Haydite and grain-slag, a new-style filter media made of grain-slag, were used as filter media for biological aerated filter (BAF) to treat synthetic wastewater in parallel. The aim of this work was to compare the ammonium nitrogen removal performance of two BAF reactors at the different influent pH values and ammonia nitrogen load rates. It could be demonstrated that the BAF packed with grain-slag presented higher ammonia removal efficiency than that with haydite. The grain-slag BAF and haydite BAF had ammonia nitrogen removals in the range of 90.63%-63.46%,75.62%-42.23%, respectively, under the ammonia nitrogen concentration varying from 16 mg/L to 64 mg/L Higher ammonia nitrogen removal in the BAF with grain-slag was attributable to its buffering pH effect capacity by stripping calcium carbonate (CaCO3). In addition, the detection of the amount of hetero bacteria and nitrobacteria of two biological aerated filters in five stages also showed that grain-slag medium was more suitable to the attached growth of nitrobacteria, which is helpful to the improvement of nitrification performance in grain-slag BAF. Crown Copyright (C) 2010 Published by Elsevier B.V. All rights reserved.