Capacitive deionization (CDI) integrated with monovalent cation selective membrane for producing divalent cation-rich solution

In this study, a newly developed capacitive deionization (MCDI) integrated with a monovalent cation permselective exchange membrane was evaluated for selective removal of monovalent and divalent cations. A variety of solution chemistries including cation composition, total dissolved solids (TDS), and feedwater pH were investigated. The removal selectivity was worsened when TDS concentration increased and pH decreased. Based on the experimental observations in this study, the optimum operating conditions (i.e., time, voltage, and flow rate) were recommended for maximum selectivity. The energy consumption of MCDI process was compared with NF process by collecting NF plant data from the previous literature. It was found that the MCDI process was more effective for selective removal of monovalent cations, and used less energy than the NF process under optimum conditions. Based on the results, a novel concept of employing MCDI process producing Ca²⁺-rich water as a post-treatment integrated with typical NF/LPRO softening process was proposed for preventing the corrosion in the pipe distribution system.