An empirical model for COD removal in a biological aerated filter (BAF) in the presence of 2,4,6-trinitrophenol (TNP) was developed, which related effluent COD to influent COD or hydraulic loading rate along the bed height. The overall reaction rate for substrate biodegradation could be described as pseudo first order. The experimental data of COD removal against reactor height were used to calculate the parameters in the empirical model. The COD concentration at different reactor height was expressed as a function of influent COD concentration and hydraulic loading rate, ln(C-0/C)=0.124H / QC(0)(0.77) and ln(C/C-0)=-5.63H / L-0.94, respectively, under the experimental condition. The models may be used to predict the COD removal profiles along the reactor height at different hydraulic loading rates and influent COD concentration for design, selection and sizing of BAR