A modified airlift reactor with a net draft tube (ALR-ndt) was developed and studied for the aerobic biodegradation of naphthalene (naph.) by Ralstonia eutropha. The performance of the ALR-ndt was investigated at different initial naph. concentrations by free cells of the bacterium. The results showed that complete degradation of naph. was achieved up to a concentration of 90 mg L⁻¹ but partial degradation was observed at 120 and 150 mg L⁻¹ of naph. due to inhibition which led to a significant decrease in the biodegradation rate. The maximum specific biodegradation rate was 0.0543 h⁻¹ achieved at the initial naph. concentration of 30 mg L⁻¹. The Haldane inhibition equation was chosen as the appropriate kinetic model representing the degradative ability of the cells in the presence of substrate inhibition.

Then, the obtained kinetic model of biodegradation is coupled with a three-dimensional (3D) transient computational fluid dynamics (CFD) model to simulate the transient behavior of the batch naph. degradation in the ALR-ndt. The simulation results of the two-phase fluid flow were predicted as the naph. concentration distribution profiles through the bioreactor and compared to those obtained from the experimental measurements. Accordingly, the results show logical trends and satisfactory agreement for different substrate concentrations with relative error less than 6.4% in terms of removal percentage. The obtained results confirmed the applicability of the 3D CFD model which integrated bio-kinetics together and highlighted the effective role of the net draft tube. Thus, the implemented model enables accurate bioreactor design with an inexpensive and less time-consuming method even in large scales.