Rheological characterisation of thermally hydrolysed waste activated sludge
Creators: name: Hii, Kevin
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 400410
subject: Wastewater treatment processes
classificationCode: 401212
subject: Non-Newtonian fluid flows (incl. rheology)

description: Hydrothermal sludge processing is a branch of sludge treatment technologies finding increased adoption in modern sewage treatment processes. These technologies involve the use of elevated temperature conditions to desirably alter sludge characteristics in the liquid phase. Benefits of these processes, such as thermal hydrolysis pre-treatment, include increased biogas production during anaerobic digestion, improved sludge dewaterability, sterilization of sludge, and improved transport operations due to desirable rheological enhancements. Sludge rheology plays a significant role in the design and operation<br>of these sludge-handling processes. Despite this, rheological studies related to sludge in hydrothermal processing conditions is very scarce. Therefore, a better understanding of sludge’s rheological properties, especially at the high temperature conditions encountered during hydrothermal processing is required for better optimization of these processes.<br><br>This study investigates the rheological characteristics of thickened waste activated sludge (WAS) in thermal hydrolysis (TH) processes. Using in-situ rheometric measurements, changes in the sludge’s flow properties due to the impact of treatment conditions (temperature, time, and sludge concentration) were examined. These changes were related to the solubilisation of sludge organics, measured by the chemical oxygen demand (COD) of sludge. Based on these observations, equations were derived to predict the rheological properties of sludge at various conditions of TH and its link to COD. Furthermore, the<br>viscoelastic properties of the thickened, untreated and thermally-treated sludges were studied in order to characterize sludge’s solid-like properties. A correlation was proposed to associate sludge’s viscoelastic data to flow curve data such that oscillatory measurement techniques could be used to collect steadyshear data which are traditionally obtained via rotational measurement.<br><br>In-situ rheological measurements revealed WAS behaved as a shear-thinning, yield stress fluid which could be described by the Herschel-Bulkley model. Despite elevated treatment temperatures (80 – 140 °C), Newtonian flow behaviour was not observed at any time in the sludge. The flow behaviour of the sludge at all treatment conditions examined could be described by a single master curve. This means sludge’s flow behaviour was governed by a similar network of physical interactions regardless of its concentration or treatment conditions. As a result of TH, the apparent viscosity, η, and Herschel-Bulkley rheological<br>parameters (yield stress, σc, and consistency index, k) were reduced irreversibly. The extent of this reduction followed a linear relationship with treatment temperature. The in-situ values of η, σc, and k were up to 92% lower compared to measurements after the thermally-treated sludge is cooled to ambient temperature.<br><br>In-situ measurements also showed that η, σc, and k reduced gradually during TH at constant temperature, following a logarithmic relationship with treatment duration. This meant the solubilization effects of TH were a time-dependent process. At constant time, reduction of η, σc, and k in situ were described by a linear relationship with increasing sludge temperature (80 – 140 °C). At constant treatment time and temperature, η, σc, and k were increased with sludge concentration (7 – 13 wt%) following a power-law relationship. The effectiveness of sludge solubilisation during TH was not impacted by varying sludge concentrations, since the reduction of η, σc, and k were nearly constant between the different sludge concentrations.<br><br>The solubilisation of sludge organics also followed a logarithmic time-dependent behaviour as shown by increasing values of COD in the soluble phase of the sludge during TH. This increase in sludge’s soluble COD (sCOD) indicated that the rheological changes observed during TH were due to the disintegration of sludge’s network structure. Accordingly, linear correlation existed between the reduction of rheological parameters (η, σc, and k) and the increase in sCOD. This correlation indicates the rheological measurement of sludge in situ can be used as a means to monitor the performance of hydrothermal processes.<br><br>Viscoelastic measurements of the untreated and thermally treated sludges revealed gel-like behaviour in the linear viscoelastic region. The frequency and creep response of the sludges were described using a fractional Kelvin-Voigt model (FKV). With increasing treatment temperatures, the storage (G’) and loss (G”) moduli were reduced linearly, further indicating a weakening of structural components in the sludge, such as extracellular polymeric substances. This is reflected in the decreasing value of energy of cohesion, Ecwhich describes the strength of the three-dimensional sludge network.<br><br>Viscoelastic data, as obtained from dynamic oscillatory measurements of the sludge could be related to the flow curve data of the sludge, as obtained via steady-shear measurements. A modified Cox-Merz relationship related the complex viscosity, η*(ω), to the steady shear viscosity, η(γ̇), by applying shift factors. More notably, raw values of dynamic viscosity, η’(ω), were nearly equal to η(γ̇) at equivalent shear rates. This meant that oscillatory measurement could readily describe steady-shear, flow data. Besides that, the yield stress of the sludge could also be estimated from the above shift factors and values of G’.<br><br>Finally, the engineering implications of this observations and results in this study were discussed, highlighting the significance of correct determination of rheological parameters which are needed in the design and operation of unit operations.