Sludge composting : influence of the waste physical preparation on initial free air space, air permeability and specific surface | Compostage de boues : influence de la préparation physique du substrat sur sa porosité, sa perméabilité à l'air et sa surface spécifique

The effectiveness of aerobic biodegradation of organic matter during composting and the stability or maturity of the end product particularly depend on the initial physical characteristics of the waste. On one hand, the accessibility of the micro-organisms to the biodegradable organic matter present within the waste is influenced by physical parameters. On the other hand, these physical parameters impact the aeration conditions, and as a consequence the substrate oxygenation during the composting treatment. From an operational point of view, the substrate physical preparation before composting treatment, for example by modification of the wastes granulometry, of the moisture content, or of the mixture ratio (sludge / bulking agent), is then a key step in order to optimize the process management. These operational parameters will particularly affect the specific surface which is essential for the accessibility of the biodegradable organic matter, and the permeability and free air space which determine gas transfer and oxygen supply. In this context, the aim of this work was (i) to develop and validate an experimental apparatus for the measurements of air porosity (FAS) and permeability (K) in a solid waste medium, (ii) to characterize with this tool mixtures of sludge and bulking agent considering several bulking agent size class, mixture ratio and moisture content, (iii) to determine correlations between the studied physical parameters. This work takes part in a larger project named ESPACE currently being carried out in partnership with Suez Environment and IMFT (French public research centre), partly financed by the ANR (French National Research Agency). An experimental apparatus, allowing the characterization of about 40L solid samples and combining features of an air pycnometer and an air permeameter, was developed and validated. The validation, consisting in a comparison between the theoretical and experimental free air space and air permeability values obtained with spherical beads, showed the good accuracy of the measurements. The application of this tool on agro-food wastewater sludge and bulking agent mixtures led to the determination of major physical operational parameters in sludge composting. The bulking agent particle size and moisture content appeared as the most influent parameters whereas the mixture ratio did not have a significant impact. An increase on moisture content generated an increase in bulk density and a decrease in FAS, K, Kozeny-Carman constant and specific surface. When the bulking agent particle size increased, K increased and Kozeny-Carman constant and specific surface decreased, whereas this operational parameter did not have a significant impact on bulk density and FAS of mixtures. The linear nature of the relationships between bulk density and moisture content, FAS and moisture content, FAS and bulk density was observed. This last relationship, equal to FAS (%) = -93.69 BD (kg/L) + 102.38 (R2 0.98), permits the accurate prediction of FAS of the studied mixtures using bulk density, whatever the bulking agent particle size, the mixture ratio and the moisture content were. A statistical exploitation of the results remains to carry out. Moreover, the physical characteristics of mixtures have to be related to respirometric results obtained on the same mixtures, in order to understand the influence of initial moisture, particle size and ratio on biodegradable potential and biodegradation kinetics. Composting trials in pilot-scale reactor could also be correlated in order to study the influence of the same physical parameters on the composting behaviour. This future exploitation of the whole results will help to understand and model the impact of physical parameters on wastes biodegradability, biodegradation kinetics and composting treatment.