Nanofiltration for Seawater Softening: An Emerging and Economically Viable Process

Abstract

Nanofiltration (NF) to remove sulfate from seawater is an established but not well-understood process. The selectivity of NF membranes to separate divalent hardness ions from seawater is influenced by operating pH and NF membrane morphology. In this respect, NF membrane morphology is not as generic as reverse osmosis (RO) membrane skin layer morphology. For applications that require a high hardness rejection, it is important to prevent NF membrane fouling and to operate at low pH, preferably 6 or less. Ultrafiltration (UF) pretreatment delivers consistent feed quality to NF membranes to ensure continuous long-term operation without membrane fouling. Under these conditions, NF is economically viable compared with conventional softening processes. To understand differences among various NF membranes and optimize an operating process envelope, the authors advocate a pilot trial in applications in which NF membranes are of economic value.