Energy Efficiency of Drying Kinetics Process of Hydroxide Sludge Wastes in an Indirect Convection Solar Dryer

The treatment of sludge, which is produced from raw water treatment, has been considered as an ecologically sensitive problem in terms of energy and pollution. In this context, this study presents many series of solar drying tests that were carried on three samples of sludge obtained from several drinking water treatment plants in Marrakech-Safi regions, Morocco. The main aim is to develop further management processes of these wastes such as upgrading or storage. A partially solar indirect convective dryer operating in forced convection was used to run experiment tests. The temperature of air has ranged from 50 to 90°C and the relative humidity from 24% to 46%. The drying airflow was kept at about 0.0833 m³/s. An estimation of the mass diffusion coefficient and the activation energy during vacuum drying besides the characteristic convective drying curve were established. In addition, six semi-theoretical and/or empirical and statistical models were used to predict experimental data. It was found that the temperature was the most important factor during the drying process. Furthermore, the obtained results of mass diffusion coefficient and activation energy were found consistent with those already existing in the literature. The mathematical model called “Midilli–Kucuk” was found to be the most appropriate to describe the drying curves of the three samples. The total energy consumption was also studied. It showed a downward tendency with the increase in temperature. The efficiency of drying energy ranged from 4.89% to 7.5%. Although it was found to be low, it can be improved by increasing the load.