Abstract: An apparatus for treating a fluid to remove at least part of a component or components from the fluid in a reciprocating flow ion exchange system. The apparatus includes a vessel having a central holding chamber that is enclosed at each end by an end cap. A packed bed of ion exchange particles is held within the central holding chamber through the use of upper and lower retaining means so that a particle free cavity is created above and below the particle bed. The flow of fluid is distributed substantially evenly across the end of the particle bed by the pressure drop through the bed when one of the particle free cavities is filled with pressurized fluid.