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 . influent water is purified by passing through a plurality of purification mediums before discharge for use . the purification capabilities of the system can be tailored to the user &# 39 ; s need and a soiled or defective core member may be replaced without compromising the integrity of the system by simply changing interchangeable core members .