Improving sludge dewaterability through freeze-thaw pretreatment: Ice crystal morphology controls consolidation and rheological enhancement.

Municipal sludge, characterized by its high-water-content and viscous texture, poses significant environmental challenges due to inefficient dewatering and poor flowability. The freeze-thaw (F/T) method is an effective and environmentally friendly pretreatment approach. It is crucial to apply rheological analysis to examine the influence of refrigeration temperature on dewatering effects and to investigate the underlying mechanisms. This study explores the effects of refrigeration temperature during F/T conditioning on sludge dewaterability, consolidation, rheological properties, and microstructure. By observing the growth process of needle-shaped ice crystals in sludge under a microscope, it is noted that sharp ice crystals depend on flocs for growth, and upon thawing, the flocs contract. Sludge exhibits better consolidation and drainage performance at -28 °C, with the optimal consolidation and permeability coefficient. The compression and fluidity performance of sludge after freezing at -36 °C are better. The water content of the frozen sludge at -28 and -36 °C can be reduced to below 60% after pressure filtration. This is due to the relaxation of the sludge structure, which weakens the strength of the floc network. The findings indicate that reduced refrigeration temperatures enhance the puncturing of floc aggregates by ice crystals, leading to disruption of the floc structure and aggregating particle. Rheological analysis provides further insight into the mechanisms involved in F/T conditioning during sludge dewatering.