A water purification system and method for the use and maintenance service of said system, whereby a combination of a primary ion exchange resin element, a secondary ion exchange resin element, a water quality monitor placed so as to indicate that the primary resin element is exhausted and the secondary resin element is being used, activated carbon elements, sterilizing or sanitizing filters, and sanitizing components such as UV radiation, or other sanitizing technologies, is used to produce purified water for consumption or other intended use. Both the primary and secondary ion exchange resin elements are comprised of one or more mixed bed ion exchange resin unit(s), or one or more paired set(s) of anion and cation exchange resin units arranged in series. The system's redundant ion exchange resin configuration allows purification to continue after the resin in the primary resin element is exhausted and can no longer remove unwanted impurities in the water. A monitor is used to detect when maintenance service of the primary resin element is required. The redundant or secondary resin element continues to remove impurities from the water, allowing the user and/or maintenance service provider flexibility and tractability in scheduling system maintenance without significant interruption of on-line supply of purified water.