Freezing-duration effects on landfilled sludge conditioning via vacuum preloading: EPS-mediated dewaterability, consolidation, and microstructural evolution.

Landfilled sludge has accumulated extensively worldwide, necessitating effective dewatering and volume reduction treatments. Freezing coupled with vacuum preloading is a proven in situ technique. Prior studies have predominantly focused on freezing temperature, with limited attention to the role of freezing duration. This study investigates how freezing duration and variations in extracellular polymeric substances (EPS) regulate sludge mechanical properties, vacuum consolidation behavior, and microstructural changes. Results show: (i) specific resistance to filtration (SRF) and capillary suction time (CST) declined significantly with freezing time-after 96 h, SRF and CST were reduced by 57% and 72.8%; (ii) longer freezing enhances EPS disruption, especially in loosely bound EPS (LB-EPS) and soluble EPS (S-EPS), correlating with improved dewaterability; (iii) freezing reduced void ratio and increased compression index, while boosting consolidation coefficient and hydraulic conductivity by 1-2 orders of magnitude; (iv) cumulative drainage at 96 h was 6.6 × that of raw sludge and 2.4 × that at 0 h, yielding 54.9% volume reduction and water content drop from 86% to 57.3%; (v) prolonged freezing duration increases overall particle size and the abundance of large pores and mesopores, yielding a denser and more uniform sludge structure; and (vi) total EPS content exhibits strong linear correlations with compressibility and consolidation properties, vacuum dewatering characteristics, and microstructural alterations, with EPS disruption markedly influencing sludge engineering performance. Overall, extending freezing duration substantially enhances sludge mechanical and consolidation-drainage properties, providing critical guidance for efficient and environmentally sound in situ management of landfilled sludge.