Convective drying of waste water sludge and sawdust-sludge mixtures in a pilot-scale fixed bed was
studied. Drying was performed in a cross-flow convective dryer using 500 g of wet material extruded
through a disk with circular dies 12 mm in diameter. The structure of the bed mainly shows volume
shrinkage and crack formation during drying. Several characterization techniques were used over a wide
range of scales from nm up to mm. The overall bed structure was imaged with X-ray macrotomography,
at a resolution of 0.36 mm per pixel. Single extrudates were scanned with X-ray microtomography, at a
resolution of 41 mm per pixel. Pore structure of the dried samples were characterized by mercury
porosimetry (7.5 nm < dp < 150 mm). Results show significant structural changes on all scales with
increasing amounts of sawdust: shrinkage decreases, crack formation increases, and the pores become
larger. This confirms the benefits of sawdust addition for sludge drying applications.