In pressure-controlled membrane filtration (e.g. in the field of wastewater treatment), mass transfer across the membrane is affected by multiple resistances before and behind the membrane surface. In the case of asymmetric membranes used in ultrafiltration and reverse osmosis, one has a combination of local transport resistances. The resistance of the active layer is mostly rate-limiting. In reverse osmosis and in some cases of ultrafiltration, permeate fluxes are low and concentration polarization is insignificant because of the high diffusive back-flow of the small molecules. All components, even those at the surface of the membrane, remain soluble; here the membrane itself controls the mass transfer.

In every case where a concentration polarization profile can occur the effect can be controlled more or less by the flow conditions along the surface of membrane and it is therefore influenced by membrane module design (Section 12.4).

We will now discuss resistances to mass transfer and the most common model for the calculation of the concentration c3 at the membrane surface in combination with mass transport mechanisms for solution-diffusion and the pore model.

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