Threshold flux in concentration mode: Fouling control during clarification of molasses by ultrafiltration

Clarification operations are vital to extract high-quality products from cane molasses. Ultrafiltration (UF) is very efficient for this purpose, although membrane fouling limits its large-scale application. In this work, five commercially available polyethersulfone UF membranes were evaluated for the clarification of molasses, whereby 30-100 kDa membranes were found to be the most suitable for this purpose. In order to minimise fouling formation during molasses clarification, the threshold flux concept was for the first time applied to dilution-concentration membrane processes. The threshold flux in concentration mode could be determined by correlating the average transmembrane pressure, average fouling rate, and specific energy consumption with the permeate flux at the same feed. An optimal dilution factor of eight was found based on such threshold flux concept and, only above the threshold flux, higher retentate concentrations and longer concentration times resulted in obvious fouling aggravation. Moreover, polydopamine coating and subsequent polymer (polyethyleneimine, chitosan oligosaccharide, alginate dialdehyde) grafting were used to tune the membrane surface properties to further control the membrane fouling. It was found that membranes modified with positive charges were more prone to molasses fouling, where the effect of the membrane surface properties on fouling was significant only above the threshold flux. However, pore narrowing after surface modification may increase the foulant retention and accumulation on the membrane, thus exerting certain influence on the threshold flux. The threshold flux value for the clarification of molasses with 50 and 100 kDa UF membranes was found to be 27-32 L m(-2) h(-1). The outcome of this work not only provides a guide to study threshold fluxes for industrial fluids at high concentrations, but also clarifies the effect of surface modification on the concentration polarisation/fouling formation during the clarification of cane molasses.