Abstract
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.
Highlights
The majority of natural waters contain a certain amount of hardness, causing potential fouling, scaling and taste issues
Capacitive deionization (CDI) is applicable to water softening due to the preferential electrosorption of divalent hardness ions over monovalent ions [10,11] and the technology has been investigated for softening brackish waters [12,13,14,15,16,17,18]
The objectives of this work are to 1) extend our previously built membrane capacitive deionization (MCDI) process model [32] to incorporate excluded ion volume effects and compare the respective simulation results to those achieved with the original model, 2) compare selectivity and removal efficiency of hardness ion under varying operating duration, 3) explore the trade-offs between selectivity and removal efficiency of hardness ions in a cut-off constant voltage (CV) mode MCDI and 4) analyze the cell performance, energy consumption and feasibility of applying MCDI to soften waters of various purposes, including industrial cooling tower blowdown water and domestic tap water
Summary
The majority of natural waters contain a certain amount of hardness (i.e., divalent ions, primarily calcium and magnesium), causing potential fouling, scaling and taste issues. Removal of hardness is common in waters for industrial, agricultural and domestic use. Traditional techniques used for softening hard waters include ion-exchange [2], chemical and electrochemical precipitation [3,4], nanofiltration [5,6] and electro-membrane processes [7]. CDI is applicable to water softening due to the preferential electrosorption of divalent hardness ions over monovalent ions [10,11] and the technology has been investigated for softening brackish waters [12,13,14,15,16,17,18]
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