Concentration Polarisation and Osmotic Pressure

Concentration polarisation is the accumulation of solute due to solvent convection through the membrane. Concentration polarisation leads to a reduced permeate flux, either in the form of an osmotic pressure on the feed side (thus reducing the effective transmembrane pressure), or due to the 

If concentration polarisation effects are strong, the solute in the boundar) layer can reach its solubility limit and cause irreversible fouling. Depending on the concentration of different solutes (in this case, calcium and organics), aggregation, gel formation, coagulation, or precipitation may occur after an initial adsorption. The concentration of solute in the boundary layer is determined by the equilibrium between convective transport of solute to the membrane and the backdiffusion of the solute into the bulk solution, due to the concentration gradient. 

The solute mass transfer coefficient is required to calculate the solute concentration at the membrane surface. This mass transfer coefficient is in theory independent of the membrane. Equation (7.6) describes the film model relationship between flux and concentration of solute at the membrane surface. 

Equation (7.7) describes flux as a function of the osmotic pressure, caused by concentration polarisation. 

Concentration polarisation may be of particular importance in NF because of the relatively high flux (compared to RO) and high rejection (compared to UF). Due to low concentrations of solute in surface water treatment applications, concentration polarisation is often neglected. In this study the extent of concentration polarisation is estimated for conditions typical of surface water membrane processing. 

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Overall, the flux ratios (J/Jwo, solution flux after filtration of 120 mL of solution relative to the pure water flux before the experiment) correspond well to the salt rejection. This indicates a concentration polarisation and osmotic pressure effect due to the accumulation of ions at the membrane surface and an increase in cell concentration. This flux decline was fully reversible. The flux of the CA-UF membrane consistently increases after salt filtration, probably due to an increased hydrophilicity after ion adsorption in the membrane. Rejection of calcium is stable and generally does not increase with the concentration in the feed cell. 

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