Natural zeolite and expanded clay were used as filter media for biological aerated filter (BAF) to treat municipal wastewater in parallel in whole three test stages. The stage one test results revealed that zeolite BAF and expanded clay BAF have COD and NH3-N removals in the range of 84.63-93.11%, 85.74-96.26%, 82.34-93.71 %, and 85.06-93.2%, respectively, under the conditions of water temperature of 20-25 degrees C and hydraulic load of 2-3 m(3)/(m(2) h). At the following stage two, the influent NH3-N concentration was increased to about double value of the stage one, and it was investigated that the effluent NH3-N of expanded clay BAF increased significantly and then gradually restored to normal condition in 2 weeks, while the effluent NH3-N of zeolite BAF kept stable. At stage three, the low reactor temperature has also different effects on these two BAFs, under conditions of water temperature of 7-10 degrees C, hydraulic load of 2-3 m3/(m(2) h), zeolite BAF and expanded clay BAF have COD and NH3-N removals in the range of 74.5-88.47% (average of 81.57%), 71.73-88.49% (average of 81.06%), 71.91-87.76% (average of 80.49%), and 38.41-77.17% (average of 65.42%), respectively. Three stages test results indicated that the zeolite BAF has a stronger adaptability to NH3-N shock load and low temperature compared to expanded clay BAF In addition, the detection of the amounts of heterobacteria and nitrobacteria of two biological aerated filters in three stages also showed the zeolite filter media was more suitable to the attached growth of nitrobacteria, which is helpful to the improvement of nitrification performance in zeolite BAF. (C) 2006 Elsevier B.V. All rights reserved.