An experimental and numerical study demonstrated that completely filling the resin bed is essential if a radial-flow ion-exchange column is to work properly. The first phase of the study developed and verified a finite-element model of both an infinitely long column and finite-height column that was completely filled with resin. The porosity and tortuosity of the support columns of the ion-exchange column were calculated using a least-squares analysis of experimental pressure drop versus flow rate data in an empty column in conjunction with a theoretical analysis based on a layered-cylinder model. Numerical analysis of a partially tilled column (i.e.. one in which the top 2% of the resin bed was empty headspace) demonstrated that approximately 9% of the fluid would flow through the headspace. This fluid would never come in contact with the ion-exchange resin and would thus leave the column untreated. Modifying the column so that the top 10% of the support membranes was impermeable changed the flow lines such that the headspace was filled with a stagnant fluid and that, after this space was filled, all subsequent fluid entering the column flowed through the ion-exchange bed. This provides a method in which one could ensure that all the fluid would flow through the ion-exchange resin and be treated even if the column could not be or was not completely filled.