High recovery desalting of brackish water

Membrane reverse osmosis (RO) desalination of inland brackish water can augment dwindling fresh water supplies. To minimize water production costs and reduce brine management challenges, the limit imposed by mineral salt scaling on RO water recovery must be overcome. The goal of the present research was to develop high recovery brackish water desalting processes using an approach involving multi-step RO with intermediate concentrate demineralization (ICD). Water recovery enhancement by secondary RO desalting was enabled through precipitation removal of scale precursors from the primary RO concentrate. Mineral scale precursors of CaSO4 and CaCO3 were the primary focus of this research given their predominance in inland brackish water.;In order to determine the necessary conditions for scale-free RO operations, a membrane scaling diagnostic approach was developed, utilizing optical imaging to directly characterize and monitor scale formation. These studies served to assess the impact of membrane type, antiscalant, concentration polarization, and solution chemistry on membrane scale formation. Direct scale characterization was demonstrated to be important for ensuring scale-free RO operation. The effects of spatial variation of local concentration polarization and adsorption processes involving precursors of multiple mineral salts were identified as being important factors in formulating cost-effective antiscalant treatment against membrane scaling.;Two ICD methods were investigated: chemical precipitation and precipitation induced by crystal seeding (seeded precipitation). Batch demineralization studies were conducted to elucidate the impact of antiscalant, supersaturation level, crystal seeding, and precipitation sequence on demineralization kinetics. Although chemical precipitation was determined to be chemically intensive, chemical precipitation was a flexible approach, characterized by fast kinetics and capable of scavenging antiscalants. Seeded precipitation, which did not employ dissolved chemical reagents, was kinetically unfavorable due to the adverse impact of antiscalants on precipitation. Nevertheless, sequential combination of chemical and seeded precipitation balanced chemical use with favorable demineralization kinetics.;High recovery desalination process feasibility, attainable range of water recovery, and process economics were assessed through theoretical process analysis, bench-scale demonstration, and data analysis of pilot- and production-scale systems' operation. The present research demonstrated that high recovery desalting of brackish water via multi-step RO with ICD is both technically and economically feasible for field deployment.