THE ROLE OF AN EQUALIZATION BASIN IN A CONSTRUCTED WETLAND TREATMENT SYSTEM FOR FLUE GAS DESULFURIZATION WASTEWATER

Coal-fired power plants have begun implementing flue gas desulfurization (FGD) scrubbers to reduce sulfur dioxide and nitrous oxide emissions to meet air quality standards. Scrubber systems utilize calcium carbonate saturated water and organic acids (i.e. dibasic acid) to precipitate sulfur compounds. Calcium sulfate (gypsum) is often removed from the slurry and used in the production of wallboard. The resulting FGD wastewater and associated particulates produced from this process must be treated before discharge. Constituents of concern in FGD wastewater include chlorides, arsenic (As), mercury (Hg), selenium (Se), and sulfates. If not removed, high concentrations of these elements may cause toxicity to organisms in receiving systems. Our research has investigated the use of constructed wetland treatment systems (CWTS) for FGD wastewater. A pilot-scale CWTS used in the current investigation includes a simulated equalization basin (EQ basin), where the wastewater is cooled and particulates settle, followed by a series of wetland microcosms. Specific research objectives focused on the EQ basin are: (1) to characterize the particulates in terms of organics and inorganic constituents including minerals; (2) to determine settling rates of particulates in the simulated EQ basin and removal rates and extents for As, Hg, and Se; and (3) to assess toxicity of sediments (settled FGD particulates) in the simulated EQ basin. In addition to a large fraction of gypsum crystals, other components such as fly ash are present in the sediments. Settling of wastewater particulates (>2 microns) occurred within the first 12 hours, and removal of the targeted constituents was related to concentrations of the elements within the wastewater. Toxicity experiments using Pimephales promelas (fathead minnow) indicated that toxicity was present at the sediment/water interface of the EQ basin. Results of our investigation demonstrate that treatment of FGD wastewater occurs within the EQ basin by settling and cooling. Therefore, the EQ basin plays a vital role within CWTS used for treating FGD wastewater.