Deposition of calcium carbonate (CaCO3) and calcium sulfate (CaSO4) causes serious processing problems and limits theproductivity of seawater reverse osmosis (RO) desalination. The interactions between CaSO4 and CaCO3 in the dynamicseawater RO systems have been neglected previously because conventional studies mainly focused on individual compounds ormixed compounds in batch systems. The present work evaluates composite fouling behavior of CaSO4 and CaCO3 in a dynamicRO unit. The fouling experiments were performed at constant pressure and velocity by a partial recycling mode which permeatewas withdrawn from the system during the recirculation of retentate to simulate the increasing of water recovery level. Thefouling phenomena were monitored by the decline of flux. Scanning electron microscopy (SEM) with a combination ofelemental dispersive x-ray microanalysis (EDS), and x-ray powder diffraction (XRD) was used to identify the morphologicalfeatures, chemical compositions and crystalline phases of foulants.The interactions of CaSO4 and CaCO3 were investigated by the comparison between individual CaSO4 or CaCO3 fouling andcomposite fouling, and by varying SO42-/HCO3- molar ratio of the feed. A recently developed approach, Scaling PotentialIndex (SPI) incorporated with measured concentration polarization modulus (CP), for assessing the fouling tendency ofinorganic salts on the membrane surface was validated in the dynamic tests. In addition, the effectiveness of two generic scaleinhibitors, polyacrylic acid (molecular weight =2100, PA) and sodium hexametaphosphate (SHMP) were evaluated. Some of thehighlights of the obtained results are as follows:••The precipitation kinetics, morphology and adhesive strength of composite scales were different from pure precipitates••CaSO4 precipitated as gypsum while CaCO3 precipitated as two crystalline phases: calcite and aragonite••The crystalline phases as well as precipitation kinetics were affected by SO42-/HCO3- ratio••Scaling Potential Index was able to predict the fouling tendency of CaSO4 and CaCO3 accurately••The dosage of PA and SHMP was effective to mitigate foulingResults of this work are significant, not only because they have made contribution to the fundamental understanding ofcomposite inorganic fouling in RO membrane systems which was ignored previously, but also because they may play a key rolein the development of scale control.
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