Last update:
08/30/2006
Membrane Mineral Scale Formation
Membrane fouling due to mineral scale formation is a problem often encountered in the reverse osmosis membrane desalination of surface water, groundwater and agricultural drainage (AD) water. Concentration polarization can result in the concentration of mineral salt ions exceeding the solubility limit. As a consequence, mineral salt precipitation may occur in solution, and also crystallize directly on membrane surfaces. Deposition of bulk mineral salt crystals onto the membrane surface and direct surface crystallization lead to flux decline and can result in the loss of membrane functionality. Although various mineral salts have been reported to be associated with membrane surface scale formation, scaling by calcium sulfate dihydrate (gypsum) is most common with AD water, surface and ground water. Scale mitigation strategies have generally relied on optimization of operating conditions to reduce concentration polarization of scale formation and the use of antiscalant additives to retard the onset of mineral salt crystallization.
In the present study, a diagnostic approach will be utilized to elucidate the impact of operating conditions on membrane scaling and the direct impact of antisclants on membrane mineral scale formation. Studies will be conducted with calcium sulfate dihydrate (gypsum) and calcium carbonate as model systems and with field water from various locations in California. RO scaling studies will be conducted with supersaturated model solutions in a plate-and-frame reverse osmosis system. Studies will be conducted in a total recycle and also in a permeate withdrawl mode. Flux decline and rejection will be monitored as a function of time. The effect of surface scaling will also be quantified in terms of the degree of membrane surface covered by mineral salt scale. The distribution of scale along the membrane surface will be determined from image analysis of optical images of the scaled membrane surface. The membrane surface will also be characterized by SEM and AFM imaging in the non-contact mode.
Results from the study will assist in the development of a detailed scale formation model for spiral-wound RO elements. The model developed will be geared towards optimizing the operation of RO modules in field applications so as to maximize product water recovery while reducing the potential for scale formation.
For additional information on membrane desalination research check the web site of the UCLA Water Technology Research Center
References:
Shih, Wen-Yi, Anditya Rahardianto, Ron-Wai Lee, and Yoram Cohen "Morphometric Characterization of Calcium Sulfate Dihydrate (Gypsum) Scale on Reverse Osmosis Membranes," Journal of Membrane Science, submitted (2004).
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Fundamental areas: AFM, surface engineering, nano-technology, crystallization, polymerization, macromolecular science, electrochemistry.
