Treatment of low-pH and sulfate-containing wastewaters in biological inverse fluidized-bed reactors under different COD/SO42- conditions

The feasibility of removing sulfate in two lactate-fed sulfidogenic inverse fluidized-bed reactors operated with different COD/sulfate ratios was investigated. Low-density polypropylene beads were used as carrier material for the development of a sulfate-reducing biomass within the reactors at a 10% fluidization degree. During the 242 days of operation, the robustness of the system was studied by suddenly decreasing the feed pH to 3.00. After a start-up phase of 35 days, in reactor 1, operated with a COD/sulfate ratio of 0.67 throughout the experiment, COD removal and sulfate reduction efficiencies reached the highest values of 75% and 35%, respectively. During the reactor operation, higher efficiencies were not achieved due to the accumulation of acetate and the microbial competition between sulfate reducers and other microorganisms. In reactor 2, a 97% sulfate reduction efficiency was obtained with a COD/sulfate ratio of 4.00 in slightly acidic feed pH conditions (pH ≈ 5.00). The process failed when the feed pH was intentionally decreased to 3.00.