Date of Award

5-2009

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Legacy Department

Environmental Toxicology

Advisor

Rodgers, John H

Committee Member

Elzerman , Alan W

Committee Member

Hensman , Carl E

Committee Member

Bridges , William

Committee Member

Kurtz , Harry

Abstract

Federal laws regarding ambient air quality are currently requiring industries to reduce emissions of sulfur dioxides (SO2). Coal-fired power plants have therefore begun implementing flue gas desulfurization (FGD) scrubbers that utilize a highly oxygenated water stream (calcium carbonate saturated water) to transform sulfur gases into soluble anion species (e.g. sulfite and sulfate). This FGD process also transfers potentially toxic constituents including arsenic, cadmium, chemical oxygen demand, copper, mercury, selenium, chloride, sulfates, and zinc into the scrubbing water. These scrubber waters, referred to as FGD waters, present an industrial problem due to the large volumes produced (378,000 to 1,900,000 L/day) and regulations regarding their discharge such as National Pollutant Elimination and Discharge System (NPDES) permits. Constituents that exceed NPDES permit discharge limits or can adversely affect sentinel toxicity testing species must be treated before discharge and were referred to as constituents of concern (COC) in our research. A plausible treatment alternative for FGD waters is remediation utilizing constructed wetland treatment systems (CWTS). Problematic constituents including metals, metalloids, nutrients (i.e. nitrogen and phosphorus), herbicides, pesticides, and generic organics (e.g. oil and grease compounds) have been decreased to acceptable discharge limits using CWTS. In order to design pilot-scale CWTS for FGD waters, we measured and identified the COC for all FGD waters used in this research. COC in these FGD waters were cadmium (Cd), chlorides (Cl), nickel (Ni), mercury (Hg), and selenium (Se) (Chapter Two), arsenic (As), Cd, chemical oxygen demand (COD), Cl, copper (Cu), Hg, Se, and zinc (Zn) (Chapter Three), Hg and Se (Chapter Four), and Se (Chapter Five). While the design of pilot-scale CWTS differed during this research, all systems targeted the removal of metals (Cd, Cu, Hg, Ni, and Zn) and metalloids (Se and As) through microbial reductive pathways in reducing reactors (-200 to 0 mV) and targeted oxidative pathways in the oxidizing reactors (0 to +150 mV). Pilot-scale CWTS are shown to decrease the identified COC in these FGD waters and provided removal rates in order to understand the scaling potential of these systems. Additionally, it was confirmed that pilot-scale CWTS were successful for decreasing the toxicity of FGD waters with co-management techniques for chlorides. Since FGD waters can differ based on site of production and can contain elements or compounds that limit the treatment of COC such as selenium and mercury, organic carbon additions were evaluated for enhancing the performance of CWTS for Se and Hg in two FGD waters. Organic carbon (e.g. molasses, glucose, and trypicase soy broth) additions can enhance the reduction and removal of Se forms in surface waters, but required testing for its application to remediate Se and Hg in FGD waters. Data indicated that sucrose and yeast culture additions could significantly increase the removal of Se in FGD waters using pilot-scale CWTS. Based on these results and laboratory experiments with organic carbon additions, we amended a full-scale CWTS with additions of sucrose and yeast culture. To understand if Se removal was enhanced with these additions, Se measurements were compared between the amended CWTS series and an un-amended CWTS series. This study confirmed that Se and nitrate removal could be significantly enhanced with additions of sucrose. Based on measurements of biochemical oxygen demand, microbial activity was also enhanced and suggests this was an important removal pathway for Se and nitrate. Data presented in this dissertation provide strategies to not only decrease risks associated with FGD waters, but can be applied and transferred to other waters contaminated with metals and metalloids. By increasing our knowledge of approaches to mitigate risks in contaminated waters, we may improve the capture and sequestering of problematic constituents.

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