Solar drying process for sewage sludge in a drying bed: A case study in Tunisia

The aim of this work is to investigate and model the sewage sludge drying kinetics in a lab‐scale drying bed by varying the sludge layer thickness during winter and summer. During each season, five experiments were carried out with increasing the sludge thickness by 0.5 cm each time. To achieve moisture content on a dry basis of 12%, the sludge drying period varied between 120 and 312 h in winter and between 72 and 168 h in summer. Thus, by increasing the sludge thickness, the drying time increased. The different curves of the moisture content evolution and the drying rates were analyzed. The drying curves show that the warm‐up phase (phase 0) and the constant rate phase (phase 1) are absent during the winter and summer seasons. Regardless of the sludge thickness, only the falling rate period was observed. In addition, the moisture content dropped quickly throughout the first 24 h because of the elimination of interstitial water. The drying characteristic curve has been represented by a 3rd degree polynomial. Mathematical equations were used to describe the experimental curves of drying kinetics. The logarithmic model was the most reliable for simulating the changes of reduced moisture content versus drying time in winter and summer.