Effect of dissolved solids on freeze-thaw conditioning

The purpose of this study was to investigate the effect of dissolved solids on the size of aggregated particles produced by freeze-thaw conditioning of alum sludge. The dissolved solids content was varied by adding 0-2000 mg/L NaCL to samples of alum sludge. The effect of the dissolved solids was observed by taking photographs of thin sections of each frozen sample and measuring the resulting aggregated particle sizes. The results of this study indicate that a relatively small amount of dissolved solids (500 mg/L NaCl or less) will cause ice crystal growth to change from columnar to dendritic. As a result, the mean aggregated particle size was reduced by approximately 50%. These results explain why the aggregated particles from alum sludge are larger and easier to dewater than those produced from wastewater sludge. Generally, alum sludge contains very little dissolved solids, so crystal growth is columnar. Conversely, wastewater sludges usually contain a significant amount of dissolved solids so that crystal growth becomes dendritic. Thin sections photographed between cross polarizers reveal that most of the aggregated particles were trapped within individual ice crystals and not at the crystal boundaries.