Apparatus and method are disclosed for introducing ion exchange or other particulates into compartments of an already-assembled electrodeionization or comparable stack by modulating a flow of slurry into the compartments with slugs of gas such as air. The air propels liquid through the cells, scavenging ponded liquid so that the particulates (which are retained, e.g., by a strainer or obstruction, in compartment of the apparatus) are deposited as well-packed beds to fill the compartments. Pressurized air filling protocols may deliver discrete slugs of slurry between bursts of air, and the direction of filling may be periodically reversed to diminish particle bed non-homogeneities or settling gradients that arise during transport. The slugs of air may be applied in the direction of slurry flow, in the reverse direction, or both. Different slurries may be transported in a sequence to form layered and packed beds of enhanced utility. An apparatus of the invention has filled compartments of enhanced packing, and is ported or has its passages arranged so that ion exchange material may be filled, or may be replenished by a fluidized flow according to a method of this invention.