Treatment of a Fenton-treated leachate was carried out by denitrification of nitrate using an upflow anaerobic sludge blanket (UASB) biofilm reactor and subsequent biofiltration of the residual COD using a biological aerated filter (BAF). Methanol was selected as the optimal external carbon source because of its good performance, low cost and simple operability. Central composite design (CCD) and response surface methodology (RSM) were used to evaluate the effect of three factors namely hydraulic retention time (HRT), nitrogen concentration and COD/N (C/N) ratio on denitrification performance. Process optimization matched well with the model prediction and allowed 94% reduction of COD and up to 98% removal of total nitrogen (TN). The 21-day biofiltration experiments in a BAF showed that an effluent with COD concentration below100 mg/L and TN concentration less than 40 mg/L was achieved. Our results indicated that the combination process of UASB-biofilter/BAF is effective for nitrate and residual carbon removal. (C) 2010 Elsevier Ltd. All rights reserved.