The traditional two-stage anoxic-oxic (A/O) activated sludge process might be inefficient in pollutants removal and could not ensure the effluent quality. By installing polyurethane foams as carriers and inoculating specialized bacteria in the oxic compartments, the activated sludge systems could be transformed into bioaugmented biofilm processes to enhance the removal efficiency to recalcitrant pollutants. Optimal environment should be provided for the bioaugmented bacteria during systems' start-up. In the present research, dissolved oxygen (DO) concentration was studied as a crucial environmental factor on the performances of the bioagumented systems. The results indicated that the system adopted lower DO concentration took less time to start-up, performed higher pollutants removal efficiency and stronger resistance to shock loadings compared to the system with higher DO level. This was the first attempt to evaluate the importance of DO concentration on the start-up of the two-stage bioaugmented A/O biofilm process. (C) 2009 Elsevier Ltd. All rights reserved.