Effect of elevated nitrate and sulfate concentrations on selenate removal by mesophilic anaerobic granular sludge bed reactors

Simultaneous removal of selenate (SeO42-), nitrate (NO3-) and sulfate (SO42-), typically present in Se-contaminated wastewaters, by Eerbeek anaerobic granular sludge, was investigated in batch and continuous bioreactor experiments. Batch experiments showed that SeO42- removal was enhanced to 91% in simulated wastewater with SeO42- + NO3- + SO42- (1 : 40 : 100 SeO42- : NO3- : SO42- molar ratios) compared to simulated wastewater with SeO42- alone (67%). SeO42- removal was severely impacted by high concentrations of SO42- (SeO42- : SO42- > 1 : 300). Removal of SeO42-, NO3- and SO42- at a 1 : 40 : 100 ratio was studied in a 2 L lab-scale upflow anaerobic sludge blanket (UASB) reactor operated at 20 [degree]C, a 24 h hydraulic retention time and a 2 g COD L-1 day-1 organic loading rate using lactate as the electron donor. The removal efficiencies were stabilized at 100, 30 and 80% for NO3-, SO42- and total Se, respectively, during 92 days of UASB operation. The total Se removal efficiencies dropped to 47% or even to a negative value when, respectively, SO42- and NO3- were sequentially excluded from the influent. Speciation of Se, particularly the microbial production of colloidal Se0 levels, was influenced by both SO42- and NO3-. The results presented here demonstrate that UASB reactors are capable of removing SeO42- in the presence of millimolar concentrations of NO3- and SO42- typically found in Se-contaminated wastewaters.