Electrochemical softening of concentrates from an electrodialysis brackish water desalination plant: Efficiency enhancement using a three-dimensional cathode

Abstract

The electrochemical softening method to remove hardness from water has not been applied in desalination practice due to a high cathodic area requirement. In this work, the use of a 3D stainless steel wool cathode is proposed to overcome this technical limitation. An extensive comparison between the 3D cathode and a 2D Ti mesh has been presented, showing higher hardness removal for the 3D one. Experiments have been conducted with waters similar to concentrates derived from a brackish water treatment by electrodialysis. In addition, the method has been proved to be efficient for different water compositions in terms of hardness, alkalinity or the presence of an anti-scalant. The main influencing parameters (flow rate and current density) have been studied and it can be concluded that lower flow rates (below 1.2 L h−1) give rise to a better efficiencies and 100 A m−2 is the optimum current density. Moreover, the precipitate was characterised by SEM, EDX and XRD showing that Ca2+ is removed as calcite and aragonite (CaCO3), whereas Mg2+ is precipitated as brucite (Mg(OH)2). Finally, long-term experiments revealed that the 3D stainless steel cathode has a better performance than the 2D Ti mesh, but only at short times.