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

Abstract

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.