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Performance of pilot-scale constructed wetland treatment systems for flue gas desulfurization waters

¹ Clemson University, 261 Lehotsky Hall, Clemson, South Carolina 29634; dereke{at}clemson.edu
² Clemson University, 261 Lehotsky Hall, Clemson, South Carolina 29634
³ ENTRIX, 102 East Main St. Pendleton, South Carolina 29670
⁴ Frontier Geosciences, 414 Pontius North Ave, Seattle, Washington 98109

Derek Eggert received his B.S. degree from The Citadel in 1999 and his M.S. degree from Clemson University in 2004. Currently, he is a Ph.D. candidate at Clemson University working with John Rodgers in the field of environmental toxicology. His research involves risk characterization and remediation of constituents of concern in flue gas desulfurization (FGD) waters.

John Rodgers received his Ph.D. from Virginia Polytechnic Institute and State University in 1977. Currently, he is a professor at Clemson University, director of the Ecotoxicology Program in the Department of Forestry and Natural Resources, and codirector of the Clemson Environmental Institute. His research involves a quest for accurate risk characterizations and development of sustainable risk mitigation tactics.

George Huddleston is a project scientist at ENTRIX, Inc., and has 9 years of professional experience involving environmental toxicology and ecological risk management. He specializes in the assessment of natural resources and the development of strategies for mitigating ecological risks. He received his B.S. and M.S. degrees from Eastern Kentucky University and received his Ph.D. in 2001 from Clemson University.

Carl E. Hensman received his Ph.D. in chemistry from New Mexico State University in 1998. Since then, Hensman has been focused on technology development to reduce the anthropogenic impact on the environment, while maintaining all stakeholder interests. He currently holds the position of Vice President of Technology Development at Asemblon Inc. (Redmond, Washington), a company focused on surface molecular chemistry and hydrogen economy.

Federal laws regarding ambient air quality are currently requiring industries to reduce emissions of sulfur and nitrous oxides. Coal-fired power plants have therefore begun implementing flue gas desulfurization (FGD) scrubbers that use a highly oxygenated water stream (calcium-carbonate-saturated water) to transform sulfur gases into soluble anion species (sulfite and sulfate). The chemical compositions of FGD waters are dependent on the FGD scrubber design, coal types burned, chemical additives, and scrubbing solution source. The FGD waters contain potentially toxic elements including arsenic, cadmium, chemical oxygen demand (COD), copper, mercury, selenium, chloride, sulfates, and zinc. Therefore, these waters must be treated before discharge into a receiving system because of constituents that can elicit toxicity. The specific objectives of this research were to (1) characterize FGD waters in terms of chemical composition and constituents of concern, (2) design constructed wetland treatment systems (CWTSs) for the remediation of constituents of concern in FGD waters, and (3) measure the performance of CWTSs for formulated and actual FGD waters based on discharge criteria established by the United States Environmental Protection Agency and regulated by National Pollutant Discharge Elimination System permits. The FGD waters are characteristically high in total dissolved solids (calcium, chloride, magnesium, and sulfate), are semineutral in pH, contain high concentrations of total suspended solids, and contain several potentially toxic constituents. Constituents of concern were identified as cadmium, COD, chloride, copper, mercury, selenium, and zinc. Pilot-scale CWTSs were designed based on biogeochemical data, and each system contained an equalization basin and two reducing and oxidizing wetland reactors in series. Three FGD waters were introduced in the pilot-scale CWTS, and the performance was assessed by measuring targeted constituents of concern (mercury and selenium) and the toxicity of pre- and posttreatment waters. Results from these studies indicate that mercury and selenium concentrations in FGD waters can be decreased using CWTSs, and, with an appropriate comanagement of low-ionic strength water for chloride concentrations, toxicity of posttreatment samples is decreased to acceptable discharge limits.