Heat and mass transfer in landfills: Simulation of the pile self-heating and of the soil contamination

The unsteady 2D heat and mass transfer of waste piles in landfill sites in which self-heating of piped material occurs and the effect on the soil below them are studied. A simplified mathematical model that includes temperature, water content, oxygen and solute (ammonium) concentration is proposed. Inside the pile, the biological and chemical reactions increase the temperature whilst consuming oxygen. Here the solute and water content are not solved for. In the soil, ammonium leaching from the landfill site into the surrounding soil is considered. Therefore, a diffusion partial differential equation (PDE) system without cellulose consumption and with source terms is considered and only temperature, oxygen and solute concentration are solved for. An Arrhenius equation is used to calculate the effect of temperature on the solute hydrodynamic parameters of the soil. The mathematical model is solved for the ammonium, oxygen concentration and temperature in the soil by means of the finite volume method. In the soil the water content is considered to be in steady state. Comparisons between analytical and numerical results for the 2D solute equation allow the algorithm to be validated. A mesh study is carried out in order to verify the algorithms convergence and to decrease the simulation time. The effect of the soil type on energy generation and mass transfer in the pile and the soil hollow depth in which the waste pile is placed are analyzed. When the soil thermal diffusion is lower, the time over which self-heating inside the pile takes place is so shorter, as expected. The hollow depth where the waste pile is placed may be related with the solute concentration near the interface air/soil and infiltration depth in the soil. In all cases, the ammonium concentration infiltrated through the hole located in the landfill base, at the interface air/soil, on both sides, has not increased.