Nanofiltration as pretreatment for scale prevention in seawater reverse osmosis desalination

Seawater contains high concentrations of sparingly soluble salts which can cause scaling of the membrane surface, which in turn can limit the productivity and water recovery potential of seawater reverse osmosis (SWRO). Nanofiltration (NF) pre-treatment of seawater can prevent scaling via preferential removal of scale-forming ions. Several studies have shown that the rejection of scale-forming ions can vary considerably depending on the membrane. The main objective of this study was to test a number of commercially available NF membranes with synthetic seawater in order to compare their performance and identify optimum membranes and operational conditions for the removal of compounds that are known to cause scaling (CaSO4 (s), Mg(OH)2, CaCO3 (s), etc.) in SWRO. Six membranes, supplied by several manufacturers, were tested in a laboratory-scale plant at various trans-membrane pressure differences. For the majority of membranes, the sulphate rejection was superior at 90%, which is very important for the prevention of scaling. Following on from the obtained results, a selection of the most suitable membranes for scaling prevention in seawater desalination can be done.