Due to high chemical oxygen demand (COD), ammonium-N content and presence of toxic compounds such as heavy metals, direct biological treatment of landfill leachate results in low removal efficiencies. In order to improve biological treatability of the leachate, coagulation-flocculation and air stripping of ammonia were used as pre-treatment. Pre-treated leachate was treated biologically using an aeration tank operated in fed-batch mode in the absence and presence of powdered activated carbon (PAC) as adsorbent. PAC addition improved COD removal significantly especially at concentrations above 0.5 gl(-1). However, improvements in COD removals were marginal for PAC concentrations above 2 gl(-1). Nearly 86% COD removal was achieved with 2 gl(-1) PAC added biological treatment whereas, COD removals by only biological oxidation and only PAC adsorption were nearly 74 and 38%, respectively at the end of 30 h of fed-batch operation. An empirical equation was developed to describe the contribution of adsorption over biological treatment as a function of PAC concentration. (C) 2003 Elsevier Science Ltd. All rights reserved.