The catalytically oxidized olive mill wastewater (OMW) was subjected to continuous anaerobic treatment using two treatment schemes. The 1st step in both schemes was an up-flow anaerobic sludge blanket (UASB) reactor (2 01). The 2nd step was either a hybrid UASB reactor or a classical one (101, each). The 1st stage was operated at constant hydraulic retention time (HRT) of 24 h. The organic loading rate (OLR) varied from 3.4 to 4.8 kgCOD/m(3) d depending on the quality of the pretreated wastewater. The results obtained indicated that, the 1st step UASB reactor achieved a COD percentage removal value of 53.9%. Corresponding total BOD(5) and TSS removal were 51.5% and 68.3%, respectively. The results obtained indicated that the hybrid UASB reactor as a 2nd step produced better quality effluent as compared to the classical one. This could be attributed to the presence of the packing curtain sponge with active biomass in the sedimentation part of hybrid UASB reactor which minimizes suspended solids washout, consequently enhancement of the efficiency of the reactor. Available data showed that a two stage system consisting of a classical and a hybrid UASB reactor operated at a total HRT of 48 h and OLR of 2.0 kgCOD/m(3) d provided promising results. Removal values of COD(total), BOD(5) (total), TOC, VFA, oil and grease were 83%, 84%, 81%, 93% and 81%, respectively. Based on the available data, the use of a two stage anaerobic system consisting of a classical UASB reactor followed by a hybrid UASB as a post-treatment step for catalytically oxidized OMW is recommended. (C) 2008 Elsevier Ltd. All rights reserved.