Biotreatment of acidic zinc- and copper-containing wastewater using ethanol-fed sulfidogenic anaerobic baffled reactor

The treatment of acidic (pH 6.5–3), sulfate- (2–3 g/L), Zn- and Cu- (total metal 0–500 mg/L) containing wastewater was studied in a four-stage anaerobic baffled reactor (ABR) at 35 °C for 250 days. Ethanol was supplemented (COD/SO₄ ²⁻ = 0.67) as carbon and electron source for sulfate reducing bacteria. Sulfate reduction, COD oxidation and metal precipitation efficiencies were 70–92, 80–94 and >99%, respectively. The alkalinity produced from sulfidogenic ethanol oxidation increased the wastewater pH from 3.0 to 7.0–8.0. The electron flow from organic oxidation to sulfate averaged 87%. Decreasing feed pH to 3 and increasing total metal concentrations to 500 mg/L did not adversely affect the performance of ABR and sufficient alkalinity was produced to increase the effluent pH to neutral values. More than 99% of metals were precipitated in the form of metal-sulfides. Accumulation of precipitated metals in the first compartment allowed metal recovery without disturbing reactor performance seriously.