Selective salt recovery from reverse osmosis concentrate using inter-stage ion exchange
A treatment process for reverse osmosis concentrate was developed to recover selected salts and increase fresh water recovery. The process utilizes cation and anion exchange to exchange all ions in the concentrate stream for sodium and chloride. The sodium chloride stream, with reduced scaling potential, can be treated further by a second reverse osmosis stage or another volume reduction technique to recover additional fresh water. The resulting concentrated sodium chloride stream can be used as ion exchange regeneration solution. Regeneration solutions from the cation and anion exchange columns are mixed to precipitate specific salts. In order to demonstrate the concepts behind this process, several phases of research were conducted. First, ion exchange batch isotherms were measured to characterize resin selectivity under ionic strength conditions common to reverse osmosis concentrate. A mathematical model was developed in which these isotherms were used to predict breakthrough curves. Regressions relationships developed from the batch tests were used in conjunction with the model to predict the number of bed volumes to breakthrough of calcium, magnesium, and sulfate. Model and regression relationships were verified by a series of column experiments. Attempts to separate ions during the regeneration process were not successful. Ion distribution within a loaded column was characterized using a specially designed column from which resin samples could be taken along the longitudinal axis. Third, simulated cation and anion regeneration solutions were combined to precipitate selected salts which were analyzed to determine their constituents. Finally, the process was tested with a continuously operated pilot scale system. Mixing of simulated and pilot generated cation and anion regeneration solutions resulted in precipitation of calcium sulfate when mixed at pH below 4.5 and mixed carbonate salts when pH was not adjusted. The pilot system can produce 12 kg of precipitate per cubic meter of regeneration solution and recover approximately 45% of the calcium and 28% of the sulfate.