Abstract: An apparatus and method for the purification or water using a multistage purification system which can be connected to a source of potable water under pressure. The multistage purification system has a cylindrical pressure vessel supported on a circular leg which is capable of stable disposition on a level surface such as a countertop. The pressure vessel has a top member and a base member connected in sealed engagement. The base member has an inlet for receiving water into the pressure vessel and an outlet for discharging reject water from the pressure vessel. Housed within the pressure vessel is a plurality of purification mediums disposed in concentric disposition. In the core area of the pressure vessel, an interchangeable core member may be removably connected to the pressure vessel. Each interchangeable core member houses an additional medium for purifying water and a pure water outlet through which the purified water passes for use by the user.