AbstractThe development of a new semi‐continuous method for operation of a fixed bed contactor is described. The technique, termed reciprocating flow, utilizes density differences to achieve the shunting of solutions in alternating directions through a fixed bed.One of the possible flow arrangements was investigated using a strong acid ion exchanger and the calcium/sodium system. It was found that the technique was feasible for the partial removal (80–60%) of calcium from solutions having concentrations ranging from 400 to 2000 ppm. of calcium carbonate.Regenerant consumption and water requirements compare favourably with present commercial practice. However, a small regenerant leak of 1–14 meq./l. into the solution is unavoidable. Resin bed and column heights can be made a small fraction of those presently used in commercial installations.Analyses of some of the important factors limiting efficiency indicate that highly concentrated solutions may be effectively treated by the reciprocating flow tech