Draining solar drying of sewage sludge: Experimental study and modeling

The draining solar drying is considered as an innovative solution in the field of sanitation. The objective of this work is to investigate the feasibility and model the kinetics of the sludge solar drying in a lab-scale draining greenhouse by varying the air flow rate during winter and summer. The first experiment was conducted without ventilation. Then, a series of experiences was carried out with increasing the air flow. The drying period varied between 3 and 6 days in summer and between 9 and 14 days in winter. By increasing the air velocity, the drying time decreased. For the drying rate curves, in summer, only the falling rate period existed. However, in winter, the curves did not follow any regular shape of drying kinetics. Nine mathematical equations existing in the literature were tested to identify the thin-layer drying kinetics of sewage sludge inside the draining greenhouse. The Midilli-Kucuk model was the most reliable for describing the evolution of reduced moisture content versus drying time in hot and cold periods. However, for different drying conditions, the Wang and Singh model was the least suitable. The results revealed also that the effective diffusivity increased when the air flow rate and the ambient temperature increased.