Theoretical Analysis of Constant Voltage Mode Membrane Capacitive Deionization for Water Softening

Water softening is desirable to reduce scaling in water infrastructure and to meet industrial water quality needs and consumer preferences. Membrane capacitive deionization (MCDI) can preferentially adsorb divalent ions including calcium and magnesium and thus may be an attractive water softening technology. In this work, a process model incorporating ion exclusion effects was applied to investigate water softening performance including ion selectivity, ion removal efficiency and energy consumption in a constant voltage (CV) mode MCDI. Trade-offs between the simulated Ca²⁺ selectivity and Ca²⁺ removal efficiency under varying applied voltage and varying initial concentration ratio of Na⁺ to Ca²⁺ were observed. A cut-off CV mode, which was operated to maximize Ca²⁺ removal efficiency per cycle, was found to lead to a specific energy consumption (SEC) of 0.061 kWh/mole removed Ca²⁺ for partially softening industrial water and 0.077 kWh/m³ removed Ca²⁺ for slightly softening tap water at a water recovery of 0.5. This is an order of magnitude less than reported values for other softening techniques. MCDI should be explored more fully as an energy efficient means of water softening.