CONSOLIDATION OF SANITARY LANDFILLS

Transcription

1 CONSOLIDATION OF SANITARY LANDFILLS Sarubbi, Alejandro Juan (*) University of Buenos Aires, Argentina, Engineering Department. More than 14 years of experience in Project and Works related with the preservation, diagnosys, remediation, urbanization and recovery of the Environment in different countries of Latinamerica (Argentina, Brazil, Colombia, Chile, Mexico, Perú, Uruguay and Venezuela), and in Solid Waste Management (Villa Dominico Landfill operating ton/day, Gonzalez Catán Landfill with 2.300ton/day, security landfill in Veracruz, Mexico for steal dust, etc.). Interamerican Director of Industrial and Dangerous Wastes Division, AIDIS, since 1999 and Member of the Argentine Council for the Sustained Development (CEADS) since Sanchez Sarmiento, Gustavo University of Buenos Aires, Argentina, Engineering Department. Dirección (*): Alberti PB 2. Buenos Aires Argentina - Tel.: (54-11) Fax: (54-11) sar@techint.com SUMMARY The deformations observed in large Sanitary Landfill (SL) are due to several factors, i.e. climatological, geological (base layer of soils), hydrogeological (aquifers movements), operative (acquired compaction and settlements), historical (sequence of disposal), etc. A SL operated in sequential and correlated steps involves a large-scale consolidation process that can be appropriately modeled as a two-dimensional plane strain model, which exhibits many common features with the one-dimensional Terzaghi consolidation problem. The analysis considers finite-strain effects, the garbage s permeability varying with the void ratio and pore leachate, and garbage moisture to flow through it. When a vertical expansion of the landfill occurs, a second layer of garbage creates a new stress framework. The finite element analyses can predict the deformation of the SL final cover as the consolidation process advances and will enable architects and engineers to design future SL. The present paper describes the application of Abaqus/Standard to the modeling this SL. Key Words-Sanitary landfill, biogas, mathematical model, consolidation process, vertical expansion, porous media, leachate. 1. INTRODUCTION A Sanitary Landfill (SL) is a structure designed following engineer techniques for mechanical stability and based on sanitary rules for avoiding potential environmental impacts, so that it is friendly with the ecological media and the human beings. Basically, the SL is a confined recipient for final disposal of solid wastes and its associated liquids. These liquids come from percolation through garbage, from the moisture, and from other products resulting from the physical-chemical-biological reactions. One of these products is the biogas generated by the decomposition of principally- the organic matter. The SL is located over a geological stripe of clay that shows uniform and homogenous characteristics of impermeability. For that reason the area has been selected geologically for installation of the SL, that is, for having the waste s liquids controlled. The stripe of clay avoids the contamination of phreatic water and acquifers. The disposal of solid wastes is done in different thin stripes so that a major compaction and density is achieved, and several properties appear: Better compaction implies greater density of garbage in the SL. Shorter differential settlements. Less probability of external water incoming the SL, and less generation of new leachate. Minor effort in maintenance of the cover and of the SL in general in its last stage (Post-Closure, once the disposal and cover of the wastes is finished). 1

2 Less cost associated with the disposal of garbage. Once the first stage of landfilling with wastes is over, a second one - called Vertical Expansion is put over. This last process implies to dispose a second layer of wastes with its corresponding intermediate and final cover, creating a new stress framework. These concerns defines the design of the SL. The slopes of the cover, maximum height of the landfill, and time of use, are the main factors that affect the final cost of the infrastructure and the price per ton of garbage disposed. There are also serious environmental and sanitary considerations, e.g. the catastrophic collapse of a SL in Santa Fe de Bogotá, that cause the sliding of 1,000,000 Tons of garbage into a nearby river, and then a huge cost of remediation came up. 2. OBJECTIVE OF THE PRESENT STUDY Our objective is to study the effects of construction and operation of a SL over the solid wastes and the soil strata. A SL operated in sequential and correlated steps involves a large-scale consolidation process that can be appropriately modeled as a two-dimensional plane strain model, including garbage s permeability varying with the void ratio and pore leachate and garbage moisture s flow. The finite element analyses can predict the deformation of the SL final cover as the consolidation process advances and will enable to design more cost effective SL. The present paper describes the application of Abaqus/Standard to the modeling of a typical SL [2]. 3. MODEL DEFINITION A mathematical model may be defined by a combination of a number or subsystems, each of which is understood independently, into a whole assembly which is not otherwise predictable. SL are extremely complex environments in which bacteria are both controlled by in and exert processes over the local ecosphere, and simple laboratory experiments have not been able to replicate the full extent of these interactions. Basically, the steps needed to construct a computer model, are as follows: 1. Decide what information is being sought 2. Identify the external controllable influences. 3. Select the main internal factors involved in the system. 4. Describe the relation between the internal factors (its own interactions) and the external influences. 5. Translate the mathematical description into computer code BASIC CONCEPTS The SW are described by the combination of three components, each characterized by a different hydrolysis and biodegradation rate. The hydrolysis rate is first order with respect to solid organic carbon, and Monod kinetics were assumed to model the growth of both acidogens and methanogens. The hydrolysis products are summed and follow the same subsequent biochemical processes with acetic acid acting as a surrogate for all acids generated. Carbon exists in one of seven forms: solid carbon, aqueous carbon, acidogenic biomass carbon, methanogenic biomass carbon, acetate carbon, carbon dioxide carbon and methane carbon. Each layer of the SL is treated as a batch bioreactor and the overall production is the sum of gas generated by individual layers. Initially, the void volume is occupied by nitrogen and carbon dioxide with 80% and 20% mole fraction respectively. This implies that oxygen has been depleted and the anaerobic phase has started. The SL is analyzed as a homogeneous isotropic porous medium. The gravitational term of Darcy s law is neglected for gas flow. Pressure and concentration gradients are negligible in the horizontal direction, which is justifiable for large SL away from their boundaries. The SL bottom and side walls are assumed impermeable, thus gas flow out of SL occurs only across its top surface, in vertical direction. The gas mixture consist of three components: methane, carbon dioxide and nitrogen. Atmospheric oxygen that may be present in the upper layers is negligible. Gas concentrations at the top of the landfill are assumed to be 2

3 equal to the ambient concentrations and the gas pressure equal to the atmospheric pressure. The gas mixture viscosity and the diffusion coefficients are assumed to be independent of the gas mixture composition CHARACTERISTICS OF THE MODEL The two-dimensional plane strain models exhibits many commons features with the one-dimensional Terzaghi consolidation problem, but performed with solid waste. The vadose zone extends from the land surface to the leachate table, with moisture movement through this zone onto the leachate collection system. The analysis considers finite-strain effects and the garbage s permeability varying with the void ratio. The change of permeability with the void ratio is physically realistic as garbage is compressed, in a vertical expansion operation, and becomes harder for pore leachate and garbage moisture to flow through it. When a vertical expansion of the landfill occurs, a second layer of garbage creates a new stress framework. TABLE 1: Properties assumed for the materials of the model Material Permeability [m/s] Specific weight [kg/m 3 ] Young Modulus [Mpa] Poisson s ratio Void Ratio 1 1.0x x x x x x The finite element analyses can predict the deformation of the sanitary landfill final cover as the consolidation process advances and will enable designers and architects to design future Sanitary Landfills. According to the classical form of the Darcy equation, the model proposes different seepage gradients in the field of solid wastes, different effective stresses and consolidation of garbage due to the sequential form of disposal (garbage disposed at different times). The model has been calibrated with real numerical data collected at a big landfill in the outskirts of Buenos Aires, Argentina, and information from other cities. As it is shown in figure No. 1, the model is composed by the materials indicated in table Figure 1. - Stripes in the sanitary landfill model. 3

4 4. DESCRIPTION OF THE CONSOLIDATION PROCESS The stripe of clay below the SL, and the stripes of wastes and the cover soils described above, behave as pore materials with some saturation degree. When the different stripes of solid wastes and soils are disposed, the underground clay is compressed and the water inside is thrown away. This process is known as consolidation (Lambe et al., 1976). During the consolidation process, the quantity of water that enters an element horizontally is less than the one that goes out, so that the continuity condition of the Filtration Theory is not more applicable. The stress or pressure of consolidation at a moment that is applied to the first stripe of garbage is almost received completely by the water that fills the pores of the clay. At this first moment, the stripe of clay has a hydrostatic overpressure almost equal to the consolidation stress. As time goes by, the overpressure of the water lowers and therefore the effective pressure increases. At any point of the clay stripe that is consolidating, the value of the hydrostatic overpressure u at a certain moment can be determined by the following equation: u = γ h w equation (1) where: h is the hydraulic charge compared with the phreatic water level; and γ w is the specific weight of the water and/or the leachate. After a long time, the hydraulic overpressure u reaches a zero value and all the consolidation pressure is transformed in effective pressure that is transmitted grain to grain of the soil. Denoting by p to the consolidation pressure at a certain point, the equilibrium condition requires: p = p + u equation (2) where p is the pressure of consolidation that, at a defined moment, is being transmitted grain to grain and u is the hydraulic overpressure that corresponds to the same moment. Because in equation [2], p is a constant value, the progress of the consolidation at a certain point can be visualized with the variation of u at that point or, with the variation of h through imaginary piezometers distributed inside the clay stripe. It is considered here that this stripe of clay can drain freely through its upper limits (solid wastes) and through its lower limit (stripe of lime-sandy soil with a high coefficient of permeability k, such as 1.0x10-4 m/s). The calculation of the speed of consolidation and of the degree of consolidation U (per cent) has the following simplification hypothesis: The coefficient of permeability k is constant at any point of the stripe of clay that consolidates and varies with the progress of the consolidation process. The coefficient of volumetric compressibility is also constant at any point of the stripe that consolidates and does not vary with the progress of the consolidation process. The water inside the clay stripe and the leachate inside the solid waste, only flows in the vertical direction, and the consolidation stress p is constant at any height of the stripes of clay and/or garbage. According to the Darcy s Law: h 1 u V = k.i = k = k z γ w z equation (3) the consolidation differential equation to solve is [4]: k γ m w v 2 u u = 2 z z equation (4) 4

5 5. CHARGE S SEQUENCE APPLICATION The process simulated consists of the following steps: Step 1: Stripe of clay (material 1): Pre-existing base (geologically disposed). Step 2: First stripe of solid wastes (material 2): Applied over the stripe of clay (material 1). Step 3: First stripe of cover (material 3): Applied over the stripe of solid wastes (material 2). Step 4: Second stripe of solid wastes (material 4): Put over the first stripe of cover (material 3). Step 5: Second stripe of intermediate cover (material 5): Applied over the stripe of solid wastes (material 4). Step 6: Third stripe of cover final (material 6): Distributed over the second stripe of intermediate cover (material 5). 6. DESCRIPTION OF THE MODELING PROCESS The analysis of saturated soils and wastes requires solution of coupled stress-diffusion equations. The coupling is approximated by the effective stress principle, which treats the saturated soil and waste as a continuum, assuming that the total stress at each point is the sum of an effective stress carried by the soil skeleton and a pore pressure in the fluid permeating the soil. This fluid pore pressure can change with time (if external conditions change, such as the addition of a load to the soil or waste). The gradient of the pressure through the soil, which is not balanced by the weight of fluid between the points under discussion, will cause the fluid to flow. The flow velocity is proportional to the pressure gradient in the fluid according to Darcy s Law GEOMETRY AND MODEL The discretization of the semi-infinite, totally loaded strip of soil and wastes is shown in figure 2. The reducedintegration plane strain element with pore pressure, CPE8RP, is used in this analysis. Reduced integration used because it gives more accurate results is less expensive than full integration. The only mesh convergence studies where those recommended by Gibson et al (1970). Pore pressure values are obtained by linear interpolation of values at the corner nodes of an element TIME STEPPING As in the one-dimensional Terzaghi consolidation solution, the problem is run in two steps. In the first step, the load (stripe of waste: material 2 and its final cover: material 3) is applied and no drainage is allowed across the top surface of the mesh. This one increment step establishes the initial distribution of pore pressure which will be dissipated during the second step. As consolidation is a typical diffusion process, initially the solution variables change rapidly with time, while at later times more gradual changes in stress and pore pressure are seen. Therefore, an automatic time stepping scheme is needed for any practical analysis, since total time of interest in consolidation is typically orders of magnitude larger than the time increments that must be used to obtain reasonable solutions during the early part of the transient. Then this process is reproduced with the second floor of wastes (material 4) and its stripps of final and intermediate cover (material 5 and 6) The time stepping is then summarized in the following table. TABLE 2: Steps and time of the process. Step Description Time Step-1 Rapid Charge I1 2 months Step-2 Rapid Charge I2 10 days Step-3 Consolidation 13 years Step-4 Rapid Charge I3 3 months Step-5 Rapid Charge I4 15 days Step-6 Consolidation 7 years 5

6 7. RESULTS The study of the model allows to make an analysis through the following principal variables: void ratio, spatial displacement and pore pressure under different steps of time. We describe at follows these variables separately VOID RATIO As it can be seen in the sketches of the figure 2, the void ratio varies with the increment of the charges applied and as time goes by. In the first sketch when the layer of garbage is disposed, an accomodation process has occured (after 2 months) and the solid wastes located in the inferior part of the SL change their void ratio. The clay also reduces its void ratio as a consequence of the pressure produced by the garbage layer and a lateral movement of the liquids contained in its pores. In the second sketch, no much difference is observed by the distribution of the soil cover layer. The consolidation process (see third sketch) produces little variation because of the small pressure produced by the layer of solid wastes and cover soil. These two layers work as gravitational charges (weight of the garbage and of the cover soil), and do not represent important charges for the consolidation process even though the time is quite large (10 years). This result can be confirmed in old SL built 20 years ago. In the fourth sketch, the new layer of garbage (more weight over the first layer of garbage) is disposed, and a new accomodation of the void ratio occurs. The next sketch shows the consolidation process at the first increment step (the total period is of 5 years), and the two accomodation processes can be observed in the first old layer of garbage, as well as in the second -more new/fresh- layer of garbage. Finally, the last sketch represents the situation of the SL after 5 years of consolidation process, when an important part of the physical, chemical and biological transformations have taken place SPATIAL DISPLACEMENT AT NODES At figure 3, it is represented the different stages of the spatial displacement at the nodes of the mesh. In the first and second sketches the first solid waste layer and soil cover have been applied. There is a principal displacement in the central part of the SL. In the perimetral area of the SL the displacements are minor and it can easily be seen that there is a direct relation with the capacity of accomodation, drainage of the leachate contained inside the pores and the weight of the materials considered. In the third sketch, again, the consolidation of these two layers plus the clay below, have no much changes. When we applied the second layer of wastes (rapid charge in 3 months), the fourth sketch shows the incrementation of the displacement in the central part of the SL, where there is a bigger capacity to absorb pressure and deformations. Repeating the study in the final stage of discharging the second layer of wastes, the displacement can be seen in the fifth sketch. Applying the soil final cover does not change much (sketch sixth), and the consolidation process after 5 years shows the final seventh sketch PORE PRESSURE The pore pressure is related with the void ratio, the liquid contained (leachate) and the charges applied. In the first sketches there is no change because the pore pressure is absorbed by the liquid (both the one inside the clay as well as the leachate contained into the solid wastes). After the second layer of wastes is put, there is a change in the perimetral part of the SL. The perimetral embankment is taken all the forces made by the different layers of soils and wastes. As these charges are vertical, they produced horizontal efforts that are absorbed by the drainage of the liquids and the accomodation of the particles of the soil in the embankment (see sketch four). 6

7 The consolidation process of the all layers in the period of 5 years produces the last two skeches. In these sketches an homogenization of the pore pressure can be observed. This confirms that the wastes behave with similar properties to some soils. 8. CONCLUSIONS The prediction of the time history of the physical, chemical and biological transformation of the solid wastes inside a SL can be done through this presented model that show the direct relation with some parameters. The principal parameters selected are void ratio, displacement and pore pressure, varying with time. The vertical settlements of the soils and the garbage under its own charges show a good correlation with the real settlements measured in the field. The model created allows to explain the accomodation process of the soils and the wastes and their liquid associated, confirming the consolidation process with the wastes as time goes by. It is very important to identify the steady state of the SL so that it can be closed, works of maintance could be reduced (lower costs) and new activities (for recreational purposes) involving the community could start without any impact for the environment. REFERENCES Bear, J. (1972) Dynamics of Fluids in Porous Media. New York, U.S.A. American Elsevier. Bryers, J. D. (1984) Structured modeling of the anaerobic digestion of biomass particulates. Biotechnology and Bioengineering 27, El-Fadel, M., Findikakis, A. N. & Leckie, J. O. (1989) A numerical model for methane production in man aged sanitary landfills. Waste Management & Research 7, El-Fadel, M., Findikakis, A. N. & Leckie, J. O. (1996) Numerical modeling of generation and transport of gas and heat in sanitary landfills: I. Model Formulation, Waste Management & Research 14, El-Fadel, M., Findikakis, A. N. & Leckie, J. O. (1996) Numerical modeling of generation and transport of gas and heat in sanitary landfills: Formulation, II. Model Application. Waste Management & Research 14, El-Fadel, M., Findikakis, A. N. & Leckie, J. O. (1997) Numerical modeling of generation and transport of gas and heat in sanitary landfills: III. Sensitivity Analysis. Waste Management & Research 15, Gibson, R.E., Schiffman, R.L. & Pu, S.L. (1970) Plane Strain and Axially Symmetric Consolidation of a Clay Layer on a Smooth Impervious Base, Quaterly Journal of Mechanics and Applied Mathematics, vol. 23. Kumar, S. (2000) Settlement prediction for municipal solid waste landfills using power creep law. Soil and Sediment Contamination, U.S.A., Lambe, William T. & Whitman, Robert V. (1976) Mecánica de Suelos, Editorial Limusa. Sarubbi, A., Ronnow, M. & De Luca, M. (1991) Estudio de la Calidad de los Residuos Sólidos de la Ciudad de Buenos Aires, CEAMSE Facultad de Ingeniería - U.B.A. Ingeniería Sanitaria y Ambiental, Asociación Argentina de Ingeniería Sanitaria y Ciencias del Ambiente - AIDIS, Buenos Aires, Argentina. Sarubbi, A., Sanchez Sarmiento, G. & Jouglard, C. (2001) Consolidation of Sanitary Landfills, ABAQUS Users Conference, Maastricht, The Netherlands. Sarubbi, A., Sanchez Sarmiento, G. & Jouglard, C. (2001) Consolidation of Sanitary Landfills, ABAQUS Users Conference, Buenos Aires, Argentina. Sarubbi, A. (1997) Leachate Treatment Plant, González Catán Landfill, Argentina. CEAMSE-SYUSA. Unique Work registered N , National Direction of Patents. Tchobanouglous, G., Theisen, H., Vigil, S. (1993) Integral Solid Wastes Management, Mac Graw Hill. Terzaghi, K. & Peck, R. (1983) Mecánica de Suelos en la Ingeniería Práctica. El Ateneo. Theisen, H., Nissen, J., Sarubbi, A. & García Marquez, C. (1996) Villa Dominico Landfill Vertical Expansion Project. Leachate Treatment System Design, Water Environment Federation (WEF) WEFTEC 96-69th Annual Conference Exposition, Dallas, Texas, U.S.A. Williams, N. D., Pohland, F. F., McGowan, K. C. & Saunders, F. M. (1987) Simulation of leachate gefneration from municipal solid waste. Georgia Institute of Technology, School of Civil Engineering, Atlanta, Georgia, U.S.A. 7

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