Surfactant-stabilized oil separation from water using ultrafiltration and nanofiltration

The treatment of oily wastewater containing well-stabilized oil emulsions remains a challenge. As gravity-based separation methods cannot effectively remove emulsified oil droplets with sizes below 10 mu m, water polishing steps need to be applied to reduce the organic load of the treated effluent. Membrane-based separation processes ensure high permeate quality, but are prone to fouling when treating oily wastes. Surfactants play an important role in producing well-stabilized oil emulsions, but there has been limited work on the effect of surfactants on membrane fouling. To fundamentally understand the surfactant effect on fouling, we studied the fouling of model oil (hexadecane) emulsions stabilized by anionic, cationic and nonionic surfactants in a crossflow filtration system using either ultrafiltration (UF) or nanofiltration (NF) membranes. For this, we investigated the impact of membrane surface and emulsion properties on the different fouling mechanisms observed in our experiments. OF filtration experiments revealed that emulsions stabilized with cationic surfactant quickly fouled negatively charged UF membranes due to electrostatic attraction, while anionic and non-ionic surfactants stabilized emulsions experienced less fouling. In NF filtration tests, membranes exhibited exponential fouling when filtrating all types of surfactant stabilized emulsions. Permeate quality confirms that the NF process achieves better rejection of surfactants than the UF process. However, cationic surfactants are able to pass through NF membranes because of their positive charge. When salt is added to the water, electrostatic forces collapse, and the differences between the surfactants are quenched. Our experimental results combined with theoretical calculations reveal that initial membrane fouling by emulsified oil drops is dominated by a cake layer formation. However, once a critical pressure drop across the cake layer is achieved, membrane wetting occurs, which results in irreversible membrane fouling in both UF and NF.