High-rate biological selenate reduction in a sequencing batch reactor for recovery of 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 for 495 days. The high rates (419 +/- 17 mgSe L-1 day(-1)) were recorded between day 446 and day 495 for a hydraulic retention time of 6 h. The maximum conversion efficiency of selenate amounted to 96% with a volumetric conversion rate of 444 mgSe L-1 day(-1), which is 6 times higher than the rates reported in the literature thus far. At the end of the experiment, a highly enriched selenate reducing biomass had developed, with a specific activity of 856 +/- 26 mgSe(-1) day(-1) g(biomash)(-1), which was nearly 1000-fold higher than that of the inoculum. 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. The produced elemental selenium partially accumulated in the reactor as pure (>= 80% Se of the total mixture of biomass sludge flocs and flaky aggregates, and similar to 100% of the specific flaky aggregates) selenium black hexagonal needles, with cluster sizes between 20 and 200 mu m. 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 high selectivity. (C) 2021 The Author(s). Published by Elsevier Ltd.

Automated summary

A high-rate process for biological selenate reduction to elemental selenium was developed, showing high selectivity and conversion efficiency, which may serve as the basis for a high-rate technology to remove and recover pure selenium from wastewater or process streams with high selectivity.