Modeling Scaling Prevention and Attainable Recovery Using Hypothetical Calcium-Permeable Reverse Osmosis Membranes

In many reverse osmosis processes, extensive pretreatment is needed to remove scale-forming species (e.g., calcium and/or sulfate), only for similar species to be added to the permeate to mitigate pipe corrosion. In this study, we investigate the application of a hypothetical calcium-permeable reverse osmosis (CPRO) membrane that allows only water, calcium, and chloride to permeate, with our models simulating the impact on gypsum scaling, salt passage, water recovery and permeate water quality. As one potential design concept, we utilized known mechanisms of carrier-based membranes to model the calcium and chloride ion separation at various ligand (carrier) concentrations, equilibrium constants, and feed compositions that vary in calcium to chloride ion ratios. Our analysis compares the gypsum saturation indexes between CPRO and RO membranes in equilibrium, coupon-scale, and module-scale scenarios. At high ligand concentration and equilibrium constants, the CaCl2 flux reaches a maximum due to the dependency of the ligand-salt concentration gradient (i.e., the driving force for permeation) on feed and permeate side salt concentrations. In chloride-rich solutions, the excess chloride leads to a high ligand-salt concentration gradient and an exponential rise in Ca2+ passage. The Cl– concentration has a critical role in influencing Ca2+ passage, highlighting an important parameter in the CPRO membrane and process design. Furthermore, with chloride-rich solutions, gypsum scaling could be avoided with little or no pretreatment. While our analysis reflects just one hypothetical design concept for CPRO, this ability to essentially avoid scaling in certain situations suggests that materials and process research in this space is warranted.