BACKGROUND: The aim of this study was to assess the feasibility of coupling photocatalysis and a biological treatment for the removal of azo dyes from aqueous effluents. Biological processes do not always appear relevant for dyes removal, owing to the low or total absence of biodegradability of this class of pollutants. RESULTS: During photocatalysis pre-treatment, a decrease in the chemical oxygen demand (COD) indicated oxidation of the target compound and thus a change in the chemical structure; better biodegradability or less toxicity could then be expected. However, the concomitant decrease in dissolved organic carbon (DOC), characteristic of a high mineralization yield, led to nearly constant COD: DOC ratios, which was unfavorable for an increase in biodegradability. It was confirmed by the low values found for the ratios biological oxygen demand (BOD5) to COD, which remained in the range 0.09-0.19, namely below 0.4 after photocatalytic reaction. Moreover, toxicity increased or remained at a high level after irradiation of the azo dyes for 3 h, and decreased only for Orange II, from toxic (EC50 = 53%) to moderately toxic (EC50 = 76%). CONCLUSION: An integrated process involving photocataysis and biological treatment to treat azo dyes appeared unsuitable under the conditions tested and may only be considered for Orange II among the four dyes tested. (C) 2010 Society of Chemical Industry