OICC Press International Journal of Energy and Environmental Engineering 2251-6832 3 1 (December 2012) 2012 10 17 Box-Behnken design application to study enhanced bioremediation of soil artificially contaminated with spent engine oil using biostimulation strategy 10.1186/2251-6832-3-31 EN Samuel E Agarry Biochemical Engineering and Biotechnology Research Laboratory, Department of Chemical Engineering, Ladoke Akintola University of Technology, Ogbomoso, Oyo State, 260001, NG Oladipupo O Ogunleye Biochemical Engineering and Biotechnology Research Laboratory, Department of Chemical Engineering, Ladoke Akintola University of Technology, Ogbomoso, Oyo State, 260001, NG Journal Article 2012 10 17 Abstract This work studies the biodegradation of spent engine oil in soil using Box-Behnken design under response surface methodology. NPK fertilizer (inorganic nutrient), Tween 80 (nonionic surfactant), and pig manure (organic nutrient) concentrations were used as independent biostimulant variables, while total petroleum hydrocarbon (TPH) and hexavalent chromium (Cr (VI)) reductions as dependent variables (response) in a 42-day remediation period. A statistically significant second-order quadratic regression model for TPH and Cr (VI) removal was obtained. The coefficient of determination ( R 2  = 0.9995 for TPH and 0.9988 for Cr (VI)) and probability value ( P