In this study, various chemical and advanced oxidation processes were tested for enhancement of anaerobic biodegradation of olive mill effluent (OME). The experiments were carried out in a 20 L lab-scale ACE reactor packed with cross-flow filter material made of water-resistant cardboard with a specific surface area of similar to 300 m(2) m(-3). The ACF reactor was operated at chosen OLR conditions and fed with different pre-treated OME samples for 240 days (717 days in total). All pretreatment options used in this study improved the anaerobic biological degradation in terms of COD, phenolics and color removal efficiencies. Best effluent quality (5700+/-250 mg COD per liter) was obtained by using the Fenton process as a pre-treatment. This effluent value is still higher than the official discharge limit for COD (4000 mg COD per liter) in Turkey. Therefore, additional final treatment (e.g. membrane filtration) may be required before discharging into the sewer line.