Understanding concentrations, saturation indexes, and induction times of a batch reverse osmosis system
Reverse osmosis (RO) is one of several methods for water desalination. Over the years the energy efficiency of these systems have increased greatly and many variants of RO have emerged. The method that provides the most energy efficient system is batch RO. Batch RO provides the ability to recycle the retentate that is produced until it reaches a concentration where it can no longer be used. Additionally, batch RO can keep the pressure and osmotic pressure of the system balanced to prevent the need of external energy recovery devices. One challenge about this system is the supersaturation levels which can occur in the recycled retentate. With a high enough concentration nucleation will begin to occur causing scaling in the membrane reducing system efficiency. In this study, a 15-hour batch cycle is observed to analyze the concentration of calcium sulfate at the membrane, the saturation index of gypsum, and the potential induction times over the trial. After the sixth hour in the trial, a batch developed a concentration at the membrane of about 10 mM higher than any other batch creating a supersaturation solution with an induction time near the length of time the batch ran for. Subsequent batches revealed that each took almost twice as long to complete, suggesting that scaling of the membrane had occurred during the batch with the highest saturation index. These results help explain why: concentration at the membrane, saturation index, and induction times are important metrics to consider to run a batch RO system more efficiently.