Removal of hardness ions from tap water using electromembrane processes

Performances of electromembrane systems were investigated for removal of the hardness materials in a tap water. Six cells of ED, EDR, and EDIR systems with an electrode area of 12.5 cm ´ 8.0 cm were operated. The results showed scaling formation due to water dissociation on the surface of cation exchange membrane, but EDR and EDIR enabled to avoid the scaling problem. EDIR operation lowered the resistance and power consumption compared to EDR due to the conductance of ion exchange resins. This study demonstrated the feasibility of EDR and EDIR processes for water softening.

This study showed the performances of EDR and EDIR systems as an alternative process of ion exchange for removal of hardness ions in tap water. Operation of an EDR system for the removal of hardness material could overcome the scaling problem of ED on the cation exchange membrane surface. The high removal efficiency and high recovery rate of EDR and EDIR process showed the feasibility as a softening process. EDIR operation showed the reduced potential drop compared to the EDR system due to the increased conductivity in the system. The EDIR operation showed cost effectiveness by reducing power consumption. However, pH and conductivity varied in an Current efficiency or removal rate (%) illustrated in Fig. 9. In the EDIR system, a bipolar interface was formed on the contacting region between cation exchange membrane and anion exchange resins. The water dissociation in an EDIR system even at a low current density becomes dominant on the cation exchange membrane due to a lower limiting current density (LCD) than that of anion exchange membrane. Moreover, a high potential drop in the bipolar interface dissociate water molecules into hydrogen and hydroxide ions [11]. The hydroxide ion on the surface of cation exchange membrane in the diluate compartment induces increment of pH and conductivity. The pH rise under the reversal condition is also explained by the same water dissociation between cation exchange membrane and anion exchange resins as shown in Fig. 9. Fig. 10 shows the removal rate and current efficiency for the operation of EDIR. Removal rates of EDIR were similar to the EDR process while scale formation was avoided. The total power consumption was calculated to examine the cost effectiveness of each process using the following Eq. (3): Current efficiency Removal rate of Mg(II) Removal rate of Ca(II) Removal rate of Na(II) Operation time (h) Fig. 10. Current efficiency of EDIR process.