Project B6

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Lifetime prediction and phenomena in landfills

Landfills are capsuled constructions serving the purpose to separate waste depositions from the surrounding environment. During operation and aftercare the embedded waste is exposed to various physical effects, which induce flow processes of leachate and landfill gas, heat transport and mechanical deformation. In many cases large quantities of organic matter exist inside the landfill body and are subjected to biochemical reaction processes.

So far there are hardly any reliable computer models available to analyze the physical processes in landfill bodies over a long period of time. In many cases observations of emissions and experience values are employed to get an approximate estimation of the long-term landfill evolution. Therefore, the development of capable analytical models for the investigation of processes inside of landfills is of great importance.

As illustrated in Fig. 1, many different factors of influence play an important role in the landfill behavior. Obviously, waste degradation and landfill emissions are strongly related. Reaction proc­esses depend on environmental conditions like temperature, pH-value and substrate concentration. Transport is driven to a certain extent by the production of gas and leachate. Waste deg­radation affects pore geometry and distribution and leads to changing hydraulic parameters like porosity and permeability.

Fig. 1: Phenomena in landfills

Landfills are composed of a variety of inorganic and organic components, among which biodegradation occurs under proper conditions. Landfill gas and leachate may accumulate and pollute the envi­ronment, if barriers are missing or insufficient. This research project deals with the development and application of a multiphase flow and transport model for the analysis of processes inside of landfills:

·        Analysis of temperature distribution.

·        Aerobe and anaerobe degradation of organic substances in the deposit and its development in time.

·        Balance of components of leachate, gas, and organic substances in the deposit and their temporal and spatial distribution.

·        Flow processes of fluids in porous media.

·        Deformation resulting from degradation in the deposit.

Multiphase flow and multicomponent transport processes

The basic equations for multiphase flow and transport in porous media are known from literature. The 3-D transport equation is obtained by mass conservation in a control volume. The control volume is referred to as the Representative Elementary Volume (REV) and is the smallest homogeneous model unit. For every component mass conservation is formulated and applied as model equation on the macroscopic level.

Storage/Evolution    Advection             Diffusion/Dispersion      Sources/Sinks

       (Ac · z),t       +        div (Au · z)       +       div (Aλ · grad z)       +       q(z)          =   0   .

The advective transport coefficients are governed by using a generalized Darcy law for multiphase flow. To describe the interaction between various fluid phases as well as capillary forces empirical approaches are applied. Diffusive transport due to concentration gradients is described by Fick's law. Temperature is considered to be equal for all phases, what is a proper assumption for characteristic flow velocities in landfills and consistent with local thermodynamic equilibrium.

Local mass balance and reaction kinetics

The model equations describing the temporal change of organic compounds (termed as sub­strates) are mainly based on Monod kinetics, according to which a set of differential equa­tions have been formulated.

The temporal change of the substrate concentrations s is described by the product of the common reaction rate matrix R(s) and the substrate-specific stoichiometric coefficient matrix M. The derived process matrix B(s) contains all information necessary for the mathematical description of transient reaction processes including biochemical substances, biomass and temperature development. Sources and sinks ŝ are related to global transport of mass and energy.

,   ....................         .

All processes are assumed to happen simultaneously in the boundary region between solid and fluid phases of the microscopical pore structure of the embedded waste. Whenever there is a change in environmental conditions – oxic/anoxic, redox potential, temperature or alkalinity – the model is able to describe new resulting processes or vanishing proc­esses in an adaptive way.

Numerical schemes

The numerical solution of the strongly nonlinear and coupled transport and reaction equations can be obtained using an adaptive 3D model. The weak form of the model equations is discretized by means of a finite element method in space domain. Integration in time is done with an implicit Eulerian scheme. Numerical stabilization of the spatial discretization is obtained by applying the fully upwinding method. The interaction of global transport and local reaction is considered implicitly. The resulting overall model is able to investigate global and local sensitivities with respect to char­acteristic processes and parameters.

Results

The rate of degradation of cellulose in a locally closed system is solely dependent on initial values of the substrates and conditions. Temperature and pH-value are controlled by heat release and the actual concentration of available substrates. Figure 2 shows the evolution of substrates for the case that initial values correspond with conditions of experiments in landfill simulation reactors. The described degradation model is compared to the model from literature that only captures the evolution of temperature and organic substance while landfill gas is just a product. The more detailed model, which is developed in cooperation with project B5 considers complex coherencies of more than 20 substrates, biomasses and temperature, see Fig 2.

Fig. 2: Comparison of models: Model from literature (left) and own model (right)

 

For practical use and coupling with 3-D transport models the detailed model has to be reduced to the most characteris­tic degradation processes and variables. Conducting sensitivity analysis main reactions and substrates may be identified and implemented into a separate model. In Fig. 3 results obtained by applying the reduced model are represented. Here, hy­drolysis, acetogenesis and methanogenesis are considered to be the most important reaction processes.

 

Fig. 3: Reduced model

Comparing Figs. 2 and 3, the evolution of substrates of the reduced model is very similar to the detailed degradation model. It follows that reduced degradation models may be accurate enough for a coupling of local reaction with global transport.

 

In many cases, realistic simulations of coupled transport and reaction processes in landfills are only possible on the basis of multidimensional analysis. The developed model allows the calculation of arbitrarily shaped two-dimensional structures with the adaptive, iso-parametric element concept. The possibilities of the model are demonstrated with the represented two-dimensional landfill section, utilizing  symmetries to the vertical boundaries, see Fig 4.

Fig. 4. Geometry and FE-mesh of landfill segment

This is an example of a common landfill section. At the beginning the pore space is filled to 60% with water and to 40% with landfill gas. Degradable organic substance is concentrated to 50% in the solid phase of the characterized subsection. Degradation of organic matter follows the reduced degradation model.

Since the landfill body is not yet sealed above, rain water – modeled as sources in the top level element layer – can penetrate the waste. At the lower boundary an inclined base liner with special Neumann boundary conditions and a drainage layer with Dirichlet boundary conditions are modeled in such a way that leachate can flow off only in the area of the left-hand side. The simulation period amounts to 50 days during which  a rainfall event occurs on days 10-11.

 

Fig. 5 shows the saturation of the gaseous phase in the pore space. Mobile water flows downward unimpaired and accumulates over the bottom liner and the drainage layer. For water saturations less than 40% the water phase is nearly immobile. With penetration of rain water the gas saturation decreases and the liquid phase is mobile again. With the degradation of organic substances the produced landfill gas flows out of the characterized subsection into the remaining area and hinders water penetration into the subsection. The water preferentially flows around the area instead, reaching here nearly full saturation.

   day 9                           11                       12                          50

 

Fig. 5: Evolution of gas phase saturation

Contact:

Prof. Dr.-Ing. D. Dinkler ? Institut für Statik ? Beethovenstr. 51 ? D-38106 Braunschweig
Tel:+49(0)531/391-3667 ? Fax:+49(0)531/391-8116 ? eMail: statik@tu.bs.de