Comparison of membrane fouling at constant flux and constant transmembrane pressure conditions

Membrane fouling is often characterized in the laboratory by flux decline experiments, where an increase in transport resistance due to accumulation of foulants on and/or in a membrane is manifested as a decrease in permeate flux with filtration time at fixed transmembrane pressure. However, many industrial microfiltration and ultrafiltration applications operate at constant permeate flux, and there are few reports comparing these modes of operation. In this study, emulsified oil fouling of polysulfone ultrafiltration membranes was studied using both constant permeate flux and constant transmembrane pressure experiments. Mass transfer resistance changes during fouling were compared between constant flux experiments and constant transmembrane pressure experiments performed at an initial flux equal to the flux imposed during the constant flux experiment. At low fluxes, the transport resistance and its change with permeate volume per unit area agreed within experimental error regardless of operational mode. In contrast, at high fluxes, the change in membrane resistance with permeate volume per unit area was much higher in constant flux than in constant transmembrane pressure experiments. The threshold flux, defined recently as the flux at which the rate of fouling begins to increase rapidly, separates the regimes of good and poor agreement between the two types of experiments. The weak form of the critical flux, below which spontaneous adsorption is the only significant resistance imposed by foulant, was also observed.