High chemical oxygen demand (COD), high ammonium-N content and presence of toxic compounds such as heavy metals result in low removal efficiencies in direct biological treatment of landfill leachate. Raw landfill leachate was subjected to pre-treatment by coagulation-flocculation and air stripping of ammonia before biological treatment. Pre-treated leachate was subjected to biological treatment in an aeration tank operated in repeated fed-batch mode in the absence and presence of powdered activated carbon (PAC) as adsorbent. Three different cycles of I x 30 h, 3 x 10 h, and 5 x 6 h were used in the absence and presence of 2 g l(-1) PAC. The results clearly indicated that PAC addition has improved COD removal significantly as compared to PAC-free biological treatment. Repeated fed-batch treatment also resulted in lower effluent COD and higher COD removal efficiencies as compared to single-cycle operation. The feed COD of 7000 mg l(-1) was reduced to final COD values of 365 mg l(-1) and 875 mg l(-1) in repeated fed-batch experiments with 5 x 6 h cycles in the presence (2 g l(-1) PAC) and absence of PAC, respectively. Repeated fed-batch operation with 3 x 30 It cycle length resulted in percent COD removal and final COD content of 94% and 285 mg l(-1), respectively, with 2 g l(-1) PAC addition. (C) 2003 Elsevier Inc. All rights reserved.