Bioremediation of oil-coated stones

The suitability of bioremediation techniques for on-site cleaning of oil-coated gravels was investigated. The oil was present as a I mm thick, paste-like layer containing small sand particles. Two types of fixed-film reactors were studied. In one, stones were placed in an aerated tank of water; in the other, water was continuously recirculated over a packed bed of stones. The reactors were inoculated with a culture acclimatized to the oil on the stones. Comparable oil removal and degradation rates were achieved in both reactors. The maximum extent of oil removal achieved was around 60%, by which point the residual oil had changed color, become flaky and detached readily from the stones. The effects of various factors on reactor performance were studied. Physical processes were found to play a significant role in oil removal. Nutrient supplementation led to increased initial oil removal and degradation rates but had less effect on the amount of oil finally removed. As expected, decreases in operating temperature lowered degradation rates; they also caused a marked and unexpected decrease in levels of oil removal, Except at the start, rates of bioremediation appeared to be controlled by diffusion processes within the oil itself.