Desalination at many inland locations has increasingly become a necessity for sustaining the quality of current traditional water resources, as well as for exploiting underutilized non-traditional brackish water. The technical and economical feasibility of RO desalting require operation at high product water recovery (>75-90%) to minimize the volume of generated concentrate (i.e., brine) and thus to improve the options for concentrate management. At high water recovery, however, dissolved mineral salts (e.g., CaSO4, BaSO4, CaCO3) may become concentrated beyond their solubility limits and may crystallize in the bulk and onto surfaces (e.g., reverse osmosis membrane). Mineral crystallization may severely limit the attainable water recovery of various desalination processes, including reverse osmosis (RO).

In the present study, a novel demineralization process for desalination concentrate is proposed to enable water recovery enhancement. Chemical demineralization of RO concentrate by precipitation softening (i.e., calcium carbonate precipitation), followed by secondary RO desalting of the demineralized concentrate, has been previously demonstrated to enhance water recovery in Colorado River Water desalting up to 95% [1]. However, application of this desalination approach for brackish water of high gypsum scaling propensity, particularly those with low carbonate content (e.g., Agricultural Drainage water in California's San Joaquin Valley), would require uneconomical chemical usage. For this case, an alternative and more promising demineralization process is proposed, involving accelerated precipitation of dissolved mineral salts, which are kept in meta-stable supersaturation due to threshold inhibition effects of polymeric additives (i.e., antiscalants). The present demineralization process relies on sustainable recycle of crystal seeds and an economical antiscalant deactivation approach with minimal impact on subsequent membrane operations (e.g., does not contribute to fouling). In order to address these critical issues, precipitation experiments, using synthetic RO concentrate solution that contained a generic antiscaling agent, were conducted in a batch reactor. In addition to online monitoring of calcium activity and pH, the evolution of crystal size distribution was measured by an electrical zone sensing method. Experimental results revealed that significant “deactivation” of antiscalants (up to 86% was observed) can be achieved by the present approach. Furthermore, sustainable recycle of crystal seeds was enabled due to antiscalant deactivation. The relationships between antiscalants deactivation, seed crystals concentration, and demineralization kinetics were quantified experimentally. A mathematical kinetic model linking important process variables was proposed to provide a framework for optimization of the process. The results from this study indicate that demineralization of high sulfate RO concentrate by accelerated precipitation is technically feasible and is a promising option for enhancing product water recovery.

[1] C.J. Gabelich et al, High-recovery reverse osmosis desalination using intermediate chemical demineralization, J. Membr. Sci. 301 (2007) 131