The effect of salinity on the pressure susceptibility of the NF270 membrane

To avoid structural changes within nanofiltration membranes during operation, pre-compaction of filtration membranes is usually performed. However, even after pre-compaction, the NF270 membrane has previously been shown to display a level of pressure susceptibility in pure water systems, particularly evident at low pressures. For the first time, this study provides experimental evidence for the effect of salinity on the pressure susceptibility of the NF270 membrane. Permeability was shown to decrease with increasing salinity up to 189 mM MgSO4, with the largest reduction (22 %) observed at the lowest MgSO4 concentration (31.49 mM MgSO4). A significant reduction (35 %) in the membrane susceptibility was also observed following the introduction of MgSO4 to a concentration of 31.49 mM. A mathematical expression, developed for pure water systems, was modified to account for salinity effects and fitted the experimental data well for concentrations up to 0.2 M. These results are explained by compaction of the membrane polymer, due to either charge neutralisation at the membrane surface, or electric double layer compression, or both. However, further increases in salinity had no significant effect on membrane susceptibility, suggesting that salt induced membrane compaction occurs at very low concentrations.