Abstract:
Coal Combustion Products (CCPs) also known as coal ash/residues are generated when coal is burnt to generate electricity or steam. CCPs include fly ash, bottom ash, flue gas desulfurization (FGD) materials (FGD solids, FGD Gypsum), spray dryer ash etc. In 2012, approximately 40 percent of the CCPs generated in United States were beneficially used while the remaining 60 percent of CCPs were disposed in surface impoundments and landfills. According to American Coal Ash Association (ACAA), the production of CCPs has grown from 59.5 million Mg in 1974 to 114.7 million Mg in 2013.
The leachates produced at the CCP landfills may contain high concentration of toxic substances such as heavy metals, very high and low pH conditions and high salinity. Therefore, United States Environmental Protection Agency (USEPA) require the CCP disposal facilities to have a composite liner consisting of a geomembrane overlying a 0.6 m thick compacted clay liner (CCLs) to prevent the potential contamination of groundwater. Hydraulic conductivity of CCLs should be < 1 × 10-9 m/s. The regulation
allows the use of alternative materials (e.g., Geosynthetic Clay Liners, GCLs) in lieu of CCLs, provided that they have equal or lower hydraulic conductivity than CCLs.
Conventional sodium bentonite (Na-B) GCLs consist of a thin layer of sodium bentonite sandwiched between two geotextiles. GCLs are easy to install, save air space due to less thickness (5 to 10 mm), and have low hydraulic conductivity (< 10-10 m/s). The low hydraulic conductivity of Na-B GCLs is due to swelling of bentonite layer that clogs the void spaces and results in narrower and tortuous flow paths. However, aggressive leachates with high salinity, polyvalent cations or extreme pH can inhibit the swelling of Na-B GCLs leading to high hydraulic conductivity. Studies have shown that polymer modified GCLs consisting of bentonite-polymer (B-P) mixtures have higher chemical compatibility and lower hydraulic conductivity to aggressive leachates than Na-B GCLs.
Recently, the CCP leachate data collected from more than 130 CCP disposal facilities have shown much more aggressive leachates than those studied by previous researchers. Therefore, the purpose of this study was to evaluate the hydraulic performance of B-P GCLs to new CCP leachates. Index properties and hydraulic conductivity of B-P GCLs was studied in detail. Swell index and hydraulic conductivity tests were conducted on six B-P GCLs permeated to six synthetic CCP leachates. B-P GCLs were selected with varying polymer loading ranging from 0.5% to 12.7%, to evaluate the effect of polymer loading on the hydraulic conductivity of B-P GCLs. The six synthetic CCP leachates were created based on data collected from more than 130 CCP disposal facilities to represent the typical and worse scenarios observed in the field.
