Modelling of moisture-dependent aerobic degradation of solid waste S. Pommier a , D. Chenu b , M. Quintard b , X. Lefebvre a, * a UMR5504, UMR792 Inge ´nierie des Syste `mes Biologiques et des Proce ´de ´s, CNRS, INRA, INSA, F-31400 Toulouse, France b Institut de Me ´canique des Fluides de Toulouse, Av. du Professeur Camille Soula, 31400 Toulouse, France Accepted 7 May 2007 Available online 3 July 2007 Abstract In landfill, high temperature levels come from aerobic reactions inside the waste surface layer. They are known to make anaerobic pro- cesses more reliable, by partial removal of easily biodegradable substrates. Aerobic biodegradation of the main components of biodegrad- able matter (paper and cardboard waste, food and yard waste) is considered. In this paper, two models which take into account the effect of moisture on aerobic biodegradation kinetics are discussed. The first one (Model A) is a simple, first order, substrate-related model, which assumes that substrate hydrolysis is the limiting step of the process. The second one (Model B) is a biomass-dependant model, considering biological growth processes. Respirometric experiments were performed in order to evaluate the efficiency of each model. The biological oxygen demands of shredded paper and cardboard samples and of food and yard waste samples prepared at various initial water contents were measured. These experimental data were used to identify model parameters. Model A, which includes moisture dependency on the maximum amount of biodegraded matter, is relevant for paper and cardboard biodegradation. On the other hand, Model B, including moisture effect on the growth rate of biomass is suitable to describe food and yard waste biodegradation. Ó 2007 Elsevier Ltd. All rights reserved. 1. Introduction Modelling of the biochemical, physical and chemical processes inside landfills has been widely reported (Young, 1989; El Fadel et al., 1996; Haarstrick et al., 2001; White, 2004). Most models focus on the description of these pro- cesses in the anaerobic period, when the biodegradable organic waste is transformed into biogas. Nevertheless, just after the disposal of the refuse, aerobic reactions are involved within the surface waste, which is fed with oxygen by diffusion from the atmospheric air (Lanini et al., 2001). No model of which we are aware takes this initial transition period into account. However, the role of the aerobic reactions and their impact on the subsequent anaerobic processes have been clearly identified and discussed. First, these reactions are highly exothermic. The released heat is responsible for the great rise of the refuse temperature observed within landfills (Farquhar and Rovers, 1973; Zanetti et al., 1997; Lanini et al., 1997). Lefebvre et al. (2000) showed, from a heat balance on a landfill, that more than 97% of the heat comes from the aerobic reactions. Conse- quently, aerobic reactions lead to the propitious tempera- ture (50–60 °C), which is important for subsequent anaerobic biological activity. It obviously goes along with a partial consumption of the rapidly biodegradable sub- strates. This is the second positive effect of the aerobic reactions: high organic acids and hydrogen accumulation, known to inhibit anaerobic refuse stabilisation (Cappai et al., 2005), can be avoided. Aguilar et al. (1999) showed relationships between the landfilling practice (especially the air exposure time of the surface refuse layer) and both the temperature level and the fraction of oxidized sub- strates at the landfill closure. Consequently, initial conditions (temperature, substrate content) for anaerobic activity result strongly from the aer- obic reactions kinetics. Their modelling and their integra- tion within an overall landfill model is a major issue for correctly predicting the biogas emission and the bio-stabil- isation of organic wastes given variable landfilling practices. 0956-053X/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.wasman.2007.05.002 * Corresponding author. Tel.: +33 5 61 55 97 57; fax: +33 5 61 55 97 58. E-mail address: xavier.lefebvre@insa-toulouse.fr (X. Lefebvre). www.elsevier.com/locate/wasman Available online at www.sciencedirect.com Waste Management 28 (2008) 1188–1200