Composting is an aerobic process used to stabilize the organic matter (OM) which leads to the production of gaseous emissions (NH3, N2O, CH4, CO2) and the loss of nitrogen. The variety of the practices and the differences in nature of the substrates modify the kinetics of degradation of OM, the final quality of the produced compost and the share of emissions in the form of gaseous pollutants. To optimize the composting process, it is required to predict these transformations or to do some empirical test. This thesis analyzes the interactions between the main biological, biochemical, physicochemical and thermodynamic processes which explain the OM stabilization and the gaseous emissions of CO2, H2O, NH3, and N2O. Focus is done on windrow composting with passive aeration. The method is based on dynamic semi-empirical modeling of the process and experimentations at semi-industrial scale. In a first part, we analyzed the impact of the interactions between the key factors of composting, i.e. bioavailability of carbon and nitrogen, moisture and porosity, on the kinetics and the stoechiometry of the gaseous emissions. This analysis was based on data gathered in controlled conditions and on data observed on a platform located on La Réunion Island over periods ranging from 20 to 80 days. The hierarchy of the factors was established: the effect of porosity coupled with the effect of moisture plays the most important role in the regulation of the transformations of OM and the gas emissions. The effects of porosity, moisture and bulking agent on OM transformations observed on field conditions confirmed the observations in controlled conditions. The repeatability differences observed in controlled conditions on temperatures or mass balances were much lower than the repeatability differences observed in field conditions. In the second part, a dynamic model of composting was developed simulating the stabilization of carbon and nitrogen as well as the emissions of CO2, H2O, NH3 and N2O. This model is composed of four coupled modules. The first simulates the kinetics of oxidation of OM by a heterotrophic microbial population producing carbon dioxide and heat. The second describes heat exchange occurring during composting. These exchanges are at the origin of the passive aeration of the windrow and the internal kinetics of temperature and water evaporation. The third module describes the transfer of oxygen within the windrow modulated by porosity and moisture. The fourth module simulates the transformations and the stabilization of nitrogen as well as the kinetics of ammonia emissions (NH3) and nitrous oxide (N2O). The whole system representing a windrow of composting in passive aeration is considered as having homogeneous composition and thermal properties. During the thermopilic phase, the first factor limiting the organization of OM is the availability of nitrogen. The decrease in porosity induces a reduction in the gas losses through the increase in the organization of OM. The initial OM fractionation and the initial microbial biomass are the key factors to predict the kinetics of organization of carbon and nitrogen. The parameters specific of the emissions of NH3, H2O and N2O are initialized according to the nature of the substrate and the physical characteristics of the windrow. The modelled effects of porosity, moisture content, bulking agent on the OM transformations can be used to develop the industrial use of the model to optimize the composting of animal effluents.
