Coupled electrochemical crystallization-electrocoagulation-flocculation process for efficient removal of hardness and silica from reverse osmosis concentrate

The benign electrochemical technologies are the promising methods for the reuse of reverse osmosis concentrate (ROC) by softening and removing silica, whereas its low treatment performance and high energy consumption limit its wide industrial application. In this study, an undivided electrochemical crystallization cell with the extraction of acid anode boundary layer was developed for the production of the alkaline effluent, where the removal of hardness and silica occurred by the homogeneous crystallization reactions. This process was further improved in the electrocoagulation cell because coagulants enhance the removal of crystalline particles and accelerate the settling of the crystals. Under optimal conditions, the system showed a removal efficiency of 90 % for total hardness, 76 % for Mg hardness, and 82 % for silica, with an energy consumption of only 2.4 kWh m - 3 . The removal of Ca hardness in the forms of calcite and aragonite was impeded by the presence of Mg 2+ . But the coexistence of silica promoted Ca hardness removal since the reaction between Mg 2+ and silica with the formation of sepiolite mitigated the inhibitory effect of Mg 2+ on CaCO 3 precipitation. The coupled electrochemical crystallization-electrocoagulation-flocculation technique provides an energy -efficient solution for the removal of hardness and silica from ROC, therefore improving the water utilization in industry.