Removal of scale-forming ions by a novel cation-exchange electrochemical system—A review

Abstract

Electrochemical removal of scale-forming ions from hard water is based on the generation of a high-pH environment around the cathode. Although the possibility of electrochemical scale removal has long been recognized, industrial application of this technique is rather restricted. Limitations of the conventional electroprecipitation technology are the high cathode area requirement, the existence of a limiting current density beyond which the precipitation rate remains unchanged and the cumbersome need for periodic cleaning of the cathode surface. A novel electrochemical technique overcomes the above limitations. In conventional equipment, the cathode performs two functions: it generates alkalinity on the boundary layer in immediate contact with the electrode and also serves as a scale deposition surface. The basic concept of the electrochemical cation-exchange system (ECM system) is locating the anode and cathode in two cells separated by a cation-exchange membrane. In this case, a high-alkaline environment is generated throughout the whole volume of the cathode compartment. By transferring the alkaline solution to a separate crystallization vessel containing seeds of the precipitating species, the crystal deposition surface is now the extensive area of the seeds rather than the restricted area of the cathode. The aim of the present paper is to review performance data and capabilities of the ECM technology. The order of magnitude reduction in cathode area requirement is illustrated by CaCO3 scale precipitation data. Further improvement of the ECM technology by integration of bipolar membranes is shown to provide higher energy efficiencies and to enable complete prevention of the undesirable process of chlorine generation. Application of the ECM technique for phosphate removal from secondary treatment effluent containing calcium and magnesium hardness is described. Finally, the ECM technique is shown to have a high potential for the removal of heavy metals from industrial wastewaters.