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 °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.