Influence of membrane and colloid characteristics on fouling of nanofiltration membranes

Colloidal fouling is still one of the major impediments for the implementation of membrane processes, e.g. in the purification of surface water. Effects of fouling for several representative nanofiltration membranes during filtration of several types of colloids in different circumstances (pH, ionic strength) are presented in this study. Four different colloidal solutions (silica–aluminum) were selected to investigate both the influence of colloid size and colloid charge.The most pronounced colloidal fouling was observed for the hydrophobic membranes, for which a dense cake layer was formed on the entire membrane surface. In addition to the hydrophobicity, membrane roughness also plays a secondary role during filtration of small colloids. These fouling effects were proven both by scanning electron microscopy (SEM) and atomic force microscopy (AFM) measurements. Not only the membrane, but also the colloid characteristics are crucial to control membrane fouling. Large colloids with a negative surface charge seem to be the most beneficial for a nanofiltration process at neutral pH. Changing the pH changes the interaction forces between membranes and colloids and also between the colloids themselves, which is in full agreement with the DLVO theory. Another way to adapt the interaction forces is to increase the ionic strength of the colloidal solution. It was shown that a higher ionic strength leads to more membrane fouling in case of membranes with small pore sizes.