Long-term performance of a UASB reactor treating acid mine drainage: effects of sulfate loading rate, hydraulic retention time, and COD/SO42- ratio.

Acid mine drainage (AMD) is among the most serious threats to water and the typical alkali-based treatment costs are high. This study's main objective was the establishment of a highly efficient biological process using an upflow anaerobic sludge blanket (UASB) reactor to treat AMD based on a shorter hydraulic retention time (HRT) and lower organic matter input. The process was evaluated for a long-term operation (739 days) in terms of the influence of HRT (14-24 h), metal addition, sulfate loading rate (0.5-2.6 g SO42- l-1 d-1), and the COD/SO42- ratio (0.67-1.0) using ethanol as the only electron donor at a pH of 4.0. Neutral effluent pH was achieved throughout the time apart from operational modifications. The reduction in HRT from 24 to 16 h and an increase in the sulfate loading rate (SLR) up to 2.25 g SO42- l-1 d-1 improved the sulfate removal to (92.1 ± 1.8)% with 80% chemical oxygen demand (COD) removal. However, the sulfate reduction was less than 80% when the HRT and SLR was changed to 14 h and 2.6 g SO42- l-1 d-1, respectively. The oxidation of organic matter by sulfate reduction was greater than 50% regardless of the conditions imposed but the use of ethanol to treat AMD was more efficient when either the HRT was 16 h (1.5 g SO42- l-1 d-1) in the presence of Fe, Zn, and Cu or the HRT was 14 h (2.6 g SO42- l-1 d-1) but the COD/SO42- ratio was reduced to 0.67. The fully optimized conditions of the UASB reactor were set at an HRT of 16 h, SLR of 1.5 g SO42- l-1 d-1, and a COD/SO42- ratio of 1.0.