Large scale composting model

International audience

Английский. One way to treat the organic wastes accordingly to the environmental policies is to develop biological treatment like composting. Nevertheless, this development largely relies on the quality of the final product and as a consequence on the quality of the biological activity during the treatment. Favourable conditions (oxygen concentration, temperature and moisture content) in the waste bed largely contribute to the establishment of a good aerobic biological activity and guarantee the organic matter stabilisation with limitation and control of odorous and greenhouse effect gaseous emissions. Several approaches (0D biochemical reducing, see Pommier et al. 2007, effective 1D modelling coupling transport and biochemical) have been made to understand the behaviour of such systems. In this paper we will present a 2D numerical model using Darcy scale equations for heat and mass transport coupled with a biochemical reactive scheme. Then, we will solve that system (using experimental measurements on reactivity and transport coefficients) with a commercial code (COMSOL TM). The model described here is based on the biological model presented in Trémier et al 2005 coupled with an upscale transport model detailed in Hénon 2008 which takes into account the major components of the gas phase: N₂, O₂, CO₂ and also H₂O. This is a crucial point because of: - The reaction rate, depending on the moisture content (humidity comes from the initial condition of the sludge but also from the reactive scheme because reactions produce water), - heat content, very sensitive to the evaporation rate in the sludge. It has been shown in Pujol et al 2011 that the impact of drying could be important on the reactivity but also that the pseudo component air could not be sufficient to represent the drying in the sludge. The process studied was a closed reactor composting process (180 m³ rectangular box) with positive forced aeration. The air was blown from the bottom of the reactor, via two ventilation pipes. In the upper part of the reactor, air was sucked and led to a biofilter treatment system. The treated waste was a mixture of sewage sludge and bulking agent that was composted during four weeks without turning. Several informations were recorded during the treatment like temperature evolutions at different locations (see Henon et al. 2009 for more details about the temperature recording). We have validated this code by comparing the temperatures obtained through the simulations with those recorded during the experiments. After this step of validation and a discussion on final composition of the organic matter in the experiments compared to the ones estimated by simulations, we have used this numerical model as an optimization tool. Modifying the initial, boundary and operating conditions we have been able to determine the best conditions to this particular composting process. A whole set of conditions is discussed in the paper.