Abstract
An effective bioaugmentation system for oil-contaminated soil under low-temperature conditions was developed with a rotational slurry bioreactor. Mixtures of two Rhodococcus oil-degraders, strain A and C, which are officially permitted to be used in bioaugmentation in Japan, were inoculated and A-fuel oil was added to a final concentration of 2500 and 5000 mg/kg-slurry. Decomposition tests were carried out for the inoculated samples and non-inoculated samples by rotating at 15 °C, the annual average temperature of Japan. The residue of A-fuel oil and the number of bacteria were measured every two days. After 6 days of treatment, more than 95% of the oil was removed in the inoculated samples, which was more than three times faster than a previous degradation experiment without rotation. A semi-continuous treatment was performed by removing 90% of the treated slurry, then adding the same amount of contaminated slurry into the system without additional degraders. Ninety-four percent of A-fuel oil was successfully degraded after 6 days by this repeated treatment. This could drastically reduce the cost of preparing the degraders. Strikingly, semi-continuous treatment showed oil removal in the non-inoculated samples, indicating that the rotational slurry conditions could efficiently promote biodegradation by indigenous degraders. Our rotational slurry bioreactor accelerated the removal of oil contamination without adding further degraders provides an efficient and cost-effective method of removal of A-fuel oil using a semi-continuous system, which can be used in practical applications in areas with a cooler climate.
Highlights
Soil contamination caused by oil spills is a global problem [1,2]
To assess whether the slurry treatments and rotation could effectively work on bioaugmentation at low temperatures, we developed a lab-scale rotational slurry bioreactor imitating a concrete mixing system that can be scaled up and used in the field to remediate contaminated soils
To improve the degradative treatment of A-fuel oil in the above semi-batch, we reduced the initial amount of inoculation from 108 to 106 CFUs/g-slurry and omitted the 24 h interval for the and third cycle, the removal rates of the oil were at a similar level to the above treatment with the inoculation of 108 CFUs/g-slurry (Figures 4A and 5A)
Summary
Soil contamination caused by oil spills is a global problem [1,2]. Bioaugmentation with oil-degrading bacteria has been used as an effective treatment to clean up the contaminated soil [3,4,5]. The certainty and efficiency of bioaugmentation is not necessarily stable, because various environmental conditions, including soil composition, water content, temperature, and indigenous microbes, influence the feasibility of this treatment [6,7,8] Among these conditions, temperature was one of the most important factors, because the microbial process could be further slowed down in cooler or cold environments [7,8]. Two Rhodococcus strains, A and C, could degrade the A-fuel oil (500 mg/kg-soil) at lower temperatures (15 and 10 ◦C) as well as at 30 ◦C [16] These two Rhodococcus strains have been permitted for bioaugmentation by the Environment Agency in Japan based on the guidelines for environmental risk assessment on bioremediation (bioaugmentation) to prefectural governments. The above oil-degraders were used, which can degrade the oil even at low temperatures, and the removal efficiencies of A-fuel oil were compared using the lab-scale rotational slurry bioreactor in different conditions
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