The influence of pH, salt and temperature on nanofiltration performance

The influence of pH, KCl and temperature on the performance of an Alfa Laval NFT-50 nanofiltration membrane was evaluated by glucose and KCl retention measurements at constant flux. It was found that at constant temperature an increase in pH in the absence of KCl reduced the water permeability, and this was correlated to the electroviscous effect. The selectivity decreased with increasing KCl concentration and the decrease became more evident with increasing pH. The decrease in selectivity could be related to membrane swelling, and it appears that pH had no effect on swelling other than enhancing the effect of KCl. The influence of KCl and pH on membrane performance was evaluated as a function of temperature after reaching steady state at 50 °C. The degree of swelling was higher at 50 °C than at 20 °C, due to an increase in polymer flexibility with increasing temperature. The decrease in selectivity with increasing temperature was less for KCl than for glucose, showing that the charge effect was influenced less by temperature than diffusion through the membrane.