Emulsion system, demulsification and membrane technology in oil–water emulsion separation: A comprehensive review

The economic development, utilization of petroleum and industrial production have produced a large quantity of emulsified oily wastewater, which will pose a serious threat to human beings and the environment if it is discharged into the environment without proper treatment. The conventional treatment methods for oily wastewater mainly include physical, chemical, and biological methods. Membrane separation technology, as a representative of physical treatment technology, has become a research hotspot. A large number of studies have shown that membrane separation technology has many advantages such as low energy consumption, low cost, high efficiency, and environmental protection. However, the practical engineering application is dominated by chemical treatment rather than membrane separation technology. So, the reason why membrane separation technology has not been widely applied in practice warrants investigation. In this paper, two main issues are summarized after comprehensive analysis: (1) the mechanism of membrane demulsification and separation is still unclear, especially the migration and transformation of emulsifiers during the process of demulsification; (2) some limitations such as membrane stability and membrane pollution are still needs to be overcome. Therefore, this paper deeply analyzes the demulsification mechanism and application barriers of various demulsification technologies with the overview of emulsion system as the starting point. An important emphasis is the application status of membrane separation technology in emulsion separation and the membrane fouling problem. Finally, some important issues in this field worthy of attention in the future development are put forward.