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Theses and Dissertations--Civil Engineering


Se(VI) and Se(IV), as the two major species of selenium in water, are toxic to aquatic lives and may cause adverse health effects to humans at high levels. Biological reduction of Se(VI) is a two-stage process first from Se(VI) to Se(IV) and then from Se(IV) to Se(0) with potential accumulation of the more toxic Se(IV) due to the slower rate of the second stage.

Selenium reduction was first evaluated with batch cultures of Shigella fergusonii strain TB42616 (TB) and Pantoea vagans strain EWB32213-2 (EWB) isolated in our laboratory from sludge and coal slurry sediment samples, respectively. In order to facilitate Se(VI) reduction and reduce Se(IV) accumulation, the Se(VI)-reducing strain TB was co-cultured with a Se(IV)-reducing strain EWB. Although Se(VI) reduction rate was not affected, Se(IV) reduction was significantly enhanced with low Se(IV) accumulation in the defined co-culture. Effects of culture composition as well as nitrate and arsenate on Se(VI) reduction were also investigated. A co-culture composition of 10:1 (EWB:TB) ratio was observed to achieve the best total selenium reduction. In addition, nitrate at 50 mg/L was observed to inhibit Se(IV) reduction but not Se(VI) reduction, while arsenate at 200 mg/L exhibited slight inhibition on both Se(VI) and Se(IV) reduction.

Biokinetic parameters were optimized with a Monod-type kinetic model using batch pure culture data through the Robust Global Optimization Algorithm embedded in a computer package. Se(VI) reduction by the defined co-culture was then simulated and verified over a range of culture compositions and initial Se(VI) concentrations, respectively. An inter-species inhibition term was incorporated into the model to illustrate the competition for Se(IV) during Se(VI) reduction in the co-culture. The model showed a significant increase of Se(IV) accumulation with higher initial Se(VI) concentration. However, Se(IV) accumulation can be reduced with increasing population ratio of EWB to TB in the defined co-culture. The relatively high correlation coefficients suggested that the model was robust and applicable in simulating Se(VI) reduction by the defined co-culture.

Since activated alumina was reported to be more effective for Se(IV) adsorption than Se(VI), 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 TB 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 hydraulic retention time 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 with increasing cell mass during the operation.

Se(VI) reduction using continuous-flow reactors packed with strain TB immobilized Ca2+-alginate beads was investigated under various hydraulic retention times (HRT) and influent Se(VI) concentrations. A high removal efficiency up to 98.7% was achieved under an HRT of 5 days and an influent Se(VI) concentration of 400 mg/L. The results showed that the overall selenium removal was positively correlated to the bed height of the reactor and the HRT but not related to the influent Se(VI) concentration. The steady state was analyzed using a mathematical model based on Monod-type equations with four biokinetic parameters optimized including the half-velocity constants and maximum specific reduction rates. The relatively high correlation coefficients indicate that the model is robust and valid to simulate Se(VI) reduction in the gel-beads-packed continuous-flow system.


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