A Study On Numerical Analysis Of Pollutant Transport Of Municipal Solid Wastes And Design Method Of Safety Lining System Of Landfill

In order to effectively control the pollution and destruction on environments induced by the municipal solid wastes (MSW), modern landfill technique has been widely employed in the safety and economic treatment and disposal of MSW. In order to effectively protect the quality of ground water resource, the safety lining system of the MSW landfill is one of the most important technical issues in the design and construction of the modern landfill. Therefore it is necessary to discover the transport process of pollutants in MSW for the given condition including lining system, site hydrogeological and climate conditions, generation mechanism of contaminants as well as management condition of landfill. In fact, as an effective and economic lining system, compacted clay lining system which was popularly used in the modern countries gets more and more application in the modern landfill in China. Hence, For both two-layer structural system composed of compacted clay lining and aquifer underlying the landfill and three-layer structural system consisting of the compacted clay lining and unsaturated soil stratum as well as aquifer underlying the landfill, one-dimensional and two-dimensional computational model are respectively established and leachate migration process is analyzed numerically. Through numerical analyses, the main factors influencing the transport mechanism and migration process of contaminants are examined based on comparative studies by varying the related parameters. The studies contains the following main parts.1. One-dimensional analysis of pollutant transport for the two strata system composed of the. CCL and aquifer underlying the landfillFor the MSW landfills in which the lining system is composed of the compacted clay liner, a two-layer strata model is presented to reproduce the system which consists of the compacted clay liner and underlying aquifer. Considering all types of mechanisms including advection, hydrodynamic dispersion and sorption of soil skeleton, one-dimensional differential equation is formulated for describing the migration behaviour of pollutants in the compacted clay liner and aquifer. Together with the actual initial and boundary conditions, the resulting initial boundary-value issue is numerically solved by various effective procedures. Based on abundant numerical computations by varying various relevant parameters, the effects of the production modes of pollutant source, hydraulic conductivity and dispersion as well as various sorption behavior of the compacted clay liner, the equilibrium and linear sorption of the aquifer as well as bio-chemical degradation on the migration behavior of contaminants in both the compacted clay liner and aquifer are systematically examined. It is shown that an independent mechanism in addition to hydrodynamic dispersion and advection, the sorption of soil skeleton plays a much important role in controlling the migration behavior of pollutants. The sorption model and its parameters should be carefully chosen in the numerical analysis of pollutant migration and design of MSW landfill. Moreover, both coefficients of hydraulic conductivity and dispersion of the compacted clay liner should be duly determined according to the actual condition. Compared to the sorption effect of the compacted clay liner, the influence of sorption of aquifer on the migration behavior of contaminants can be overlooked.2. Two-dimensional analysis of pollutant transport in the three strata system composed of the CCL and saturated aquitard and aquifer underlying the landfillFor the three strata system composed of the CCL and saturated aquitard and aquifer underlying the landfill, Considering the anisotropy of permeability of geo-media and nonlinear characteristics of equilibrium sorption, based on two-dimensional computational model and finite-difference method are developed to simulate the pollutant transport process. The comparative study based on numerical analyses for a given landfill, it is shown that vertical dispersion coefficient of both CCL and aquitard have more important effet in transport of contaminants. The maximum sorption capability of the CCL has more noticeable effect on the pollutant transport.3. One-dimensional numerical analysis of pollutant transport for the in the two strata system composed of the CCL and aquifer underlying the landfill considering the reduction of varying porosity of geo-media induced by sorption.The sorption of the CCL on contaminants will reduce the porosity and the permeability of geo-media. Therefore variation of porosity of geo-media should be taken into account. Under the linear and nonlinear conditions of sorption of geo-media on contaminants, the empirical patterns of variation of porosity with sorption are given. Then for the two-strata system consisting of the CCL and aquifer underlying landfill, the controlling equation of pollutant transport is established and the actual boundary conditions are incorporated. The pollutant transport process is numerically analyzed. Through comparative analyses, it is shown that the variation of porosity induced by sorption will exert a considerable effect on the transport of pollutants, the peak concentration of pollutants in the CCL will be reduced to one-second as in the circumstance of constant porosity while the reduction of the peak concentration of pollutants in the aquifer will be more significant. As decreasing of the porosity with sorption, the dispersion will become the key mechanism governing the pollutant transport.4. One-dimensional analysis of pollutant transport for the three strata system composed of the CCL and unsaturated soil layer as well we aquifer underlying the landfillThe actual landfill site is idealized as three-layer strata composed of a compacted clay liner and an underlying unsaturated deposit and a layer of aquifer are below the landfill. For such a system, one-dimensional mathematical equation for governing landfill pollutant transport through the three-layer strata is established. In the proposed model, steady-state unsaturated moisture distributions are employed in the dispersion-advection equation to simulate the transient distribution of contaminant in the strata beneath landfill. For the compacted clay liner, nonlinear equation of equilibrium sorption for the contaminant is employed while the linear equation of equilibrium sorption is used for the unsaturated soil layer. The proposed equation of pollutant transport is numerically solved and comparative computations for varying parameters are made to examine the effect of various factors on the transport process of pollutants in each individual layer of the strata. It is shown that the existence of the unsaturated soil underlying the landfill play a significant role in contaminant migration through both the unsaturated soil layer and aquifer. With decrease of saturation degree of unsaturated soil, the migration of pollutants will be reduced. Moreover, when nonlinear sorption capacity of compacted clay liner is large, not only the concentration of contaminant migration through the three layers will decrease considerably, but also the retardation effect will get more significant, particularly in unsaturated soil layer and aquifer. Therefore the nonlinear sorption capacity of the compacted clay liner is one ofthe most important factors for governing the contaminant transport in three-layer system.5. Simplified computational method for design of fundamental properties of landfill liners in municipal solid wastesThe related factors affecting contaminant transport mechanism and process in single geomembrane liner, compacted clay liner and composite liner are evaluated. Based on the different contaminant transport mechanism respectively for single geomembrane liners, compacted clay liners and composite liners, simplified methods are presented for evaluation and design of fundamental properties of seepage-prevention liners of municipal solid landfill. Parametric computations are performed by using the proposed methods. The computed results show that the key important parameters controlling contaminant transport process include hydraulic conductivity, retardation factor, thickness of compacted clay liner, defects of geomembranes and height of leachate in landfills.