Assessment of Aquatic Toxicity and Oxygen Depletion during Aerobic Biodegradation of Vegetable Oil: Effect of Oil Loading and Mixing Regime

The potential ecological impacts of aerobic biodegradation of vegetable oils on contaminated water columns was investigated in the laboratory at different oil loadings (100, 333, and 1,000 gal acre(-1)) and mixing regimes (fully, moderately, and nonmixed microcosms). The impacts were estimated by use of the Microtox assay and dissolved oxygen concentration measurements. The results of the Microtox assay showed no major toxicity at the 100 gal acre(-1) loading. Furthermore, oxygen was not completely depleted from the water column at this oil coverage. At higher oil loadings, oxygen was fully depleted from the mixed and nonmixed water columns. A transient toxicity in the aqueous phase was observed in the case of the moderately mixed microcosms at 333 gal acre(-1) and was maintained at moderate levels (EC50 similar to 30%) in the nonmixed microcosms. A substantial increase in toxicity (EC50 similar to 10%) was observed in both mixing conditions when the initial oil loading was increased to 1,000 gal acre(-1). At all oil loadings, significant toxicity (EC50 < 2%) was found in the solid phase due to the strong partition of lipids to the biomass. Long and medium chains fatty acids associated with the measured toxicity were detected in both liquid and solid phases.