Many manufacturers are producing a 9 -inch long filter which has nearly become a standard. The cartridges, however, may contain any of the activated carbon forms and the flow direction through the cartridges may be different. The flow through cartridges filled with granular activated carbon is normally longitudinal. The water enters one end of the cylinder and flows out the other.
Cartridges containing block carbon or carbon in a pad, on the other hand, normally have radial flow.
The untreated water enters the outside of the cylinder and flows into the center. Both types, however, are normally hollow cylinders and a close inspection is needed to determine where the water enters and exits. All activated carbon filter media work best when first put in service. With use, the adsorption capacity of the carbon becomes used up and the filter no longer removes as much of the contaminants and will do a poorer job on the most difficult ones. The limits are not clearly defined.