This study aims to assess the removal mechanism of COD, TSS, and ammonia in a down-flow hanging sponge (DHS) system treating agricultural drainage water (ADW). A DHS system consists of three segments (S-1, S-2. and S-3) connected vertically in series and operated at total HRT of 2 h and OLR of 3 kg COD/m(3) d. The results of this study showed that the major portions of CODtot and TSS were removed in S-1 and S-2, while the ammonia oxidation occurred in the lower segment (S-3). This was mainly because the food/microorganism (F/M) ratio was higher in S-1 and S-2, favoring the heterotrophic biomass, while nitrifying bacteria were dominant in S-3. A simple empirical model was designed to assess the contribution of each segment of the OHS system to the removal of organic matter and the nitrification process. The experimental results were applied to verify the relationship between the model constants and reactor performance. The model results provided average removal efficiencies of 83.7%, 88.6%, 66.7%, 85.0%, and 88.9% for CODtot, CODsol, TKj-N, NH4-N, and TSS, respectively. The model was found to be a simple and useful tool for design and operation of a DHS system treating ADW in practical applications. Published by Elsevier B.V.