The treatment of oily wastewater that contains well-stabilized oil emulsions remains a challenge. As gravity-based separation methods cannot remove emulsified oil droplets with sizes below 20 µm, 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. UF 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 were fouled exponentially when filtrating all types of surfactants stabilized emulsions. The permeate quality confirms that NF process achieves better permeate quality than UF process because of its ability to reject surfactants. However, cationic surfactants are able to pass through NF membranes because of their positive charge. Our experimental results combined with theoretical calculations reveal that emulsions fouling in UF processes are due to emulsion penetration into membrane pores, while NF membrane fouling is dominated by membrane surface wetting.

Surfactant-stabilized oil separation from water using ultrafiltration and nanofiltration

253rd National Meeting (2017)

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