Membrane Compaction and Baseline

Very tight membranes such as those in NF and RO may compact when relatively high pressures are applied, and flux may also be influenced by the wettability of the polymer. Compaction can also occur as a result of the repulsive forces between membrane functional groups, which as shown by Braghetta (1995), will be dependent on the solution chemistry. However, in this section only compaction by pressure is considered. 

To determine long term water flux behaviour or compaction due to an applied pressure, MilliQ water was filtered over night and the baseline measured. This flux decline is not due to fouling. Values of 13%, 5%, 10%, and -lP/a were obtained for the TFC-ULP, FFC-SR, TFC-S, and CA-UF membranes, respectively. The filtered volume was 1.5L (about 5 to 10 times the volume used in rejection and recycle experiments) after the compaction of the membranes. 

As a result of these preliminary studies, membranes are compacted at 10 bar for one hour, then the pure water flux 0o) is usually measured at 5 bar for about half an hour. When analysing experimental 

Membrane [m- ] Average Permeability [Lm %- bar ] Average Pure Water Flux at 5 bar [Lm-% ] 

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