FEDI^® - MAKING A GOOD TECHNOLOGY EVEN BETTER

The Fractional Electrodeionization (FEDI^®) process is an advancement of EDI. It was developed by taking into account the limitations of conventional EDI described above which, if not addressed properly, lead to scaling and reduced module efficiency and reliability. There are two types of ionic impurities removed in an EDI process; strongly ionized impurities (divalent ions such as Ca, Mg, SO4, and monovalent ions such as Na, Cl and HCO3) and weakly ionized impurities (such as CO2, B and SiO2).

Conventional EDI addresses both the strongly and weakly ionized impurities in the same manner with the application of one current per module. The hardness limitations in conventional EDI essentially exist because of the alkaline conditions in the concentrate compartment of the EDI module; which can lead to hardness precipitation, even at very low values in the feed water.

Both types of ionic impurities require a different driving force (current) for movement and separation. Strongly ionized impurities require less current, whereas weakly ionized impurities require more. Rather than applying one current to the entire module the FEDI^® process differentiates the treatment of weakly ionized and strongly ionized impurities by applying different currents and voltages in a two stage process. This allows a significant portion of strongly ionized impurities, mainly the divalent ions which can cause precipitation at a higher voltage, to be removed in Stage-1. Subsequently, a higher voltage is applied for removing weakly ionized impurities in Stage-2. The rejected ions from both stages are removed, using separate reject streams, thus preventing hardness precipitation.