Conventional EDI is limited by feed water hardness, free CO2 and Silica. The EDI recovery process is dependent upon the feed water hardness.
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.
“The FEDI Process was developed by taking into account the limitations of conventional EDI. The patented dual voltage process allows for a higher flexibility and tolerance to inlet water conditions, thus lowering the risk of scaling, and improving the plant's design economics and reliability.”
Stage 2: Silica Removal Zone
Weakly ionized impurities (such as Silica and Boron) are removed in Stage-2. Higher voltage and current in Stage-2 provide efficient removal of the residual weakly ionized impurities, while a significant amount of strongly ionized impurities have already been removed in Stage-1. The higher voltage also ensures that Stage-2 will remain in a highly regenerated state resulting in superior final product water quality. The high pH feed condition in Stage-2 helps with efficient removal of Silica and Boron.
STAGE 1: Hardness Removal Zone
This section, where a significant amount of strongly ionized impurities such as hardness are removed, operates at a lower voltage and current, requiring about one third of the total power. The acidic condition in the concentrate chamber of Stage-1 prevents scale formation, thus giving a higher hardness tolerance to the FEDI process. The patented ion exchange media construction used in the module further reduces the hardness scaling potential.
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 is able to:
- Achieve a higher hardness tolerance by having distinctly different concentrate chambers with separate reject streams and thus reducing the potential of hardness scaling.
- Optimize power consumption by using higher electrical current only where required.
- Ensure the best water quality, continuously & 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.