Disentangling rheological effects on sludge dewaterability: A new perspective on carrier fluid vs. particle effects

Dewaterability of wastewater sludges is one of the key factors determining the overall cost and environmental sustainability of biosolids management. Although dewaterability and rheology are known to be linked, the relative contributions of the carrier fluid viscosity and particle-induced viscosity within complex sludge suspension are not well understood. This study investigates these mechanisms by independently modifying the rheology of digested sludge (DS) and waste activated sludge (WAS) using xanthan gum (XG), polymethyl methacrylate (PMMA), and fumed silica as viscosity modifiers. XG significantly increased carrier fluid viscosity, yield stress and consistency index, which reduced dewaterability. In contrast, PMMA improved dewaterability by promoting porous sludge cake formation, particularly at lower sludge concentrations. Fumed silica increased solids-induced viscosity (i.e., bulk viscosity) through particle aggregation but with a less severe impact than XG. XG-treated samples exhibited up to a 94% reduction in water removal efficiency (WRE) compared to controls, with WRE decreasing progressively as XG dosage increased. In comparison, fumed silica–modified samples showed up to a 50% reduction in WRE at dosages that resulted in similar viscosities than XG-treated samples. These mechanistic differences demonstrate that carrier-fluid viscosity plays a more dominant role in controlling dewaterability than particulate-induced viscosity. The findings provide fundamental insight into how liquid-phase and solid-phase rheological contributions govern solid–liquid separation in sludge systems, offering a scientific basis for improving sludge conditioning, avoiding overdosing of soluble polymers, and optimising dewatering performance in wastewater treatment operations.