BACKGROUND: The purpose of this study was to investigate the co-treatment of olive-mill wastewater (OMW) and municipal wastewater in activated sludge systems operating in the absence and presence of different adsorbent materials and to study the role of sorption and biodegradation in total phenols removal. RESULTS: Batch experiments were initially conducted to investigate total phenols' adsorption capacity on activated sludge (AS), olive pomace (OP) and powdered activated carbon (PAC). According to the results, PAC presented the best adsorption capacity. Three sequencing batch reactors (SBRs) were also operated, treating municipal wastewater and different amounts of OMW. The first SBR contained AS (AS-System), the second AS andOP (AS-OP System) and the third AS and PAC (AS-PAC System). All SBRs operated sufficiently in the presence of 1% v/v OMW, achievingmean COD and total phenols removal efficiency higher than 86% and 85%, respectively, and satisfactory settling capacity. Increase of OMW concentration to 5% v/v affected the performance of SBRs, resulting in mean COD removal efficiencies that ranged between 61% (AS-OP System) and 80% (AS-PAC System). CONCLUSION: Among the SBRs used, the AS-PAC System operated with highest performance in the presence of 1 and 2.5% v/v OMW, and showed better stability in the presence of 5% v/v OMW. Calculation of total phenols mass flux revealed that biodegradation was the principal mechanism of their removal. The highest values of mean biotransformation rates were calculated for the AS-PAC System and ranged between 2.0 and 40.6 d-1 for different experimental phases. (C) 2012 Society of Chemical Industry