In the present study, olive mill wastewater (OMW) treatment is investigated experimentally in various stage processes of advanced oxidation with ozone (O-3), photodegraclation by UV radiation, and an aerobic biodegradation. The effects of these treatment processes on the removal of chemical oxygen demand (COD) of OMW are monitored. For both single-stage treatment of O-3 and two-stage treatment of O-3/UV, the chemical oxygen demand remains quite high. In contrast, a combination of biological and UV/O-3 process for the OMW treatment seems to be a serious alternative in the reduction of the COD. In particular, biodegradation of UV/O-3 pretreated OMW found to have the highest removal levels; the percent of COD removal reaches about 91%. On the other hand, the kinetic study shows that the decay of chemical oxygen demand follows a first-order and pseudo first-order models for advanced oxidation and biodegradation processes; respectively. (C) 2009 Elsevier B.V. All rights reserved.