The Impact of Leachate From Clean Coal Technology Waste on the Stability of Clay and Synthetic Liners

This project was developed to provide design criteria for landfill disposal sites used for sludges such as those generated using the Clean Coal Technologies (CCT) tested at the Public Service Company of Colorado's Arapahoe Power Plant. The CCT wastes used were produced at the Arapahoe Plant Unit No.4 that was equipped with the integrated dry NOx/SO2 emissions control system installed under the Clean Coal Technology (CCT) Program. The investigation emphasized the potential impact of clean coal technology materials (sodium and calcium injection systems, and urea injection) on the permeability and stability characteristics of clay liner materials and the stability of synthetic liner materials. Flexible-wall permeameters were used to determine the hydraulic conductivities (HC) of the clay liner materials impacted by various compactive conditions. Tests were conducted using the waste materials overlying the clay liner materials under wet/dry cycles, freeze/thaw cycles, and over 120-day periods. The impact of CCT materials on the characteristics of the clay liner materials studied in this project was minimal. The HC measurements of the waste/clay liner systems were similar to the water/clay liner systems. HC decreased for clay liners compacted at moisture levels slightly higher than optimum (standard Procter) and increased for liners compacted at moisture levels lower than optimum (standard Procter). Although some swelling was evident in the sodium materials, the sludge materials did not have a negative impact on the integrity of the liners over 120-day tests. Wet/dry cycles tended to result in lower HC, while freeze/thaw cycles substantially increased HC for the liners tested. Tests were also conducted to assess the compatibility of synthetic liner materials with the CCT by-products. The test program was conducted using methods specified and/or referenced in EPA, SW 846-Method 9090 with some modifications. Compatibility evaluations were made using high-density polyethylene (HDPE), very low-density polyethylene (VDPE), and polyvinyl chloride (PVC) synthetic liner materials treated with baseline fly ash materials (no CCT used), sodium injection materials, calcium injection materials, and materials generated from the sodium injection/urea injection/low NOx burner control system. The synthetic liner materials were subjected to a 50:50 ratio of sludge to water for periods to 120 days at room temperature (23°C). At the end of each equilibration period, the liner materials were tested using mechanical engineering techniques and weight losses due to volatiles and extractables. Sustained increment changes in the measured physical properties of the materials over time were not observed. Some abrupt changes in strength were found at several times during the testing period. However, these aberrations seemed more indicative of isolated changes in the conditioning methods or test procedures and could be related to flaws or changes in the materials related to manufacturing conditions. After 120 days of conditioning, none of the measured physical properties varied significantly from those for the untreated liner materials. This was true for all samples regardless of the conditioning solution used. The volatiles and extractables tests for the HDPE and VDPE materials indicated that the waste materials had little influence on their overall structure. However, the extractables data suggest that PVC liner material might decompose in the waste environments evaluated. The PVC liner material reacted similarly for all treatments with about a 30% weight loss.