Internal concentration polarization ICP

All the currently available FO membranes have asymmetric structures consisting of a dense rejection layer (i.e., active layer) and a porous support layer. The water flux in FO is strongly affected by the internal concentration polarization (ICP) inside the membrane support layer (Fig. 14.2). ICP gives rise to severe loss of the effective osmotic driving force and thus the water flux. This can be either due to the dilution of the draw solution within the support layer when the membrane is placed with its active layer towards the feed solution (AL-FS) or the concentration of feed solution within the support layer when the membrane is placed with its active layer towards the... 

Figure 14.2. A schematic representation of the internal concentration polarization (ICP) in FO processes. Modified from Cath et al (2006).

Conventional FO membranes have a similar structure with UF, NF, and RO, consisting of a top thin barrier layer and a thick support layer. The drawback is due to the phenomenon of internal concentration polarization (ICP), caused by the tormous and dense support layer hindering the compensate diffusion passing through the support layer. ICP leads to a lower water flux, and it gets worse with solute concentration increase [94]. Most of the conventional FO membranes have water flux rate of less than 25 L/m h. Loeb and co-workers described that the appropriate support layer for FO should have low tortuosity, high porosity, and a thin structure [95]. 

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