Methods of treating oily sludge: A review on current trends

Oily sludge is a highly stable water-in-oil (O/W) and oil-in-water (W/O) emulsions that are mainly generated during the refinery and production of petroleum industries. This hazardous scheduled waste contains crude oil, water, solid particles, heavy metals, and heteroatoms. The physicochemical properties of oily sludge differ from one source to another depending on the oilfield type, soil composition, and storage conditions. Treating this ecological threat has been of extreme importance and the focus of thousands of studies. Many technologies and treatment methods such as solvent extraction, ultrasonication, demulsification, microwave irradiation, centrifugation, and freezing/thawing have been introduced to treat oily sludge and retrieve valuable resources. Thus, this article reviewed previous studies and current trends on the treatment methods based on the level of oil recovery and economic feasibility. Having studied different published articles, effective combined treatments were established for future studies.