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Kinetics of Mineral Precipitation and Organics Removal In Brackish Water

Brian C. McCool, Anditya Rahardianto, and Yoram Cohen. Chemical & Biomolecular Engineering, University of California, Los Angeles, 5531 Boelter Hall, 420 Westwood Plaza, Los Angeles, CA 90095-1592

Natural waters often contain natural organic matter (NOM). In many water treatment processes synthetic organic polymer additives are often used to enhance system performance (e.g. coagulants, flocculants, and dispersants). However, it is often desirable to reduce these organic concentrations prior to use. Recent studies of high recovery brackish water desalination suggested that chemical demineralization (an inter-stage process integrated with RO) has the added benefit of reducing NOM concentrations by up to 30%. The removal of organics from water is believed to occur via adsorption during precipitation of mineral salts during enhanced softening via high alkaline dosing. Mineral salt precipitation has been shown to be accompanied by the removal of organics primarily via adsorption. The interactions between organics and crystal surfaces during precipitation processes control the organic removal efficiency and these interactions merit a fundamental study of the removal process and its implication for attaining high product water recovery in RO desalting of brackish water of high scaling propensity.

In order to better understand the phenomenon of organic removal during mineral salt precipitation of RO concentrate, the removal of organics during mineral salt precipitation was studied in a model brackish water system. Laboratory precipitation studies were carried out in batch reactors with on-line monitoring of pH and mineral salt ion concentrations via ion-selective electrodes. Grab samples were taken periodically for DOC analysis, crystal size distribution measurements and imaging of mineral crystals. The results of the study served as a basis for assessing the integration of mineral precipitation with RO desalting so as to attain high recovery and thereby minimize the volume of generated concentrate.

The present results demonstrate that the concentration of organics can be significantly reduced by seeded mineral crystallization. The interaction between the organic and mineral salt precipitation was quantified via the effective rate constants for mineral salt crystallization, while varying the organic concentrations. The results indicate that organic removal during mineral salt precipitation is affected by the evolution of mineral salt crystal size distribution. In addition, the removal of mineral salt precursors was found to be sufficiently high to allow operation of secondary RO desalting while eliminating the problem of membrane scaling.