Pilot-plant treatment of olive mill wastewater (OMW) by solar TiO 2 photocatalysis and solar photo-Fenton W. Gernjak a, * , M.I. Maldonado a , S. Malato a , J. C aceres a , T. Krutzler b , A. Glaser b , R. Bauer b a Plataforma Solar de Almer  ıa-CIEMAT, Carretera de Sen es km 4, 04200 Almer  ıa, Spain b Institute of Materials Chemistry, Vienna University of Technology, Getreidemarkt 9, 1060 Vienna, Austria Received 1 December 2003; received in revised form 30 March 2004; accepted 30 March 2004 Available online 20 July 2004 Communicated by: Associate Editor Claudio Estrada-Gasca Abstract Olive mill wastewater (OMW), a highly polluted wastewater from the olive oil industry, was treated by solar photocatalysis and solar photo-Fenton. Among the tested systems the application of titanium dioxide alone was not successful. The addition of peroxydisulphate as an electron acceptor had only limited effect on degradation performance and led to high salt concentrations (30 g/l sulphate generated) and a pH value near zero. The photo-Fenton method successfully removed up to 85% COD and up to 100% of phenol index of OMW with different initial concentrations and from different sources. Two solar photocatalytic pilot-plant reactors were used; one of conventional CPC type and an open non-concentrating Falling Film Reactor. The latter, newly designed reactor worked properly and yielded com- parable results to the CPC in terms of degradation rate referred to incident UV radiation energy per solution volume. The suspended solids in the OMW hinder light from entering the reactor. Therefore, flocculation induced by a com- mercial flocculation agent was successfully applied to remove suspended solids. Application of this pre-treatment led to considerable increase of degradation rates and decrease of hydrogen peroxide consumption. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Olive mill wastewater; Photo-Fenton; Solar wastewater treatment 1. Introduction Olive mill wastewater (OMW) accrues in the pro- duction of olive oil during winter and consists of olive fruit extract and added process water. Its annual pro- duction is in the range of some million cubic meters throughout the European Union. Analytical parameters reported in literature vary by several hundred percent, as the composition depends on a broad variety of factors such as type of olives and press system. Nevertheless, OMW could be described as a wastewater with high organic load (typically 10–30 g/l TOC), high COD (up to 150 g/l reported), high salt concentration and acidic pH. The generation of huge amounts of OMW in a short time during the press season is the first problem. Second, OMW contains polyphenols up to several grams per litre, which are phytotoxic and toxic to bacteria used in common biological wastewater treatment plants (Bec- cari et al., 1999; Borja et al., 1996; Paredes et al., 1999). Furthermore, the low pH and the polyphenols’ com- plexing abilities raise the solubility of heavy metals in the environment (Bejarano and Madrid, 1992). Current treatment of OMW is done mostly by evaporation in large shallow ponds, which are often defective and leaking (Fiestas Ros de Ursinos and Borja, 1992). * Corresponding author. Tel.: +34-950-387-957; fax: +34- 950-365-015. E-mail address: wolfgang.gernjak@psa.es (W. Gernjak). 0038-092X/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.solener.2004.03.030 Solar Energy 77 (2004) 567–572 www.elsevier.com/locate/solener