Experimental and Numerical Study of Flat Plate Sludge Drying at Low Temperature by Convection and Direct Conduction

This article deals with the drying of the waste by-product of an activated sludge manure treatment plant. The studies concern the low temperature drying of planar plates 3 cm thick. The upper face of the sludge is subjected to a tangential air flow controlled for speed, temperature, and humidity. The lower face can be subjected to contact heating. The maximum temperature for air and the bottom of the product is 60 °C. After characterizing the main thermophysical and hygric properties of the product, the laboratory set-up specifically designed for this study is described. Experiments are performed to determine temperature, mass loss, and deformation evolutions for different boundary conditions. These experimental data are used to build a numerical one-dimensional heat and mass transfer model to predict temperature and moisture content fields. Sludge is assimilated with a hygroscopic porous medium, and shrinkage is not taken into account. To validate this model, several comparisons between simulated and measured data are made for different drying methods (hot or fresh air convective drying, contact drying, and combined drying) and various drying conditions.