Oyster shell and plastic ball were applied as the media of biological aerated filters (BAF) to treat municipal wastewater in two lab scale upflow BAFs. The results indicated that oyster shell BAF and plastic ball BAF had average chemical oxygen demand (COD) removals of 85.1% and 80.0%, when hydraulic retention time (HRT) was longer than 4 h, and 65.7% and 68% with HRT of 2 h, respectively. In terms of removing ammonia nitrogen (NH3-N), oyster shell BAF and plastic ball BAF had average removals of 98.1% and 93.7% for HRT longer than 4 h, and 47.2% and 65.1% for HRT of 2 h, respectively. Total phosphorus (TP) removals of oyster shell BAF were increased to 79.9% and 90.6% as the pH increased to 9 and 10, respectively, while no improvement was observed for plastic ball BAF. The effluent pH of oyster shell BAF was higher and buffered compared with that of the influent, mainly due to the CaCO3 released from the shell. The oyster shell may also be the main reason to support the BAF a higher NH3-N removal efficiency when HRT was longer than 4 h. compared with plastic ball BAF. Crown Copyright (C) 2009 Published by Elsevier B.V. All rights reserved.