Constructed treatment wetlands for flue gas desulfurization waters: Full-scale design, construction issues, and performance

Abstract

Relatively large volumes of wastewater (0.5–3 million gallons per day [mgd]) are generated by coal-fired power plants through flue gas desulfurization (FGD). Through wet scrubbing of sulfur from atmospheric emissions, numerous other potentially problematic elements (e.g., Se, Hg, As, etc.) are entrained in the water. After the separation of gypsum, the residual wastewater contains relatively high levels (mg/L) of constituents that require treatment prior to the discharge to receiving aquatic systems. Based on information gained from comprehensive pilot-scale studies, wetland treatment systems were designed and constructed at power plants in North Carolina. Constructed treatment wetland systems were designed to target the removal of Se and Hg from FGD wastewater. The systems consisted of an upstream equalization (EQ) basin followed by three parallel surface-flow wetland treatment trains. Each treatment train consisted of four stages in a series, including two wetland cells planted with a bulrush, a rock cascade, and a wetland cell planted with cattails. The total surface area of the treatment systems was 12 ac, with a hydraulic retention time of approximately 7 days. Inflows during start-up conditions contained Se (38–634 g/L), Hg (0.02–3.55 g/L) and chlorides (503–2796 mg/L). The achieved start-up performance ranged from no measurable removal to 25% removal of Se and 88–91% for Hg. Studies are ongoing to improve the removal efficiency of selenium using constructed wetlands.