BACKGROUND: The combined treatment of olive mill wastewater (OMWW) by applying the anaerobic digestion process and further treatment in a system consisting of filters and membranes is presented. The anaerobic digestion of the OMWW took place in a high rate system, the periodic anaerobic baffled reactor (PABR). Application of the membrane system aimed at purifying the anaerobic effluent. RESULTS: An increase in the organic loading rate was achieved by increasing the influent chemical oxygen demand (COD) and alternatively by decreasing the hydraulic retention time (HRT). The first option caused process failure, since the volatile fatty acids accumulation resulted in negligible biogas production. In contrast, the second change (decrease in HRT) led to stable operation that permitted the reduction of the HRT to 3.75 d and increase of the organic loading rate to 8.9 g tCOD L-1 d(-1) with satisfactory total COD removal (72%). Higher total COD removal (up to 80%) was observed at lower organic loading rates (<3.5 g tCOD L-1 d(-1)). Further purification in the membrane units resulted in a final permeate of less than 0.1 g tCOD L-1. The membrane systems proved to be more efficient on the anaerobic effluent than on the raw OMWW (the final permeate in that case contained 1g tCOD L-1). CONCLUSIONS: The anaerobic digestion of OMWW in a PABR was stable even at high organic loading rates. Filtering and membrane fractionation of the PABR effluent resulted in a final permeate stream of high quality, suitable for irrigation and/or reuse in the proposed operating scheme for diluting the OMWW prior to anaerobic digestion. (C) 2009 Society of Chemical Industry