Landfill leachate obtained from the solid waste landfill area contained high chemical oxygen demand (COD) and ammonium ions which resulted in low COD and ammonium removals by direct biological treatment. COD and ammonium ion contents of the leachate were reduced to reasonable levels by chemical precipitation with lime and air stripping of ammonia. The pre-treated leachate was subjected to aerobic biological treatment in an aeration tank by fed-batch operation. The effects of the feed wastewater COD content and flow rate on COD and ammonium ions removal were investigated. Nearly 76% COD and 23% NH4-N removals were obtained after 30 h of operation with a flow rate of 0.211 h(-1) and the feed COD content of 7000 mg COD l(-1). COD removal efficiency decreased with increasing COD loading rates. A kinetic model for COD removal was developed and the kinetic constants were determined by using the experimental data. (C) 2003 Elsevier Inc. All rights reserved.