Optimization of metal sulphide precipitation in fluidized-bed treatment of acidic wastewater

Sulphate-reducing biofilm and suspension processes were studied for treatment of synthetic wastewater containing sulphate, zinc and iron. With lactate supplemented wastewater with 170–230 mg/l Zn and 58 mg/l Fe, the following precipitation rates were obtained: 250 and 350 mg/l d for Zn in fluidized-bed (FBR) and upflow anaerobic sludge blanket reactors, respectively, and 80 mg/l d for Fe in both reactors with hydraulic retention time of 16 h. The effluent Zn and Fe concentrations remained at less than 0.1 mg/l. The alkalinity produced in lactate oxidation increased the initial pH of 2.5–3, resulting in effluent pH of 7.5–8.5. The highest sulphate reduction rate was over 2000 mg/l d. In terms of sulphate reduction, hydrogen sulphide production and effluent alkalinity, the start-up of the FBR with the 10% fluidization rate was superior to the FBRs with 20–30% fluidization rates. With increased loading rates, high recycling rate became an advantage. After process failure caused by intentional overloading, the sulphate reduction partially recovered within 2 weeks. Metal precipitates in the reactors were predominantly FeS 2, ZnS and FeS. The metal mass balance was as follows: 73–86% of Zn and Fe accumulated into the reactors and water level adjustors, 14–23% of the metals were washed out as precipitates and 0.05–0.15% remained as soluble metals. Biomass yield in the sulphate-reducing processes was 0.039–0.054 g dry biomass (VS or VSS) per g of lactate oxidized or 0.035–0.074 g dry biomass per g of sulphate reduced. The results of this work demonstrate that the lactate supplemented sulphate-reducing processes precipitated the metals as sulphides and neutralized the acidity of the synthetic wastewater.