The main problem in treating oil/water emulsion from car wash waste-water by ultrafiltration (UF) is fouling caused by oil adsorption on the membrane surface acid internal pore walls. This study demonstrates that the addition of bentonite clay can reduce the adsorption layer on cellulose acetate UF membrane, resulting in a reduction of total membrane resistance (R-t). Experiments were conducted to identify and describe three possible mechanisms: (i) bulk oil emulsion concentration reduction; (ii) particle aggregation and (iii) detachment of the adsorbed gel layer by shear force. Adsorption of oil emulsion by bentonite can lead to a significant reduction of bulk oil emulsion concentration, one of the major causes of flux enhancement. Results show that contact of oil emulsion with bentonite forms larger particles resulting in Aux increment. An optimum particle size of 37 mum, corresponds with a bentonite concentration of 300 mg/l and provided the highest Aux. Beyond this limiting concentration, flux improvement gradually declined, possibly due to the formation of packed cake of particles on the membrane surface. The presence of bentonite in the oil emulsion promotes high shear stress which acts against the gel layer. This high shear stress, caused by bentonite particles and cross-how velocity, reverses the adsorbed gel layer to the bulk of the liquid phase.