Last update:
11/09/2004
Surface Mineral Scaling of RO Membranes
Desalination of surface and groundwater, s well as seawater, using reverse osmosis (RO) membranes, is often hindered by the precipitation of bulk mineral salts and membrane surface scaling when operating at high recovery. Scaling of RO and NF membranes results in flux decline and shortened membrane life. The formation of surface scale can be the result of both deposited mineral salt crystals formed in solution and surface crystallization. Fundamental understanding of the nature of surface crystallization, relative to precipitate deposition, is crucial for the formulation of preventive measures to combat surface scaling.
The present program focuses the study of mineral surface crystal formation, down to a nanoscale level, on a number of commercial membrane surfaces as well as surrogate polymeric surfaces. Selected membranes are scaled in a laboratory RO system under a variety of operating conditions. Scaled membrane samples are collected at the end of each run and divided into segments from the entrance region to the exit channel of the membrane. Membrane segments are analyzed via optical imaging and SEM. Surface topology images of the scaled membranes are acquired using a Multimode AFM (Nanoscope IIIa SPM controllers, Digital Instruments).
Surface topology of the scaled membranes is determined by SEM and non-contact AFM imaging to obtain the evolution of surface scale along the membrane (axially in the flow direction). Studies, to date, have revealed axial profiles of mineral scale mass density that increased with distance from the entrance to the RO membrane channel -- consistent with the development of the concentration polarization profile. Our studies have confirmed that surface blockage of the membranes by mineral scale crystals is the main cause of flux decline rather than cake buildup. Surface imaging, at the early stages of scale formation, revealed the presence of mineral salt crystals that appear to originate and protrude from surface crevices or pores. However, as one progresses further towards the exit region, the membrane surface surface crystals increase in size and grow laterally. The present studies are ongoing to establish the link between membrane scaling propensity and surface properties (e.g., surface roughness and surface charge.)
Work in this part of our program is done in collaboration with the California Department of Water Resources and the Metropolitan Water District of Southern California.
Reference
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).
A composite numerical model illustrating the axial time evolution of gypsum scale on RO membrane.
A composite numerical model illustrating the time-dependent axial flux decline profiles due to gypsum scale formation.
Optical image of the axial development of gypsum surface scale Gypsum scale on a reverse osmosis membrane. Teh Figure to the right shows the scale mass density (determined by elemental analysis of segments of the membrane shown to the left).
