Journal of Biotechnology 121 (2006) 534–543 Performance and microbial communities of a continuous stirred tank anaerobic reactor treating two-phases olive mill solid wastes at low organic loading rates B. Rinc ´ on a , F. Raposo a , R. Borja a,∗ , J.M. Gonzalez b , M.C. Portillo b , C. Saiz-Jimenez b a Instituto de la Grasa, CSIC, Avda. Padre Garc´ ıa Tejero 4, 41012 Sevilla, Spain b Instituto de Recursos Naturales y Agrobiolog´ ıa, CSIC, Apartado 1052, 41080 Sevilla, Spain Received 21 February 2005; received in revised form 29 July 2005; accepted 4 August 2005 Abstract A study of the performance and microbial communities of a continuous stirred tank reactor (CSTR) treating two-phases olive mill solid wastes (OMSW) was carried out at laboratory-scale. The reactor operated at a mesophilic temperature (35 ◦ C) and an influent substrate concentration of 162 g total chemical oxygen demand (COD) L -1 and 126 g volatile solids (VS) L -1 . The data analyzed in this work corresponded to a range of organic loading rates (OLR) of between 0.75 and 3.00 g COD L -1 d -1 , getting removal efficiencies in the range of 97.0–95.6%. Methane production rate increased from 0.164 to 0.659 L CH 4 L reactor -1 d -1 when the OLR increased within the tested range. Methane yield coefficients were 0.225 L CH 4 g -1 COD removed and 0.290 L CH 4 g -1 VS removed and were virtually independent of the OLR applied. A molecular characterization of the microbial communities involved in the process was also accomplished. Molecular identification of microbial species was performed by PCR amplification of 16S ribosomal RNA genes, denaturing gradient gel electrophoresis (DGGE), cloning and sequencing. Among the predominant microorganisms in the bioreactor, the Firmicutes (mainly represented by Clostridiales) were the most abundant group, followed by the Chloroflexi and the Gamma-Proteobacteria (Pseudomonas species as the major representative). Other bacterial groups detected in the bioreactor were the Actinobacteria, Bacteroidetes and Deferribacteres. Among the Archaea, the methanogen Methanosaeta concilii was the most representative species. © 2005 Elsevier B.V. All rights reserved. Keywords: Two-phases olive mill solid waste; Anaerobic digestion; Performance; Microbial community ∗ Corresponding author. Tel.: +34 95 4689654; fax: +34 95 4691262. E-mail address: rborja@cica.es (R. Borja). 1. Introduction Olive oil manufacturing has a great importance in the Mediterranean basin. This industry has undergone important changes in the last years (Alba et al., 1990). 0168-1656/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jbiotec.2005.08.013