Abstract: A system for preparing a fluid intended for a medical procedure substantially at the time of use by mixing of at least one concentrate in powder form with water. The system includes a reservoir for a source of water, at least one vessel containing a concentrate in powder form and a concentrate fluid circuit for withdrawing a small quantity of water from the reservoir and passing same through the vessel containing the concentrate in powder form in order to dissolve the concentrate to produce a concentrate fluid, and for then conducting the concentrate fluid to a primary fluid circuit communicating with the reservoir so that the produced concentrate fluid is mixed with the rest of the water withdrawn from the reservoir. In a preferred embodiment, the vessel comprises a self-contained cartridge containing a quantity of concentrate in powder form therein which is suitable for one treatment procedure.