Hydrogen-based nitrate and selenate bioreductions in flue-gas desulfurization brine

The H₂ -based membrane biofilm reactor (MBfR) was shown to simultaneously reduce nitrate and selenate in flue-gas desulfurization (FGD) brines (15-33 g/L TDS). The MBfR also used polyester fibers, which differ from the composite gas-transfer fibers used in previous MBfR studies. Selenate reduction was nearly 100% in all media tested and was immediate, which suggests that selenate was coreduced with nitrate, possibly by the same microorganisms. A step increase in the nitrate removal flux was observed at a H₂ pressure greater than 20 psig (138 kPa) using polyester fibers. High concentrations of nitrate and sulfate had no inhibitory effect on selenate reduction as long as H₂ was available. In comparison with previous MBfR experiments treating ion-exchange brine, total electron equivalent fluxes were 29% lower and in contrast to earlier work on selenate reduction in fresh water, sulfate reduction was inhibited, likely due to the high total dissolved solids in the FGD brine or insufficient H₂ availability due to CaCO₃ or Se° precipitation onto the tighter polyester fibers.