Document Type

Technical Report

Publication Date



Leachate collection systems, ash co-disposal, waste-to-energy residuals, mineral precipitates, lysimeters, landfill leachates


Design, operation, and closure practices for Class I municipal solid waste (MSW) landfills are based on RCRA Subtitle D requirements for control of leachates and gases generated during the life of the landfill. Because leachate is generated as a result of water percolating through the landfill and waste consolidation, it contains dissolved and suspended materials that reflect the characteristics of the material it contacts. Over the past two decades significant changes have occurred in the characteristics of the waste streams that are disposed in landfills. In addition, many Florida municipalities have implemented Waste-to-Energy (WTE) systems. The WTE ash is either disposed in monofills or co-deposited with MSW and/or residuals from water and wastewater treatment facilities. Leachate collection systems consist of: underdrains, collection trenches and pipes, line clean-out ports, pumps and lift stations, and storage tanks or wet wells. Clogging of any portion of the system can lead to higher hydraulic heads within the waste zone and increase the potential for leakage through the liner. The occurrence of clogging of leachate collection systems has been attributed to several factors including sedimentation and deposition of fines, biological clogging, and chemical and/or biogeochemical precipitation. This project was conducted to investigate the impact of waste mixtures and associated characteristics of leachate chemistry and microbiology on the development of mineral precipitates that lead to clogging. The project involved three components: 1) testing of leachates and clog material from a Class I landfill; 2) conduct of laboratory lysimeter tests to compare leachate characteristics from monofills of MSW or combustion residues to leachates generated by mixtures of MSW, combustion residues, and residuals from water and wastewater treatment; and 3) conduct of batch leaching tests to evaluate dominant constituents that leach from combustion residues generated by Waste-to-Energy (WTE) facilities. Laboratory lysimeters were operated for a period of eight months. Chemical and microbiological characterization tests were conducted throughout the operating period. The dominant constituent identified in clog material from leachate collection systems was calcium, with co-precipitation of carbonates, sulfates, iron, phosphorus, and other elements. Leachates generated from MSW dominated lysimeters tended to have relatively high concentrations of organics and alkalinity and a robust microbial community that modulates with changes in the bioavailability of waste components and redox conditions. Leachates generated from ash dominated lysimeters tended to be high in ionic strength, chloride, sodium, potassium, and calcium. During the course of the lysimeter study, deposits developed in the leachate collection tubing associated with the waste mixtures (MSW, combustion residues, and treatment plant residuals). Biomass associated deposits formed in the MSW monofills, while minimal deposition occurred in the ash dominated lysimeters. Based on the data generated by this project, co-disposal of MSW and combustion residues yields leachates supersaturated in carbonate (from biological activity) and calcium (from combustion residues) leading to the formation of mineral precipitates that can clog leachate collection systems. Batch leaching tests provided a method for predicting the leachate composition associated with landfilling of combustion residues and allowed for comparison of the mineral leachability associated with different types of ash processing methods.

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Citation / Publisher Attribution

Florida Center for Solid and Hazardous Waste Management, 74 p.