in this study various two-stage processes were investigated for biological hydrogen production from olive mill wastewater (OMW) by Rhodobacter sphaeroides O.U.001. Two-stage processes consist of physicochemical pretreatment of OMW followed by photofermentation for hydrogen production. Explored pretreatment methods were chemical oxidation with ozone and Fenton's reagent, photodegradation by UV radiation, and adsorption with clay or zeolite. Among these different two-stage processes, strong chemical oxidants like ozone and Fenton's reagent have the highest color removal (90%). However, their effluents were observed to be unsuitable for both hydrogen production and bacterial growth. On the other hand, clay treatment method was selected as the optimum process that allows fast and low-cost treatment as well as its effluent found to have the highest hydrogen production potential (31.5 m(3) m(-3)). Spent-clay regeneration was also investigated on the grounds that solid waste minimization is important for the overall efficiency of this process. (C) 2008 Elsevier Ltd. All rights reserved.