Removal of Calcium and Magnesium Ions from Reverse Osmosis Concentrate Using a Two-stage Precipitation with Carbonation Process

Abstract The reverse osmosis (RO) technique has been extensively employed in the advanced treatment of industrial water and wastewater. However, this process results in the production of a significant quantity of reverse osmosis concentrate (ROC), which contains high levels of salinity and organic contaminants, thereby posing serious environmental problems. This study reported a two-stage precipitation process utilizing quicklime (CaO) and caustic soda (NaOH) in conjunction with air blowing (carbonation) for the removal of Ca²⁺ and Mg²⁺ from real brackish water ROC of factory. In stage I, the CaO precipitation-carbonation process was employed to eliminate the majority of Ca²⁺ from the ROC, while leaving Mg²⁺ virtually unaffected, yielding high-purity CaCO₃ precipitates. In stage II, the NaOH precipitation method was utilized to eliminate the remaining Ca²⁺ and Mg²⁺ from the ROC. It was demonstrated that under optimal conditions, the removal rates of Ca²⁺ and Mg²⁺ exceeded 97%. Finally, the characterization of precipitates demonstrated the generation of high-purity CaCO₃ precipitates in stage I, as well as the formation of CaCO₃ and Mg(OH)₂ precipitates in stage II. The results confirmed the feasibility of employing the two-stage precipitation with carbonation process to economically treat ROC and enable its reuse, offering valuable insights for the treatment of industrial wastewater.