Advances in the conversion of oily refinery sludge waste into biofuel: The role of efficient microbes with high tolerability to bio-ethanol

ABSTRACT The conversion of oily refinery sludge into biofuel presents a promising route for sustainable waste management and renewable energy production. Microbial-based processes offer a cost-effective and environmentally friendly approach to transform complex hydrocarbons into valuable biofuels such as bioethanol. This review summarizes recent advances in refinery sludge valorization, with a focus on hydrocarbon-degrading and ethanol-tolerant microorganisms integrated with pretreatment strategies that enhance biofuel yield and process stability. Analysis of the selected studies indicates that microbial consortia comprising genera such as Pseudomonas , Rhodococcus , Sphingomonas , and Bacillus consistently achieve significant reductions in total petroleum hydrocarbons (TPH) and chemical oxygen demand (COD). While several sludge-derived strains demonstrate a high ethanol tolerance, in some cases up to 30% (v/v). Despite these advances, the review identifies key limitations, including variability in sludge composition, inhibitory compounds affecting microbial performance, and the lack of validation beyond laboratory and pilot-scale systems. Techno-economic and lifecycle assessments suggest that refinery sludge valorization can reduce greenhouse gas emissions and support circular economy goals; however, challenges related to process scalability, operational stability, and cost-effective downstream recovery remain significant. Overall, this review highlights that while microbial-based conversion strategies show strong potential, the integration of strain engineering with process optimization and industrial-scale validation is essential for the practical implementation of sludge-to-bioethanol systems.