Unidirectional freezing of waste-activated sludge: The presence of sodium chloride

This paper experimentally addressed the discrepancies in sludge freezing literature regarding the effect of electrolyte (sodium chloride) on the subsequent freeze/thawed sludge dewaterability. Waste-activated sludge is vertically frozen at fixed freezing speeds with sodium chloride concentration as a parameter. Results obtained herein demonstrate that if the sludge is completely frozen, regardless of the electrolyte, the sludge filterability can be markedly improved. However, the associated sludge settle ability and the final sediment height do not alter much except when flocs gross migration occurs. Adding NaCl retards the flocs gross migration, thereby reducing the corresponding critical freezing speed. We speculate that the transition- layer freezing point suppression, the double-layer compression, and the initiation of wavy ice front are possible factors for the electrolyte effects. However, whether the double-layer compression is an influential mechanism in freeze/thaw treatment still remains unclear.