A novel apparatus was developed to test the ferrous iron oxidation kinetics of Leptospirillum ferriphilum in predominantly sessile culture, by combining a CSTR under wash-out conditions with a packed bed of inert ceramic saddles. Results indicate that a dense culture of sessile bacteria is established rapidly, which achieves high oxidation rates in all experiments with a yield in terms of CO₂ uptake rates comparable or higher to what has been measured in planktonic culture. However, the yield in terms of cell counts changes dramatically, indicating a substantial shift in carbon utilisation in the sessile culture. The apparatus is feasible as a method to study iron oxidation kinetics of sessile cultures, providing it is operated at sufficiently high recycle ratios.