A 2nd RO is designed to remove 99 percent salt from the raw water, EDI remove almost every possible mineral left, permeate achieve ultrapure condition for use in semi-conductor production. EDI has exceptionally strict requirements on the feed water, with conductivity under 10us/cm. then usually a second level RO is designed to work with EDI.
RO membranes also have strict demand on its feed water quality, SDI, residual chlorine, turbidity, etc. in order to protect RO membranes, a reasonable pretreatment system and an assisting dosing system must be devised to work along. The pretreatment system always consists of multimedia filter, activated carbon filter, ultrafiltration, these physical filters clear out big particles, bacteria, organic macromolecular.