On boundary layers and the attenuation of driving forces in forward osmosis and other membrane processes

Concentration polarization refers to the emergence of concentration gradients at a membrane/solution interface resulting from selective transfer through the membrane. The link between this natural consequence of permselectivity and the attenuation of driving forces across the active layer of the membranes themselves is explored for a range of selected membrane processes. Common features are highlighted through use of the boundary layer Peclet number. It is shown for the first time that one of the unique features of forward osmosis (FO) is that owing to the reverse salt flux there is a maximum Peclet number. There are two paradigmic approaches for modelling flux, one uses the overall driving force (in which case allowance for osmotic effects are expressed as additional resistances) and the other uses the net driving force across the separating layer or fouled separating layer. In FO the effective driving force, even in the absence of fouling, is limited by concentrative and dilutive concentration polarization and by reverse salt diffusion. Having expressed these as additional resistances, their relative importance is established. Comments on other forms of polarization, such as so-called temperature polarization, are included. An interesting link is made between the temperature polarization coefficient and its FO equivalent.