Biological Selenium Reduction and Recovery in Flue Gas Desulfurization Wastewater

<p dir="ltr">Enhanced understanding of anaerobic selenium oxyanion reducing bacteria (SeRB) is needed to harness their full potential for the remediation and recovery of selenium from flue gas desulfurization (FGD) wastewater (WW). This thesis evaluates the taxonomic identities of SeRB native to FGD wastewater, in simulated co-contaminant conditions, and in the treatment train associated with FGD wastewater. It also evaluates the SeRB metabolic capabilities, and the biogenic products produced. This work provides a novel resolution of selenium reduction in these systems not yet reported in literature and offers a fresh perspective of selenium remediation and recovery, one that stands as a novel contribution to the scientific discourse.</p><p dir="ltr">The first objective focuses on understanding and enriching for SeRB from FGD wastewater on defined media. This involves selecting for native SeRB microbes in a controlled environment to achieve novel resolution of these organisms, identifying previously unrecognized selenium oxyanion reducing capabilities in Anaerosolibacter alongside predominant SeRB from Mesobacillus and Tepidibacillus genera. This culturing and enrichment effort was essential for isolating and studying the specific microbial players responsible for selenium transformation and understanding their initial community composition. </p><p dir="ltr">The second objective investigates the effect of selenium co-contaminants, nitrate and sulfate, on microbial selenium reduction by testing both the FGD wastewater and the enriched SeRB consortia. This encompasses utilizing a combination of molecular ecology and biogeochemistry approaches to evaluate the SeRB and their role in these environments. This study revealed that selenium reduction extent varied significantly with co-contaminant type and concentration, with nitrate proving to be less inhibitory to selenium removal than sulfate. Notably, Mesobacillus emerged as a particularly resilient and metabolically versatile genus, demonstrating remarkable tolerance to environmental stresses such as thermal stress and co-contaminant presence in anoxic reactor conditions. Furthermore, a crucial finding was that native FGD wastewater inocula consistently demonstrated superior performance in selenium reductions compared to enriched communities, strongly highlighting the indispensable importance of preserving ecological complexity and stress adapted, functionally diverse taxa for effective real world wastewater applications. </p><p dir="ltr">The third objective investigates the treatment train of FGD systems, specifically looking for selenium reduction in the commercial bioreactor. This involved leveraging a rare opportunity to sample from a full-scale FGD biotreatment system engineered to meet the effluent limitation guidelines but currently underperforming for selenium. In addition, selenium recovery potential was evaluated by stimulating bacteria from the operating biotreatment system to evaluate the microbial consortia that emerges and characterizing the biogenic elemental selenium formed. This integrated approach allowed for a holistic understanding of the microbial mechanisms at play in a practical setting. The insights gained from identifying specific selenium oxyanion reduction genes within metagenome assembled genomes (MAGs), coupled with the comprehensive biogeochemical and particle characterization data from a real-world system, will empower wastewater treatment operators to design and manage highly optimized biological treatment systems that can effectively cultivate efficient SeRB for enhanced selenium removal and substantially increase the potential for valuable selenium recovery. </p><p dir="ltr">The main findings in this work includes identifying novel selenium reducing bacteria derived from FGD wastewater and distinct microbial consortia for selenate and selenite. This study also found that the native microbial communities of FGD wastewater are more effective at selenium removal when in the presence of selenium co-contaminants, and that the presence of nitrate is less inhibitory than the presence of sulfate in regard to selenium reduction. This body of work also comprises the first in-depth study of an operating FGD treatment train and offers insight into how microbial communities shift throughout treatment and explores how selenium can be recovered through biotreatment. Metabolic capabilities were evaluated in all objectives, identifying the presence and absence of known microbial selenium reduction genes. This work explores the potential of using FGD WW not only as a waste stream to be remediated, but also as a domestic source of selenium, with biotreatment enabling its biological sequestration and potential recovery. </p><p dir="ltr">This research provides a foundational and novel understanding of the microbial ecology and selenium transformation capacity within FGD wastewater and its biotreatment systems. By elucidating the specific SeRB, their metabolic pathways, and the influence of environmental factors and co-contaminants, this study offers actionable insights for optimizing treatment processes. Beyond achieving regulatory compliance, this work establishes a paradigm for considering FGD wastewater not merely as a waste stream, but as a potential domestic resource for valuable selenium, thereby fostering a more sustainable approach to industrial waste management and resource recovery.</p>