Experimental Study on Softening High-Calcium Sulfate Reverse Osmosis Concentrate Using Induced Crystallization Method

Reverse osmosis (RO) concentrate often contains high levels of sulfate and calcium ions due to the use of antiscalants, leading to significant calcium sulfate supersaturation and creating favorable conditions for induced crystallization. This study utilized a combination of static and dynamic experiments to investigate the key factors influencing the removal of calcium sulfate from RO concentrate via induced crystallization. The static experiments examined the effects of seed crystal concentration, stirring speed, reaction temperature, and the molar ratio of SO42− to Ca2+ on removal efficiency, with response surface methodology (RSM) employed to analyze the interactions among these factors. In the dynamic experiments, gypsum particles were used as seed crystals in a fluidized bed reactor to study the impact of initial seed crystal dosage and influent flow rate on the removal performance. Optimization strategies for stable operation were also explored. The static experiments revealed that seed crystal concentration was the most critical factor affecting removal efficiency. Under optimal conditions, the calcium ion concentration in the treated water could be reduced to 453 mg/L, achieving a removal rate of 63.8%. In the dynamic experiments, the effluent calcium ion concentration was reduced to 724 mg/L, with a removal rate of 52.5%. However, prolonged continuous operation led to a gradual increase in effluent calcium ion levels, which could be mitigated by recycling seed crystals from the settling zone back to the reaction zone. Characterization of the induced seed crystals and simulation calculations demonstrated that Ca2+ and SO42− reacted to form calcium sulfate crystals, primarily as CaSO4·2H2O, which adhered to the seed crystal surfaces. The growth of the seed crystals, indicated by an increase in particle size, correlated with the volume of water treated. This study provides valuable insights and data for the application of calcium sulfate-induced crystallization as a method to reduce sulfate and calcium ion concentrations in RO concentrate, offering a viable approach to water softening and resource recovery.