ABSTRACT
Nanofiltration (NF) and ultrafiltration (UF) stand out as two important subsets of pressure-driven membrane processes having immense application potentials. But their wider use at a commercial scale is often marred by problems of gradual reduction in permeate flux or membrane throughput over the period of operation. This is predominantly ascribed to the phenomena of concentration polarization and membrane fouling and is perhaps the single most important problem to become a recursive theme for many years now in the membrane process design and operation. The present review attempts to provide an exhaustive overview of various measures adopted by membrane scientists to address this issue. Feed pretreatment, prior to membrane process, includes coagulation, flocculation, adsorption, advanced oxidation or a few of their combinations. In addition, there exist a number of strategies for in-line mitigation of flux while the feed is inside the module. Imparting fluid instabilities and secondary flow by rotating disk modules, turbulent promoters of various geometries, low frequency acoustic irradiation, adopting flow reversal and pulsating flow, air sparging and gas slug, and membrane surface modification are some of the widely used strategies for flux enhancement. Careful manipulation of these method(s) can greatly minimize the concentration polarization and fouling propensity of membranes, thereby rendering process intensification. The literature illustrates that these core issues must be addressed for future developments in membrane systems.
