Conventional and Emerging Contaminants in Oily Sludge: Sources, Environmental Behavior, and Treatment Technologies

Oily sludge is a hazardous waste generated during petroleum extraction, transportation, refining, storage, and wastewater treatment. It contains conventional contaminants, including petroleum hydrocarbons, heavy metals (HMs), and salts, as well as emerging contaminants such as microplastics (MPs) and per- and polyfluoroalkyl substances (PFAS). Petroleum hydrocarbons mainly comprise saturated hydrocarbons, aromatic hydrocarbons, resins, asphaltenes, and BTEX compounds (benzene, toluene, ethylbenzene, and xylenes), and are commonly quantified as total petroleum hydrocarbons (TPHs), although polycyclic aromatic hydrocarbons (PAHs) are chemically a subset of petroleum hydrocarbons, they are discussed separately in this review because of their persistence, toxicity, and distinct environmental behavior. Salt-related contaminants are primarily derived from highly mineralized formation water and include chloride-, sulfate-, carbonate-, and bicarbonate-containing salts. Although numerous studies have investigated oily sludge treatment technologies, most have focused on the removal of conventional contaminants such as PAHs and HMs. In contrast, the removal of emerging contaminants, particularly MPs and PFAS, has received much less attention. This review summarizes the sources, environmental behavior, and risk assessment of these contaminants and critically evaluates physical, chemical, biological, thermal, and integrated treatment technologies according to their application scale, removal mechanisms, and limitations. Physical methods mainly achieve phase separation and concentration; chemical and thermal methods can degrade or destroy persistent organics; and biological methods are suitable for biodegradable hydrocarbons but remain limited for microplastics and PFAS. Across all categories, most processes transfer or concentrate emerging contaminants rather than fully eliminating them, and few have been validated at pilot or full scale under realistic sludge conditions. Finally, future directions for low-carbon, risk-based, and resource-efficient oily sludge management are highlighted.