Enhancing sludge hot-pressing drying by sawdust: Sensitivity analysis of heat and mass transfer physical properties of sludge, process performance, and kinetics

Hot-pressing drying shows promise in sludge treatment. Optimization often focuses on temperature and external load. However, the impact is restricted by sludge inherent properties, which can be changed by sludge conditioning. Due to coupling effects, altering individual sludge properties is difficult. Key sludge properties that significantly affect heat and mass transfer remain unclear, hampering identification of specific conditioning techniques. To address this gap, the effects of sludge specific heat, thermal conductivity, water activity, moisture diffusivity, and moisture permeability on drying performance were explored using sensitivity analysis. Sawdust conditioning and kinetics were employed to improve drying performance, validate sensitivity analysis findings, and elucidate drying mechanisms. Sensitivity analysis revealed that liquid water permeability significantly positively influenced drying performance by enhancing liquid water convection. Sawdust conditioning and kinetics demonstrated that adding 20 % sawdust increased drying flux from 31 to 62 kgwater center dot m-2 center dot h-1. The drying process included warm-up and constant rate, the first falling rate, and the second falling rate periods. Enhanced moisture transfer was dominated by breakthrough of interfacial gas film, transformation from liquid water diffusion to convection, and facilitated water vapor diffusion, respectively. Hence, this study provided novel insights, strategies, and direction of guidance for revealing sludge drying mechanisms and improving drying performance.