Effect of Operational Mode (Constant Flux or Constant Transmembrane Pressure) and Surface Modification On Oil/Water Emulsion Fouling Behavior of Polysulfone Ultrafiltration Membranes

Two methods of operating water filtration membrane systems are: (1) maintaining constant permeate flux by controlling transmembrane pressure (TMP) difference or (2) maintaining constant TMP and obtaining the permeate flux associated with that TMP.  Many literature studies of fouling of water purification membranes report data from constant TMP experiments, and industrial filtration systems often operate at constant flux.  To explore the similarities and differences between operation at constant flux and constant TMP, this presentation shows a comparison of constant-flux and constant-TMP operation of flat-sheet polysulfone ultrafiltration membranes during oil/water emulsion filtration.  For a given set of experimental conditions, the critical flux was determined using a flux-stepping method in constant flux crossflow filtration.  The membrane performance during fouling tests in both constant flux and constant TMP experiments was investigated at fluxes higher and lower than the critical flux.  Membrane resistance as a function of time and as a function of permeate volume during fouling were compared for constant flux and constant TMP operation.  At fluxes lower than the critical flux, the membrane resistance during constant flux and constant TMP fouling were similar.  At fluxes higher than the critical flux, the membrane resistances differed for the two operating modes, in some cases by more than an order of magnitude.  Possible reasons for similarities and differences in membrane behavior for each mode of operation will be discussed. 

Surface modification has been widely studied as a mean to reduce membrane fouling.  Membrane surface coating using dopamine solution (and related compounds such as hydroquinone), which undergoes polymerization under alkaline, aerobic conditions to form polydopamine, has previously been shown to significantly improve membrane fouling resistance to oil/water emulsions.  Polydopamine modification conditions (i.e., dopamine concentration, deposition time, and dopamine solution pH) influence polydopamine film properties, such as thickness, hydrophilicity, roughness, etc.  Results from studies exploring the effect of dopamine and related compounds on the fouling properties of UF and MF membranes used in oil/water emulsion filtration will be reported.