As you can imagine, there are some ions that are easily removed by the EDI process and others that are more difficult to remove. Since the EDI module works by removing charged ions from the feed water, you can see how a highly charged ion would be easily removed. Weakly charged ions are more difficult for the EDI module to take out of the feed stream. Again, this is where the design of the RO unit plays a major factor in the EDI system's performance.

Ions with a strong ionic charge, such as Na+, Cl-, Ca+2, H+, and OH-, are easily removed in the first portion of the resin bed, which is referred to as the "working" bed. Ions with weak ionic charges, such as CO2, SiO2 (Silica), and H3BO3 (Boron), are difficult for the EDI to remove and require more residence time in the second portion of the module called the polishing bed. To ensure that all of the ions are removed, the resin bed should be maximized for removal of whichever ions are more prevalent in the feed water. For example, if it is expected that CO2 levels are going to be high in the RO permeate, the EDI should be maximized so that the working bed is small and the polishing bed is large. This means proper RO design to remove as much of the "easy ions" is critical. Conversely, if CO2 and Silica are expected to be low, then the working bed can be expanded to remove a higher load of "easy" ions.