An empirical model for wastewater treatment by biological aerated filters (BAF) was developed in the study, which relates influent soluble chemical oxygen demand (sCOD) to effluent sCOD and medium bed height in order to simply engineering design and verify experimentally the suitability of this model and ascertain the relationship between the constant values calculated with the model and reactor performance. The theoretical model was based on design of contact oxidation process and conventional biofilter because these fixed-film processes show similar flow pattern. Firstly, the two parallel BAY systems packed with lava medium and expanded clay medium, respectively, were run for actual domestic wastewater at different hydraulic loading rates during the experiment. A great deal of data was then used to the empirical model, which was based on a first-order reaction, to calculate the empirical values of n (medium constant) and K (overall process constant). The experimental results show that the treatment efficiency of lava medium exceed to that of expanded clay medium, which just conform to what the model predict, the higher value of K the greater efficiency of sCOD removal, and the lower value of n the greater variation of sCOD removal indicating a poor resistance to shock loading. Next, the effect of suspended solid (SS) on model was estimated by simulated domestic wastewater to verify its feasibility. (c) 2005 Elsevier Ltd. All rights reserved.