The effects of hydraulic retention time (HRT) and sulfide toxicity on ethanol and acetate utilization were studied in a sulfate-reducing fluidized-bed reactor (FBR) treating acidic metal-containing wastewater. The effects of HRT were determined with continuous flow FBR experiments. The percentage of ethanol oxidation was 99.9% even at a HRT of 6.5 h (loading of 2.6 g ethanol L-1 d(-1)), while acetate accumulated in the FBR with HRTs below 12 h (loading of 1.4 g ethanol L-1 d(-1)). Partial acetate utilization was accompanied by decreased concentrations of dissolved sulfide (DS) and alkalinity in the effluent, and eventually resulted in process failure when HRT was decreased to 6.1 h (loading of 2.7 g ethanol L-1 d(-1)). Zinc and iron precipitation rates increased to over 600 mg L-1 d(-1) and 300 mg L-1 d(-1), respectively, with decreasing HRT. At HRT of 6.5 h, percent metal precipitation was over 99.9%, and effluent metal concentrations remained below 0.08 mg L-1. Under these conditions, the alkalinity produced by substrate utilization increased the wastewater pH from 3 to 7.9-8.0. The percentage of electron flow from ethanol to sulfate reduction averaged 76 +/- 10% and was not affected by the HRT. The lowest HRT did not result in significant biomass washout from the FBR. The effect of sulfide toxicity on the sulfate-reducing culture was studied with batch kinetic experiments in the FBR. Noncompetitive inhibition model described well the sulfide inhibition of the sulfate-reducing culture. (DS) inhibition constants (K-i) for ethanol and acetate oxidation were 248 mg S L-1 and 356 mg S L-1, respectively, and the corresponding Ki values for H2S were 84 mg S L-1 and 124 mg S L-1. In conclusion, ethanol oxidation was more inhibited by sulfide toxicity than the acetate oxidation. (C) 2004 Wiley Periodicals, Inc.