Filtration of surfactant-stabilized oil-in-water emulsions with porous ceramic membranes: Effects of membrane pore size and surface charge on fouling behavior

The management of oily wastewater produced in the petrochemical industries poses an immense challenge from both practical and environmental points of view. Filtration using ceramic membranes has been considered a promising technology to remove stable emulsions from oily wastewater; however, the separation performances are severely limited by membrane fouling. In this study, we evaluated the filtration performance of porous ceramic membranes for the treatment of oil/water emulsions. Porous ceramic membrane, having different pore sizes (0.1–1.4 μm) and made from various materials (Al2O3, TiO2–ZrO2, TiO2, and SiC), were tested in a crossflow microfiltration system using oil/water emulsions. The emulsions were stabilized by anionic and cationic surfactants. The fouling caused by filtration is discussed in terms if membrane and emulsion properties. Pore size and surface charge were the two most important prominent fouling factors during the oil/water emulsion filtrations. For membranes with small pores, serious fouling was observed, irrespective of the type of surfactant; however, these membranes exhibited excellent oil rejection. The fouling was caused by the accumulation of oil droplets on the membrane surface, which subsequently led to the formation of a cake layer and/or a continuous oil layer. The effect of surface charge became significant regarding membranes of larger pore sizes, especially when membrane pore sizes were comparable to that of the droplet sizes. When the membrane and oil droplets had the same charge, electrostatic repulsion occurred, preventing the penetration of oil droplets into the pores. Although some of the small oil droplets could enter the pores, they would be trapped at the narrowing and/or flexing inside the pores because droplet deformation was inhibited by the electrostatic repulsion. Therefore, fouling occurred on the membrane surface and inside. As a result, a severe decline in water permeation was observed, together with a moderate oil rejection. By contrast, when the membrane and oil droplets were oppositely charged, oil droplets were deformed and entered the pores, resulting in permeation through the membrane. Therefore, water permeation was high, but oil rejection was sacrificed. These results indicate that both pore size and surface charge of membranes should be optimized to achieve better filtration performance.