The shrinking water supply together with our continuously increasing demand for this resource has stimulated a renewed interest in reverse osmosis (RO) as a water treatment unit process. A wider use of RO membranes, however, is impeded by membrane fouling. The relevant physicochemical phenomena include concentration polarization, scaling, adsorptive and biological fouling, and colloidal fouling.
The goal of this study was to investigate the coupled effects of salt concentration polarization and colloidal deposition on reverse osmosis (RO) membrane fouling by colloid-bearing feed waters. The fouling can be due to residual particulate matter that passed the pretreatment stage or due to large colloidal loadings in cases when pretreatment fails or simply does not exist. By recognizing possible
interferences between rejected salts and colloids present in the feed and/or deposited on the membrane surface, the proposed work addressed the phenomenon of coupling between colloidal deposition and concentration polarization as main factors limiting the application of RO membranes. We hypothesized that the dynamics of the flux decline in RO systems can be predicted based on the knowledge of
interrelationships between colloidal fouling and concentration polarization effect and that this coupling is indeed an essential feature of the overall fouling process.
Results & Discussion
Filtration of model colloidal particle suspensions of different ionic strengths was conducted using a bench-scale crossflow filtration apparatus that comprised two flat-sheet RO membrane modules connected in parallel. The collected real-time data included permeate flux, permeate conductivity, feed suspension temperature, retentate flow rate, transmembrane pressure, and the mass of particles deposited
on the membrane surface. Based on measurements of osmotic pressure and salt flux in the absence of colloidal foulants, the concentration of rejected salt at the membrane wall and membrane salt permeability constant (B) were experimentally determined. The constant B was then used to calculate the concentration of rejected salt at the membrane wall and the resulting osmotic pressure when colloidal particles
were present in the feed.
This approach allowed for a clear identification of individual contributions of salt concentration polarization and colloidal deposit formation to the permeate flux decline. The results unequivocally pointed to the importance of the two-way coupling between these two phenomena: formation of the colloidal deposit resulted in enhanced concentration polarization of the salt while
the structure (effective porosity) of the colloidal deposits exhibited a strong dependence on the ionic strength.
Baseline rejection was recorded in the experiments when no colloidal particles were present in the feed. A short-term increase in salt rejection upon introduction of colloidal particles into the feed water was observed. We attributed this initial increase to the mixing of the salt concentration polarization layer by colloidal particles. The following decrease in rejection below the baseline value was
attributed to the effect of the enhancement of concentration polarization by the deposited layer of colloidal particles.
Individual contributions of osmotic pressure and colloidal fouling to the RO permeate flux decline can be identified by determining salt permeability constant and measuring salt transport across the membrane. By using this approach, we demonstrate that the two-way coupling between salt concentration polarization and colloidal deposition is essential in determining both permeate flux and rejection.
On one hand, porosity of the deposit and its resistance to the permeate flux are measured to be strong functions of the solution ionic strength. On the other hand, deposition of colloidal particles on the membrane surface influences salt transport to and across the membrane in a complex way.
Formation of the stagnant colloidal deposit results in the hindrance of the back diffusion of salt away from
the membrane surface. Under certain conditions, however, the presence of colloidal particles improves membrane performance. Specifically, the introduction of colloidal particles into the feed results in a short term increase in salt rejection; this increase can be sustained over the long term when feed channel spacers are used.
These findings point to the potential of using particles with low deposition propensity as "mobile mixers" to complement feed channel spacers as means of improving performance of salt-rejecting membranes.
F. Wang, V.V. Tarabara, Coupled Effects of Colloidal Deposition and Salt Concentration Polarization on Reverse Osmosis Membrane, J. Membr. Sci. (2007),
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