This study was focused on several physico-chemical and biological treatment methods that may affect the reduction of the organic load in olive mill wastewater (OMW). In this study, removal of 95% of the phenolic compounds present in OMW was achieved using sand filtration and subsequent treatment with powdered activated carbon in a batch system. This pretreatment for OMW was found to enhance the anaerobic activity of the sludge in the batch system significantly. The efficiency of organic load removal achieved by the anaerobic treatment of untreated OMW in batch reactors with tap water dilution factors below 1:10, reached approximately 65% chemical oxygen demand (COD) removal. However, in the up-flow sludge anaerobic blanket (UASB) reactor, COD removal efficiency of 80-85% was reached at a hydraulic retention time (HRT) of 5 days with an influent COD concentration of 40 g l(-1) and organic loading rate (OLR) = 8 g(-1) COD l(-1) per day. (C) 2003 Elsevier Ltd. All rights reserved.