Biotreatment of Cu- and Zn-containing synthetic wastewater was studied in sulfate-reducing up-flow (UFBR) and down-flow fluidized-bed reactors (DFBR) at 35 degrees C. The robustness of the systems was studied by stepwise increasing feed metal concentrations (total metal concentrations 25-300 mg/L) and decreasing feed pH (down to 2.0). Lactate was used as a carbon and energy source for sulfate reducing bacteria. After start-up, sulfate reduction and COD oxidation efficiencies were 60-86% and 87-95% in UFBR and, 40-88 and 55-95% in DFBR, respectively. Optimum COD/sulfate ratio for sulfate reduction was 0.85 and 1.25 for UFBR and DFBR, respectively. Approximately 70% and 55% of the electrons produced from lactate oxidation were used for sulfate reduction in UFBR and DFBR, respectively. Sulfide production and metal precipitation capacity of UFBR were higher than those of DFBR, although down-flow regime gave the possibility of metal recovery. Metals were precipitated more than 99% in both reactors. XRF analyses showed that metals were precipitated as metal-sulfides. (C) 2010 Elsevier Ltd. All rights reserved.