Biodegradability assessment of chemically washed oily sludge using indigenous bacterial isolates

Chemical hot washing can substantially reduce the petroleum content of oily sludge, but residual petroleum hydrocarbons remain and require further treatment. Whether these residual hydrocarbons retain sufficient biodegradability for subsequent biological treatment, and what characteristics of the residual hydrocarbons and indigenous microorganisms are associated with their biodegradability, remain insufficiently understood. In this study, the biodegradability of chemically washed oily sludge was evaluated by integrating sludge characterization, isolation and identification of indigenous bacterial isolates, physiological characterization, emulsifying activity assessment, and petroleum hydrocarbon degradation experiments. The chemically washed oily sludge contained 3.89% petroleum substances, with saturated hydrocarbons accounting for more than 50% of the oil phase. The saturated hydrocarbons were mainly distributed in the C13–C31 range, and the residual hydrocarbons also included polycyclic aromatic hydrocarbons (PAHs). Five indigenous bacterial isolates (S1–S5) with petroleum hydrocarbon degradation potential were obtained and identified as members of the genera Atlantibacter, Aeromonas, Shewanella, and Enterobacter. The isolates exhibited different responses to pH, temperature, and salinity, and their emulsifying activity varied among strains. In degradation experiments with an initial petroleum hydrocarbon concentration of 10,000 mg/L, the five isolates achieved 66.2%–80.2% removal after 10 days, with S5 showing the highest removal. The isolates preferentially degraded alkane compounds, whereas PAHs showed relatively limited degradation. These findings indicate that a substantial fraction of the residual petroleum hydrocarbons in chemically washed oily sludge is amenable to microbial degradation under the tested conditions and indicates that indigenous bacterial isolates may serve as useful microbial resources for subsequent biological treatment.