Insights into the drying kinetics of saline sewage sludge in far-infrared radiation drying

The drying characteristics of saline sewage sludge (SS) were examined by using a lab-scale far-infrared irradiation dryer. The effects of radiation temperatures (130, 150, 180 and 200 °C) and salt contents (0.0214, 0.178, 0.265, 0.438 and 0.731 mol kg⁻¹ NaCl) on the drying behavior were addressed. The drying rates of the sample decreased with increasing the salt content in the sample. The average drying rate of conventional sludge with 0.0214 mol kg⁻¹ NaCl within 130–200 °C decreased by 4.4% compared to the free-soluble salt sample. The average drying rate of hypersaline sludge (0.178–0.731 mol kg⁻¹ NaCl) went down by 10.1% compared to the conventional sludge. The average effective moisture diffusivity for the free-soluble salt sample in the falling rate stage was 6.42 × 10⁻⁹ m² s⁻¹, which was higher than 6.09 × 10⁻⁹ m² s⁻¹ for the conventional sludge followed by 5.62 × 10⁻⁹ m² s⁻¹ for the hypersaline sludge. The average value of the surface crack ratio (SCR) for the conventional SS increased by about 28% compared with the sample with salt content of 0.731 mol kg⁻¹ NaCl. The Modified Midilli model had the best ability in describing the change of the moisture ratio (MR) in the samples. This study could contribute a reference to design and operate the industrial drying system for SS with high salt contents. The research could provide fundamental data for identifying the heat and mass transfer mechanisms of the saline SS.