Bioremediation of a Drilling Waste-Contaminated Soil; Biotreatability Assessment and Microcosm Optimization for Developing a Field-Scale Remediation Process

Background: Petroleum drilling and exploration operations generate a significant amount of oily wastes. Comparing with physical and chemical remediation methods, bioremediation is cost-effective, highly efficient, and environment-friendly technology. Objectives: The aim of this study was to evaluate biological treatment efficiency for an oil-contaminated soil prepared from an abandoned drilling mud pit located in the Khangiran region, Khorasan Razavi Provirvce, Iran. Materials and Methods: Following chemical analysis of the contamination, isolation and phylogenetic analysis of the predominant hydrocarbon-utilizing bacteria were performed in order to elucidate microbial diversity. Afterwards, the full factorial design was used to optimize the concentration of nitrogen (0, 100, 250. and 500 ppm) and phosphate (0, 10, 25, and 50 ppm) in the soil. The prepared microcosms were incubated at 30 degrees C for a 60-day period. The total heterotrophic bacterial content was enumerated during bioremediation process. The total petroleum hydrombon concentration was analyzed after 60 days to evaluate biodegradation extent. Results: The mud pit was mainly contaminated with the weathered diesel compounds ranging from C-12 to C-30. Investigation of the indigenous microflora showed that a variety of hydrocaibon-utilizing bacteria were present in the untreated soil dominated by Actinobacteria including the genera Gordonia. Nocardia, Nocardioides, and Rhodococcus. The full-factorial soil microcosms' nutrient optimization indicated that nitrogen amendment could be the most effective at 250 ppm, while excess nitrogen would lead to a diminished microbial activity. The main effect statistical analysis demonstrated that addition of phosphate did not cause a significant improvement in the remediation process. In the optimal nutrient conditions, the total microbial count increased from 1.76 x 10(7) CFU.g(-1) to 1.24 x 10(9) CFU.g(-1) and led into 75.4 % w/w biodegradation in two months, while in the control microcosm without nutrient amendment only 36.6 % of the depletion was observed. Conclusions: These results indicate that biostimulation could be a promising candidate for a successful field-scale bioremediation process in the studied drilling mud pit.