FOULING AND RETENTION OF NANOFILTRATION MEMBRANES

Nanofiltration membranes retain substances with molar masses higher than similar to 300 g/mol and multivalent ions. The retention characteristics depend much on how much free volume there is in the membranes, which can for some membranes be related to the Aux. In this study, fouling and retention of four different nanofiltration membranes (NF40, NTR-7450, NTR-7410 and NTR-7250) were followed using different model substances. It could be seen that multivalent salts were retained better than small organic substances, but retention was much depending on the pH. As nanofiltration membranes have characteristics of both ultrafiltration as well as reverse osmosis membranes,their fouling characteristics are also rather individual. The tighter membranes foul less. Multivalent salts foul mostly only when together with organic compounds, e.g., lignosulfonate containing calcium fouls more than if it contains sodium as counterion. Maize starch containing small amounts of protein fouls more than potato starch. The NTR-7250 membranes could be modified by chloride ions at high pH to give better flux without loss of retention. Anionic model substances were retained better and fouled less due to charge repulsion effects especially at high pH