Convective drying kinetics of sewage sludge: Experimental analysis

Wastewater treatment plants (WWTP) produce large quantities of residual sludge, which is usually discharged into the environment. Sewage sludge contains a high moisture content (65-90%), which is a major obstacle to any subsequent disposal method. To overcome this problem, the drying technique appears as an alternative solution to reduce its volume, especially in developing countries where green energy is an abundant resource. In this context, the present study concentrates on the convection drying of sewage sludge, highlighting its processes of cracking and shrinkage, as well as the effective diffusion coefficients of the used samples, considering shrinkage or not. The drying tests were conducted on samples with thicknesses of 1, 1.5 and 2 cm, respectively. The temperature and drying air velocity varied between 50 and 70°C and between 1 and 2 m/s, respectively. Results show that the convective drying of sewage sludge is significantly influenced by drying temperature, hot air velocity, and sample thickness. In addition, incorporating shrinkage into the mathematical model significantly improves the accuracy of moisture transport prediction. Volumetric shrinkage was successfully modeled using a cubic polynomial function, which showed excellent agreement with experimental data (R2 = 0.988).