The effect of untreated and Fenton-treated acid dyes (C.I. Acid Red 183 and C.I. Acid Orange 5 1) and a reactive dye (C.I. Reactive Blue 4) on aerobic, anoxic and anaerobic processes was investigated. The optimum Fe2+ :H2O2 molar ratio was selected as 1:5 (4 mM:20 mM) for 10 thin Fenton treatment at pH 3, resulting in reduced chemical oxygen demand and dissolved organic carbon removal efficiencies; only acetate was detected as a stable dye oxidation end product. During anaerobic digestion, 100, 29% and no inhibition in methane production was observed for the untreated blue, red and orange dyes, respectively. The inhibitory effect of the blue reactive dye on methane production was similar to 21% after Fenton treatment. Neither untreated nor treated dyes exhibited an inhibitory effect on denitrification. Aerobic glucose degradation Was inhibited by 23-29% by untreated dyes, whereas Fenton-treated dyes had no inhibitory effect on aerobic glucose degradation. (c) 2007 Elsevier Ltd. All rights reserved.