Performance and ethanol oxidation kinetics of a sulfate-reducing fluidized-bed reactor treating acidic metal-containing wastewater.

The treatment of simulated acidic wastewater (pH 2.5-5) containing sulfate (1.0-2.2 g l(-1)), zinc (15-340 mg l(-1)) and iron (57 mg l(-1)) was studied in a sulfate-reducing fluidized-bed reactor (FBR) at 35 degrees C. The original lactate feed for enrichment and maintenance of the FBR culture was replaced stepwise with ethanol over 50 days. The robustness of the process was studied by increasing stepwise the Zn, sulfate and ethanol feed concentrations and decreasing the feed pH. The following precipitation rates were obtained: 360 mg l(-1) d(-1) for Zn and 86 mg l(-1) d(-1) for Fe, with over 99.8% Zn and Fe removal, with a hydraulic retention time of 16 h. Under these conditions, 77-95% of the electrons were accepted by sulfate reduction. The alkalinity produced from ethanol oxidation increased the wastewater pH from 2.5 to 7.5-8.5. Michaelis-Menten constants (Km) determined in batch FBR experiments, were 4.3-7.1 mg l(-1) and 2.7-3.5 mg l(-1) for ethanol and acetate oxidation, respectively. The maximum oxidation velocities (Vmax) were 0.19-0.22 mg gVS(-1) min(-1) and 0.033-0.035 mg gVS(-1) min(-1), for ethanol and acetate, respectively. In summary, the FBR process produced a good quality effluent as indicated by its low organic content and Zn and Fe concentrations below 0.1 mg l(-1).