An effective soil slurry bioremediation protocol for the treatment of Libyan soil contaminated with crude oil tank bottom sludge

• Petroleum hydrocarbons are the most widespread contaminants in the environment.
• Bioremediation of soil contaminated with oil sludge is environmentally friendly.
• Pseudomonas species are well-known as potentially microbial hydrocarbon degraders.
• Biostimulation-bioaugmentation treatment is more efficient than other treatments.

Petroleum hydrocarbons are the most widespread contaminants in the environment. Soil contamination with crude oil tank bottom sludge (COTBS) represents a significant risk to both human and environmental health. Current traditional approaches only partially resolve the issue as they are inefficient, expensive, not environmentally friendly and leave behind unwanted products; hence, there is an urgent need to develop an environmentally friendly and cost effective solution to address this issue, especially in developing countries. The aim of this study was to develop a slurry phase bioremediation protocol for the treatment of Libyan soil contaminated with crude oil tank bottom sludge (COTBS) at a minimum soil:water ratio using indigenous bacterial isolates. Two hydrocarbonoclastic bacterial isolates, Pseudomonas spp. (4M12) and Pseudomonas xanthomarina (4M14), were used in three different strategies, namely:- bioaugmentation (BA), biostimulation (BS) and biostimulation-bioaugmentation (BS-BA) to assess their ability to reduce the total petroleum hydrocarbon concentration (TPH) in COBTS contaminated Libyan soil. The results indicated that a substantial reduction in TPH was observed in all three treatments, with the BS-BA treatments showing the highest reduction (96–97%, from 30,703 to 860 and 1020 mg kg−1), followed by the BS treatment (92–93% reduction). In contrast, control microcosms showed only a 17.15% reduction (30,667 mg kg−1) in TPH concentration. The results from CO2 respiration, community fingerprinting (DGGE) and metabolic profiling (Biolog assay) confirmed increased activity in the BS/BA treatment. For example, BS-BA treatments produced the highest levels of CO2 after 90 d incubation (0.075 mg day−1 g soil−1 and 0.072 mg day−1 g soil−1) respectively, some 244–258% higher than the control (0.020 mg day−1 g soil−1). This study confirms the potential benefit of the BS-BA approach and the ability of Pseudomonas spp. isolates to significantly reduce the TPH concentration in soil contaminated with COTBS. This approach can be utilized at the commercial scale, as it has lower water requirements than traditional slurry treatments.