Bioremediation of contaminated soil by biostimulation strategy enables indigenous microorganisms in the soil to biodegrade contaminants by utilizing nutrients added to the soil. This study attempts to investigate the optimum nutrients required for bioremediation of spent-engine oil contaminated soil using Box-Behnken Design under response surface methodology. Three concentration levels of cow manure, poultry manure and NPK fertilizer were used as independent biostimulant variables, while reduction in total petroleum hydrocarbon (TPH) and total soil porosity response (TSP) as dependent variables were monitored during a 6-week incubation period. Ex-situ data generated in assessing the rate of biodegradation in the soil were used to develop second-order quadratic regression models for both TPH and TSP. The two models were found to be highly significant and their high coefficients of determination values: R2 = 0.9982 for the TPH model and R2 = 1.000 for the TSP model at p≤0.05 validate the significance of the regression models. Optimization of the biodegradation process was conducted by using numerical optimization technique. The optimum values for biostimulating agents to achieve a predicted maximum TPH removal and TSP improvement of 67.20 and 53.42%dryweight per kg of the spent-engine contaminated soil respectively were found to be as follows: cow manure 125.0 g, poultry manure 100.0 g and NPK fertilizer 10.5 g. At this optimum point, the observed TPH and TSP percentages were found to be 66.92 and 52.65%dryweight respectively. The results suggest that the biostimulation process enhanced the bioactivities of the soil microbes in biodegrading hydrocarbons from the contaminated soil.
Keywords: bioremediation; biostimulation; Box Behnken design; hydrocarbons-degrading microbes; regression models; spent-engine oil