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). Both 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 to remove weakly ionized impurities in Stage 2. The rejected ions from both stages are removed using separate reject streams, thus preventing hardness precipitation.
FEDI Two-Stage Separation
Hardness is the scaling component and the main limiting factor for feed conditions in a conventional EDI. By incorporating a two-stage separation process with different voltages, the FEDI process can:
- Achieve a higher hardness tolerance by having distinctly different concentrate chambers with separate reject streams, thus reducing the potential of hardness scaling.
- Optimize power consumption by using higher electrical current only where required.
- Ensure the best water quality, continuously, and consistently by removing a major part of the deionization load in the ‘hardness removal zone,’ while residual ionic impurities are effectively removed in the ‘silica removal zone,’ which stays in a polishing mode.