Biological treatment of selenate-containing saline wastewater by activated sludge under oxygen-limiting conditions

Selenium often coincides with high salinity in certain industrial wastewaters, which can be a limitation in the practical application of biological treatment. However, there are no studies on the biological treatment of selenate-containing saline wastewater. A sequencing batch reactor inoculated with activated sludge was applied to treat selenate in the presence of 3% (w/v) NaCl. Start-up of the sequencing batch reactor with a 7-day cycle duration and excessive acetate as the sole carbon source succeeded in removing above 98% and 72% soluble and solid selenium, respectively, under oxygen-limiting conditions. Further selenium removal experiments with a shorter cycle duration of 3 days and a stepwise decrease of acetate addition achieved soluble and total selenium removal efficiencies in most batches above 96% and 80%, respectively. Mass balance analysis revealed that selenate was converted into elemental selenium, most of which was accumulated in the sludge. Microscopic analyses also found that elemental selenium particles were primarily present as approximately 2 μm large rods, with some extremely large particles above 10 μm. Although the bacterial populations responsible for selenium removal, especially selenate reduction, could not be identified by microbial community analysis, this study reported for the first time that selenate could be biologically treated in the presence of considerable salinity, offering implications for the practical treatment of selenium in certain industrial wastewaters.

Biological treatment of selenium-laden wastewater containing nitrate and sulfate in an upflow anaerobic sludge bed reactor at pH 5.0

This study investigated the removal of selenate (SeO₄^2-), sulfate (SO₄^2-) and nitrate (NO₃^-) at different influent pH values ranging from 7.0 to 5.0 and 20 °C in an upflow anaerobic sludge blanket (UASB) reactor using lactate as an electron donor. At pH 5.0, the UASB reactor showed a 20-30% decrease in reactor performance compared to operation at pH 5.5 to 7.0, reaching removal efficiencies of 79%, 15%, 43% and 61% for NO₃^-, SO₄^2-, Se_total and Se_diss, respectively. However, the reactor stability was an issue upon lowering the pH to 5.0 and further experiments are recommended. The sludge formed during low pH operation had a fluffy, floc-like appearance with filamentous structure, possibly due to the low polysaccharide (PS) to protein (PN) ratio (0.01 PS/PN) in the soluble extracellular polymeric substances (EPS) matrix of the biomass. Scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX) analysis of the sludge confirmed Se oxyanion reduction and deposition of Se⁰ particles inside the biomass. Microbial community analysis using Illumina MiSeq sequencing revealed that the families of Campylobacteraceae and Desulfomicrobiaceae were the dominant phylotypes throughout the reactor operation at approximately 23% and 10% relative abundance, respectively. Furthermore, approximately 10% relative abundance of both Geobacteraceae and Spirochaetaceae was observed in the granular sludge during the pH 5.0 operation. Overall, this study demonstrated the feasibility of UASB operation at pH values ranging from 7.0 to 5.0 for removing Se and other oxyanions from wastewaters.

Biological removal of selenate and ammonium by activated sludge in a sequencing batch reactor

Wastewaters contaminated by both selenium and ammonium need to be treated prior to discharge into natural water bodies, but there are no studies on the simultaneous removal of selenium and ammonium. A sequencing batch reactor (SBR) was inoculated with activated sludge and operated for 90days. The highest ammonium removal efficiency achieved was 98%, while the total nitrogen removal was 75%. Nearly a complete chemical oxygen demand removal efficiency was attained after 16days of operation, whereas complete selenate removal was achieved only after 66days. The highest total Se removal efficiency was 97%. Batch experiments showed that the total Se in the aqueous phase decreased by 21% with increasing initial ammonium concentration from 50 to 100mgL^-1. This study showed that SBR can remove both selenate and ammonium via, respectively, bioreduction and partial nitrification-denitrification and thus offer possibilities for treating selenium and ammonium contaminated effluents.