Uni-directional freezing of waste activated sludges: Vertical freezing versus radial freezing

Radial freezing and vertical freezing of waste activated sludge were conducted in this work. In radial freezing, where the sludge sample is placed in a cylindrical chamber cooled at its outer wall surface, the ice front grows and migrates inwards from the rim region along a radial direction. The corresponding freezing speed decreases gradually due to the increase in heat conduction resistance through the ice layer. In vertical freezing, where the sludge was placed in sample tubes immersed vertically into a freezing pool, the ice front grows upwards at a constant freezing speed. Comparisons between floc characteristics and the changes in chemical compositions resulting from both tests make clear the role of freezing speed. That is, if only the developing ice front can reject foreign particles and thereby cause the gross migration of sludge flocs, the effects on sludge performance would be similar. The most influential factor that determines the freeze/thaw-conditioned sludge performance is discussed.