Influence of the airflow rate on heat and mass transfers during sewage sludge and bulking agent composting

The aim of this study was to characterize the gas flow during the composting, at a pilot scale, of a mixture of sludge and bulking agent, in order to model heat and mass transfers involved in the process. Thus, a closed 300-litre cylindrical pilot was fed with a mixture of wastewater treatment sludge and pine bark. Aeration was supplied from the bottom via an air blower and gases were collected at the top. Three experiments were led with constant gas flow rates and one with varying aeration rate. Temperatures within the pilot reactor were monitored all along the trials and their evolutions were discussed in term of heat transfers and parameters influencing the heat balance. Concurrently, Retention Time Distribution curvers were obtained by injecting a pulse of methane in the entering airflow and by analysing the methane concentration in the exhaust gas, every two or three days during composting. The gas flow, within the composting medium, was charaterized by a dispersion model, which is a deviation of the plug flow model. The dispersive effect of the flow was correlated to the evolution of the experimental temperature, and a convective dispersion model was used to describe the heat and mass transfers through the gas flow. These equations will be, in future work, coupled with heat production and mass degradation terms in order to model the global mass and heat balances of this composting process. Finally, axial dispersion coefficients of gases were determined and correlated with the airflow rate.