Effect of Biological Activities on Selenium Removal by Continuous-Flow Reactors Packed with Alum-Impregnated Activated Alumina

Se(VI) and Se(IV) are the two major species of selenium in water. They are toxic to aquatic lives and human beings at high levels. Biological reduction of Se(VI) is a two-stage process, first from Se(VI) to Se(IV) and then to Se(0) with potential accumulation of the more toxic Se(IV) due to the slower rate of the second stage. Activated alumina, on the other hand, is more effective for Se(IV) adsorption than Se(VI). In this study, the effect of biological activities on selenium removal was investigated using continuous-flow reactors packed with alum-impregnated activated alumina (AIAA) and cultured with a Se(VI)-reducing strain, Shigella fergusonii strain TB42616, under various influent Se(VI) concentrations and hydraulic retention times (HRTs). A selenium removal efficiency of 92% was achieved in a bioreactor with initial biomass of 2.2×106 cells/g AIAA after a 70-day operation period. Little improvement was observed by lowering the influent Se(VI) concentration from 50 to 10 mg/L, while the removal efficiency was significantly enhanced by either extending the HRT from 3.2 to 5.0 days or increasing the attached biomass during the startup. An increase in mass ratios of Se(VI) reduction by immobilized cells to adsorption by AIAA was also observed from 1:2.2 to 1:0.83 with increasing cell mass during the operation.