An integrated electrolysis-microfiltration-ion exchange closed-loop system for effective water softening without chemicals input and spent regenerant discharge

In this study, an energy-efficient and industrially scalable undivided electrolysis cell featuring the abstraction of H+ from the anode boundary layer was developed to produce acid and alkaline effluents. Then, the electrolysis cell was integrated with microfiltration and ion-exchange (IEMI) for water softening. In this system, CaCO3 crystals could form in the alkaline effluent from the electrolysis cell and then was intercepted by a porous tubular Ti microfilter in the crystallizer, and weak acid ion-exchange resin in H+-form (WAC-H) was used to further remove the hardness content residual in water. The closed-loop operation mode of the IEMI system with zero discharge of spent regenerant could be realized by using the acid effluent abstracted from the electrolysis cell to regenerate the exhausted WAC-H and then introducing the spent regenerant into the feed water for hardness precipitation again in the IEMI system. Specifically, the total hardness could reduce from 400 to 40 mg CaCO3 L-1 with a removal efficiency of 90 % at energy consumption of 1.9 kWh (kg CaCO3)-1 when an optimal current density of 10 mA cm-2 was applied. Overall, the developed IEMI system paved the ground for scaling up the applications of the electrochemical softening technology in various industrial and domestic fields.