Mechanical compression assisted conductive drying of thin-film dewatered sewage sludge: Process performance, heat and mass transfer behavior

Abstract

The improvement in heat transfer efficiency between the hot wall and sewage sludge was a critical issue to enhance the conductive drying performance. The drying behavior of thin-film dewatered sewage sludge was investigated based on a conductive dryer assisted with mechanical compression at hot wall temperatures of 120–210 °C. The heat and mass transfer behavior of the sludge in the conductive drying process alone was compared to those in the mechanical compression assisted conductive drying process at three external mechanical loads of 25, 100, and 200 kPa. The average drying rates with mechanical compression were higher than those without mechanical load and were enhanced with the increase of mechanical loads at 210 °C. The extrusion of interfacial vapor film and the reduction of sludge surface roughness was responsible for the enhanced interfacial heat transfer efficiency under mechanical compression. The effective moisture diffusivity, mass transfer coefficient, and effective thermal conductivity were enhanced by mechanical compression. The improved moisture transfer inside sludge and on the open surface, and the decreased heat transfer resistance of sludge was due to the generated pressure-driven flow and the reduced gas cavities in sludge, resulting in the higher drying rates. Additionally, this finding provided reference data for developing a new sludge drying method.