ASSESSMENT OF EQUALIZATION BASINS IN TREATMENT OF FLUE GAS DESULFURIZATION WATERS

Equalization basins are constructed retention pools used for cooling waters and settling particulates prior to subsequent treatment. These basins, also known as detention basins, have retained sewage, ash sluice waters, surface mine drainage, and surface runoff. As air emission standards are becoming increasingly stringent, equalization basins are being more widely used for flue gas desulfurization (FGD) waters produced at thermo-electric power plants. The major concerns associated with FGD waters include elevated concentrations of suspended solids and inorganics (Cl^-, Hg, Se, and As). This pilot-scale study evaluates the role of equalization basins as a component of constructed wetland treatment systems designed to remove constituents of concern in FGD waters. Two pilot-scale equalization basins were used, a 1000 gallon tank and a 20 gallon rectangular bin. Both simulated and actual FGD waters were used for the experiments. Specific research objectives include: (1) characterize FGD particulates; (2) determine settling rates for particulates in FGD waters; (3) determine if removal of Hg, As, and Se occurs within the pilot-scale equalization basins; and (4) determine the extent of toxicity of FGD water in the pilot-scale basins. Three major particle types identified within FGD particulates include gypsum as the most common, fly ash, and iron oxides. Within a typical equalization basin with a depth of 4.5 meters, 89% of the particulates (all >15 µm) will settle in 7.75 hours. Removal of targeted constituents within a pilot-scale equalization basin ranged from no removal of each element to a maximum of 69% for mercury, 9% for selenium, and 3% for arsenic. Toxicity experiments indicated that both survival and reproduction of Ceriodaphnia dubia were detrimentally affected by FGD waters in the equalization basin. This research supports the use of equalization basins for initial treatment of FGD waters and potentially other types of waters containing settleable particulates.