The effect of organic loading and aeration rates on the performance of a lab-scale upflow aerated submerged fixed-film bioreactor, packed with polypropylene media, was examined in terms of dissolved oxygen (DO) concentration and chemical oxygen demand (COD) removal efficiency using an experimental design technique for a 6-month operation period. The system could handle the organic loadings of 1-2.5 kgCOD/m(3)d with the COD removal efficiency from 74 to 90% regardless of aeration rate for synthetic municipal wastewater. In a low aeration rate of 41/min, increasing organic loadings to the values higher than 2 kgCOD/m(3)d resulted in a 16% decrease in the system performance, most likely due to the DO concentration reduction (about 57%). With increasing the aeration rate to 81/min, however, the COD removal and DO values remained constant at about 90% and 5.8 mg/l, respectively, with a slightly decrease to 83% and 4.5 mg/l for the high loading rate of 2.5 kgCOD/m(3)d. The statistical analysis with a 90% confidence interval showed the interaction of main factors on the COD removal is important only when the both main factors of the aeration and loading rates were increased. It is concluded that the system used in the present study could show an acceptable stability and performance during the long-term period particularly at the high airflow rate. (C) 2011 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.