Modeling Reactive Transport within Landfill Bioreactors

Bioreactor landfills are more and more gaining in popularity, because of their interest in waste management. Bioreactors use enhanced microbiological processes to stabilize organic waste within 10 to 15 years of implementation, instead of 30 to 50 years for classical sanitary landfills. Modeling reactive transport phenomena within bioreactor landfills is a crucial issue to improve the efficiency of those engineered landfills. The goal is to control gas and liquid flows and to optimise the valorisation of refuses as well as to decrease the stabilisation time by controlling microbiological activity. This work aims to develop a three dimensional multiphase, multi-component model, which would help landfill operators to manage their site. Our first work consisted in the development of a conceptual mechanistic model which correctly describes the different phenomena that occur during the waste biodegradation. We adopted a relatively complex microscopic model involving four phases, including a biofilm, and nine constituents. The biofilm phase is considered as a continuum where the biological reactions take place. The associated macroscopic equations are discussed using results from a volume averaging method. Micro-scale equations at the "pore" level are averaged over a representative elementary volume, to obtain the macro-scale equations based on the microstructure of the wastes. The final model couples mass balance equations with momentum and energy equations. The solid wastes are separated into four categories according to their biodegradability. Production and consumption terms are included in the model for each reactant with different kinetics according to aerobic or anaerobic conditions. They involved simplified Monod kinetics taking into account the biomass activity and the availability of substrates. Kinetic coefficients also include temperature dependence and terms taking into account oxygen availability and moisture content. On this basis, a two dimensional finite volume code is developed.