Applied transmembrane pressure water flux

Physical sieving is believed to be the major rejection mechanism for MF with water convecting through the membrane due to an applied transmembrane pressure. The deposit or cake on the membrane can act as a self-rejecting layer, and retain even smaller particles or solutes than would be expected to be removed given the pore size of the membrane ( dynamic membrane ). Thus a fouled MF membrane may have UF rejection characterisncs and flux may decline significantly due to the build-up of this deposit. 

Figure 12.11 Water flux vs. applied transmembrane pressure, AP, for PVDF (GVHP and HVHP) and PTFE (Gore, 0.2-p.m mean pore size) membranes at 25°C. (Adapted from Garcfa-Payo et al., 2000a.)...

Generally, the pure-water flux through a membrane layer, uw is directly proportional to the applied hydrostatic pressure difference (transmembrane pressure, AP) according to Darcy s law as follows ... 

Increasing the (Ap - An) term in Eq. (9a) clearly increases rejection, since the flux of solvent (water) increases proportionally to this factor, while the flux of salt is essentially independent of it, within the accuracy of the approximations of the model. A typical example of such behavior is shown in Fig. 4 as a function of feed pressure at 25°C for a brackish water feed with low salt concentration (0.5 wt % or 0.16 mol %). As expected based on Eq. (9a), when the applied transmembrane Ap equals the... 

The pure water fluxes of all UF membranes used at the transmembrane pressure applied are summarised in Table 6.1. Also shown are other membrane characteristics, including molecular weight cut-off (MWCO), estimated pore size, and measured surface charge. 

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