Abstract: An apparatus and method for operating a system of two or more fluid treatment units connected in parallel, which fluid treatment units need to be regenerated as they become depleted, whereby a method is provided for determining when to initiate a regeneration and which unit to regenerate as well as an apparatus for implementing the same. The decision when to regenerate is based on the number of fluid treatment units in service and the fractional capacity of each fluid treatment unit in service. The fluid treatment selected for regeneration is the one with the least or nearly the least remaining capacity. The apparatus comprises a plurality of fluid treatment units; control valves to selectively connect each fluid treatment unit to a source of untreated fluid, a destination for treated fluid, and a source of regenerant; and a master controller to automatically implement the method of operation by controlling the valves appropriately.