Date of Award

12-2007

Document Type

Thesis

Degree Name

Master of Science (MS)

Legacy Department

Hydrogeology

Advisor

Castle, James W.

Committee Member

Rodgers , John H.

Committee Member

Andersen , C. B.

Committee Member

Huddleston , George M.

Abstract

Coal-fired power plants are introducing flue gas desulfurization (FGD) scrubbers to reduce sulfur dioxide and mercury emissions in order to meet air quality standards. FGD scrubber systems utilize a mixture of limestone, water, and organic acids to precipitate sulfur compounds. The resulting FGD water and associated particulates often contain constituents of concern including chlorides, inorganic elements (Hg, As, and Se), and sulfates that must be treated before discharge. Constructed wetland treatment systems, consisting of an equalization basin followed by wetland reactors, present a viable option to efficiently treat FGD waters. Equalization basins are designed to cool and homogenize FGD water and settle particulates. Specific research objectives focused on equalization basins are: 1) to characterize FGD particulates in terms of elemental and mineralogical composition; 2) to determine size and settling rates of FGD particulates; 3) to determine if Hg, As, and Se concentrations within FGD water stored in an equalization basin change with time; and 4) to determine if toxicity of FGD water within an equalization basin changes during a 24 hr hydraulic retention time.
The most common FGD particle type was characterized as gypsum. Other particle types identified included fly ash and iron oxides. FGD particulates settled in an equalization basin are interpreted to have originated during coal combustion and FGD processes. The majority of FGD particulates were determined to be silt size, and settling analysis shows that 95% of these particulates settled to the bottom of a typical 2.5 m deep equalization basin within approximately 4 hrs. FGD particulates contained concentrations of Hg, As, and Se, and as particulates settled, constituents were removed from the water column. Analysis of FGD water samples indicate that aqueous concentrations of Hg and Se decreased in the pilot-scale equalization basins by 20 µg/L and 200 µg/L, respectively, during a 24 hr hydraulic retention time. Data from toxicity tests indicate that equalization basins do not decrease toxicity of FGD water to aquatic organisms. Equalization basins are necessary for initial treatment of FGD water by settling particulates, which may contain Hg, As, and Se. Additional treatment for these waters occurs in the wetland reactors.

Included in

Geology Commons

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