Insights into conditioning landfill sludge with freeze-thaw method: Effects on the physical-mechanical properties and micro characteristics

With the rapid increase of sludge production, large quantities of sludge with high water content (WC) are difficult to dispose of. It is necessary to evaluate sludge's physical-mechanical properties for waste sludge resource utilization and stability of landfills. In this study, freeze-thaw method is used to treat sludge. A series of tests, such as particle size analysis test, consolidation test, direct shear test, and SEM test, were carried out on the sludge treated by freeze-thaw. The influence of different refrigerating temperatures on the physical-mechanical properties is investigated. By analyzing the sludge morphology, particle distribution, pore change, consolidation and compression characteristics, shear characteristics, and microstructure, the influence mechanism of refrigerating temperature on sludge's physical-mechanical properties are revealed. The results show that after the freeze-thaw, the sludge particles gathered together, the water is easy to separate from solid particles. In addition, compressibility and void ratio of sludge decrease. At the same time, the permeability coefficient and consolidation coefficient increased by 1–2 orders of magnitude. When the freezing rate is slow, the sludge particles are pushed and squeezed by the ice layer, destroying the floc structure; when the freezing rate is high, the bound water can be better removed, which is conducive to forming a denser structure. Therefore, the cohesion of sludge decreases, and the internal friction angle increases after freeze-thaw treatment. Shear performance improved. The results of this study provide a reference for waste sludge resource utilization.