Process evaluation of intermediate chemical demineralization for water recovery enhancement in production-scale brackish water desalting

This study evaluated intermediate chemical demineralization (ICD) as a technique for overcoming mineral salt solubility limits that constrain the water recovery of reverse osmosis (RO) desalting. An empirical approach was developed to determine theoretical cation removal set points for ICD based on predetermined process design and operation guidelines to facilitate increasing the overall water recovery of Colorado River water desalting from 85% (i.e., in the primary RO unit) to 95% via a secondary RO (SRO) unit. Additionally, the study evaluated whether microfiltration (MF) or dual-media filtration (DMF) could serve as the filtration step prior to the secondary reverse osmosis (SRO) unit. The model suggested ranges for ICD influent total-carbonate-to-calcium ratios and ICD effluent pH to simultaneously mitigate barium sulfate, calcium sulfate, and silica scaling. MF was preferred over DMF as the filtration step prior to SRO based on significantly lower filter effluent turbidity and silt density indices. The overall PRO-ICD-SRO system operated for over 550 h at 95% total RO water recovery without evidence of scaling of the terminal RO elements, validating the cation removal set points developed through the ICD model and demonstrating the utility of ICD.

Research Highlights
► Intermediate chemical demineralization improved RO water recovery from 85% to 95%.
► An empirical approach was developed to predict cation-removal set points.
► Proper calcium removal was key to control BaSO4, CaSO4, and silica scaling.
► MF was preferred over media filtration for filtration prior to the second RO step.
► Production-scale testing operated at 95% water recovery without mineral scaling.