Removal of calcium and magnesium ions from reverse osmosis concentrate using a two-stage precipitation with...

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.