Alum Residual Floc Interactions with an Advancing Ice/Water Interface

Abstract

Freezing and thawing transform alum sludge flocs into granular particles that rapidly settle. One objective of this paper is to help modelers and designers of the freeze/thaw conditioning process visualize the process at a microscopic scale. A second objective is to report a phenomenon occurring at the ice/water interface, namely, the fragmentation of flocs by the advancing freezing front. To meet these objectives, directional freezing of alum sludge was observed using a microscope at 40× power. A Bridgman growth apparatus was designed to allow various rates of directional freezing to be observed. The interface interacted with the flocs in various ways, depending on the freezing rate. At high freezing rates, dendrites either pierced the flocs or bypassed them, effectively confining the flocs and fragmenting them. At low rates, the interface was smooth and planar. Rather than being incorporated in the ice, flocs were rejected by the planar interface. At intermediate freezing rates, the interface was rough, and flocs were fragmented as they were rejected. The fragmentation of the flocs helps explain why some researchers have noted a decrease in floc size following freeze/thaw conditioning.