High-rate biological selenate reduction in a sequencing batch reactor for recovery of highly pure hexagonal selenium

Recovery of selenium (Se) from wastewater provides a solution for both securing Se supply and preventing Se pollution. Here, we developed a high-rate process for biological selenate reduction to elemental selenium. Distinctive from other studies, we aimed for a process with selenate as the main biological electron sink, with minimal formation of methane or sulfide. A sequencing batch reactor, fed with an influent containing 120 mgSe L-1 selenate and ethanol as electron donor and carbon source, was operated at a hydraulic retention time of 6h for 67 days. The operation took place in total 490 days. The Maximum conversion efficiency of selenate amounted to 96% with a conversion rate of 444 mgSe L-1 day-1, which is 6 times higher than rates reported in literature thus far. No evidence was found for the formation of methane, sulfide, or volatile reduced selenium compounds like dimethyl-selenide or H2Se, revealing a high selectivity. Ethanol was incompletely oxidized to acetate. Batch experiments indicated that selenate was reduced stepwise, with selenite as intermediate. The produced elemental selenium partially accumulated in the reactor as a pure (≥80% of total solids) selenium precipitate, with particle sizes between 20-200 µm. These particles consisted of black crystalline hexagonal selenium. The new process may serve as the basis for a high-rate technology to remove and recover pure selenium from wastewater or process streams with a high selectivity