BACKGROUND: Ex situ bioremediation is a feasible and economical way to remove petroleum pollutants from contaminated soil or water. A baffled roller bioreactor was shown to be effective for biodegradation of diesel oil as a model petroleum pollutant. Microorganisms enriched from an industrially contaminated soil with heavy hydrocarbons were shown to be the best inoculum source for diesel biodegradation. RESULTS: The baffled roller bioreactor demonstrated better performance than control (roller bioreactor without baffles) or bead mill roller (control bioreactor filled partially with spherical beads) bioreactors. Biodegradation consisted of both fast and slow stages for degradation of light and heavy compounds, respectively. Among the tested temperatures ranging from 15 to 35 degrees C, room temperature (23 degrees C) was found to be the optimum temperature for biodegradation. The values of maximum specific growth rate and substrate yield (mu(max) and Y-xs) for the indigenous microorganisms in the baffled roller bioreactor at room temperature were found to be 0.72 +/- 0.08 h(-1) and (7.0 +/- 1.0) x 10(7) cells mg(-1) diesel, respectively. Biodegradation of diesel concentrations up to 200 g L-1 was achieved with the highest biodegradation rate of 266 mg L-1 h(-1) at the highest rotation rate of 45 rpm in the baffled roller bioreactor. CONCLUSION: Using indigenous bacteria enriched from industrial contaminated soil at room temperature, a baffled roller bioreactor is able to biodegrade high diesel oil concentrations at high biodegradation rates. (c) 2008 Society of Chemical Industry