The Influence of MSW Landfill Height in the Perched Leachate Formation

Abstract

During the last decades, the largest part of municipal solid waste (MSW) generated world wide has been disposed of in landfills without any pre-treatment. The main environmental impacts of such landfills, containing high amounts of biodegradable organic matter, are caused by emissions of liq-uid effluents and landfill gas. With no collection and treatment, leachates from landfills pollute groundwater and soils locally. Furthermore, besides the biological processes, many physical phe-nomena occur inside the landfill body which negatively affect landfill management. Indeed, the waste settlement inside the landfill body, with a consequent increase of its density, corresponds to a proportional decrease of waste permeability; such aspect may cause serious problems to leachate drainage through the waste body, with the possible formation of the so-called suspended “perched leachate zone”. The perched leachate zone is a temporary phenomenon, which evolves downwards, reaching the bottom of the MSW landfill and producing a leachate head above the leachate collec-tion system. Such phenomenon is strongly influenced by many factors and, among others, the rain infiltration rate, waste moisture content and landfill height are of great concern. Bearing in mind such considerations, the main aim of the paper is to present a mathematical modelling survey to gain insights about the role played by the landfill height and the initial MSW moisture content. To accomplish such a goal, a previously developed 1D mathematical model was employed. The math-ematical model allows the simulation of the percolation fluxes throughout a MSW landfill. Specifi-cally, the model is based on mass balance equations and considers the landfill divided in several layers evaluating the inflow to and outflow from each layer as well as the continuous moisture dis-tribution. Several model simulations were performed considering different landfill heights and MSW initial moisture. The model results showed a different response of the landfill in terms of flow rates throughout the landfill. Indeed, the landfill height content drastically influenced not only the formation of perched leachate zones but also their extension. In particular, the model results showed that there is a sort of landfill threshold height, below which, the perched leachate zone does not appear.