Batch Test Assessment of Waste-to-Energy Combustion Residues Impacts on Precipitate Formation in Landfill Leachate Collection Systems

Abstract

Disposal practices for bottom ash and fly ash from wasteto-energy (WTE) facilities include emplacement in ash monofills or co-disposal with municipal solid waste (MSW) and residues from water and wastewater treatment facilities. In some cases, WTE residues are used as daily cover in landfills that receive MSW. A recurring problem in many landfills is the development of calcium-based precipitates in leachate collection systems. Although MSW contains varying levels of calcium, WTE residues and treatment plant sludges have the potential to contribute concentrated sources of leachable minerals into landfill leachates. This study was conducted to evaluate the leachability of calcium and other minerals from residues generated by WTE combustion using residues obtained from three WTE facilities in Florida (two mass-burn and one refuse-derived fuel). Leaching potential was quantified as a function of contact time and liquid-tosolid ratios with batch tests and longer term leaching tests using laboratory lysimeters to simulate an ash monofill containing fly ash and bottom ash. The leachate generated as a result of these tests had total dissolved solid (TDS) levels ranging from 5 to 320 mg TDS/g ash. Calcium was a major contributor to the TDS values, contributing from 20 to 105 g calcium/kg ash. Fly ash was a major contributor of leachable calcium. Precipitate formation in leachates from WTE combustion residues could be induced by adding mineral acids or through gas dissolution (carbon dioxide or air). Stabilization of residual calcium in fly ashes that are landfilled and/or the use of less leachable neutralization reagents during processing of acidic gases from WTE facilities could help to decrease the calcium levels in leachates and help to prevent precipitate formation in leachate collection systems. INTRODUCTIONWaste-to-Energy (WTE) facilities provide a means to reduce the volume of municipal solid wastes (MSW) that require landfilling while yielding energy and recoverable ferrous and nonferrous metals. Byproducts from WTE facilities include combustible gases and noncombustible residues (fly ash and bottom ash). Although options have been developed for using ash residues in different construction applications, 1,2 the majority of WTE residues produced in the United States are either disposed in ash monofills or codisposed with MSW and other materials such as residues from water and wastewater treatment facilities [3][4][5] The leaching characteristics of WTE residues have been studied extensively, particularly with respect to the leachability of toxic materials from fly ashes and bottom ashes. 6,7 Less information is available on the leachability of dominant minerals that may participate in precipitation reactions. For example, a recurring problem in many landfills is the development of calcium-based deposits that can clog leachate collection systems. 8 -11 The purpose of this study was to assess the leachability of precipitatable minerals from WTE residues and evaluate the potential to form mineral deposits in...

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