Impact of Non-Degradable Organic Matter (Ndo) on Sludge Rheology and Dewaterability

Understanding the mechanisms governing sludge dewaterability is critical for optimising wastewater treatment and reducing biosolids management costs. Sludge is a complex suspension comprising a carrier liquid and a diverse mixture of inorganic particles and organic compounds. Among the organic constituents, non-degradable organic matter (NDO), such as lignin, microplastics, surfactants, thickeners, and extracellular polymeric substances (EPS), can constitute up to 80% of total sludge solids and plays a central role in determining sludge rheology and dewaterability. EPS, in particular, forms stable, gel-like structures that significantly influence water retention and the resistance of sludge to dewatering. Variations in total solids (TS), volatile solids (VS), and the nature of the organic fraction alter sludge’s physical structure and flow properties, leading to complex, co-dependent relationships between composition, rheology, and dewatering performance. This thesis examines these relationships through three interconnected research questions, using samples sourced from full-scale Australian wastewater treatment plants and applying both experimental rheology and dewaterability assessments. The first study (research question 1) investigated the influence of organic matter and EPS on the rheological behaviour and dewaterability of waste activated sludge (WAS) and anaerobically digested sludge (DS). Across 19 sludge samples, higher organic fraction (VS/TS) was associated with increased yield stress and consistency index, and with poorer dewaterability (p < 0.02). DS exhibited stronger shear-thinning behaviour than WAS, reducing viscosity at higher operational shear rates. While EPS concentration alone was a weak predictor of performance, Fourier-transform infrared spectroscopy (FTIR) revealed distinct chemical signatures, particularly protein-associated amide, carboxylic, and hydroxyl groups, that were linked to rheological and dewatering outcomes. The second study (research question 2) assessed the role of residual flocculant polymers introduced prior to anaerobic digestion. Three polymer types (synthetic powder, synthetic liquid, and biopolymer powder) were dosed for pre-thickening, and their effects on rheology, methane yield, VS destruction, and post-digestion dewaterability were quantified. Optimal dosing reduced yield stress by 20–40%, increased methane yield by 15–25% over controls, and improved digestate dewaterability. However, only 8–25% of these dewatering gains were retained after digestion, indicating partial polymer degradation. Excessive dosing, particularly of biopolymers, inhibited VS destruction due to excessive biomass aggregation. These findings highlight the potential of strategic polymer application to enhance both biogas production and downstream dewatering efficiency. The third study (research question 3) investigated the impact of sludge rheological properties on its dewaterability independent of organic matter using viscosity modifiers, in which the effects of carrier fluid viscosity and particle-induced structuring on sludge dewaterability decoupled. Xanthan gum (XG) was used to increase carrier fluid viscosity, while polymethyl methacrylate (PMMA) and fumed silica modified particle-induced structuring. XG markedly increased yield stress and consistency index, causing up to a 94% reduction in water removal efficiency (WRE). In contrast, PMMA improved dewaterability by promoting porous cake formation, while fumed silica increased bulk viscosity through aggregation but with less severe impact than XG. These results demonstrate that carrier fluid viscosity exerts a greater influence on dewaterability than particle structuring at comparable viscosity levels. Collectively, this work provides a mechanistic understanding of how sludge composition, particularly organic matter, EPS, and residual polymers, shapes rheology and dewaterability, and how these effects can be manipulated to improve treatment performance. The findings have practical implications for optimising polymer use, digestion strategies, and sludge handling processes, ultimately supporting more sustainable and cost-effective biosolids management.