Start-up performance of UASB reactors in low pH for acid mine drainage treatment

Acid mine drainage (AMD) is one of the most significant environmental liabilities of the mining industry. Biological AMD treatment is proposed as an alternative remediation method, but it is typically employed at neutral pH, which influences its costs. This study evaluated the influence of carbon sources (lactate, acetate, and ethanol) on the start-up of three upflow anaerobic sludge blanket (UASB) reactors, inoculated with non-adapted biomass. A synthetic AMD with an acidic pH of 3.0 and the addition of metals (iron, zinc, and copper) was used. The ethanol-fed reactor was the only one capable of treating AMD and stabilizing sulfate removal, achieving efficiencies higher than 90% over 55 operational days, with metals removal efficiencies of 96% for copper, 98.4% for zinc, and 86% for iron. The lactate-fed reactor, after a long acclimation period of 55 days, produced sulfide and was capable of removing copper and zinc. However, 45 days after the acclimation period, the metabolic sulfidogenic pathway ceased, and lactate fermentation began. The acetate-fed reactor was not capable of utilizing this carbon source for sulfate reduction. This reactor suffered from the acetic acid decoupling effect, which caused the collapse of biomass cells by disrupting biomass granules and also allowed fungal growth. It was evident that the choice of carbon source is also influenced by the pH of the acid drainage to be treated, as pH values lower than the pKa value can lead to the failure of the treatment process.