Salinity effects on ultrasound-assisted hot air drying kinetics of sewage sludge

Abstract

The influences of salt concentrations (0, 0.0214, 0.178, 0.265, 0.438 and 0.731 mol kg−1) and freeze/thawing treatment on the drying characteristics of sewage sludge were examined in a series of ultrasound-assisted (150 W) hot air convective dryer experiments at 80–130 °C. The average drying rate of the conventional sludge and hypersaline sludge sample through freezing/thawing treatment increased. The drying rates of saline sewage sludge decreased as the salt concentration increased. The average drying rate of the hypersaline sludge decreased by 8.6 percent compared to the conventional one. The apparent activation energy of the conventional and hypersaline sludge in the first falling rate stage of the drying were 14.27 and 15.61 kJ mol−1, respectively. Moreover, surface cracking dynamics formed as a result of drying were investigated. Our results show that the surface crack ratio during ultrasound assisted hot-air convection drying of conventional sewage sludge increased by 6.1 percent compared to that of the hot-air convection drying alone.