Electrochemical Ion-Exchange Regeneration and Fluidized Bed Crystallization for Zero-Liquid-Discharge Water Softening

This research investigated the use of an electrochemical system for regenerating ion-exchange media and for promoting the crystallization of hardness minerals in a fluidized bed crystallization reactor (FBCR). The closed-loop process eliminates the creation of waste brine solutions that are normally produced when regenerating ion-exchange media. A bipolar membrane electrodialysis stack was used to generate acids and bases from 100 mM salt solutions. The acid was used to regenerate weak acid cation (WAC) ion-exchange media used for water softening. The base solutions were used to absorb CO₂ gas and to provide a source of alkalinity for removing noncarbonate hardness by WAC media operated in H⁺ form. The base solutions were also used to promote the crystallization of CaCO₃ and Mg­(OH)₂ in a FBCR. The overall process removes hardness ions from the water being softened and replaces them with H⁺ ions, slightly decreasing the pH value of the softened water. The current utilization efficiency for acid and base production was ∼75% over the operational range of interest, and the energy costs for producing acids and bases were an order of magnitude lower than the costs for purchasing acid and base in bulk quantities. Ion balances indicate that the closed-loop system will accumulate SO₄^2–, Cl^–, and alkali metal ions. Acid and base balances indicate that for a typical water, small amounts of base will be accumulated