The incoming city water quality and the amount of pure water needed are the two most important factors that will help determine this answer. If your incoming total dissolved solids is less than 350 microsiemens and you are using greater than 3000 gallons of pure water in a month, then it becomes more cost effective to install a Reverse Osmosis Machine ahead of your portable exchange DI system. This will lengthen the life of the portable exchange deionization tanks as the Reverse Osmosis membranes will remove 98-99% of the dissolved ions.