Rheology is the science of describing the flow and deformation of materials and provides valuable information about the flow behaviour of liquids and the deformation behaviour of solids.

One useful piece of information which can be gathered from rheological measurements is a fluid’s flow behaviour. A fluid’s flow behaviour can be either Newtonian or non-Newtonian. In Newtonian flow, the fluid’s shear rate is proportional to its shear stress – in other words, how ‘fast’ the fluid flows is directly proportional to the ‘amount’ of force applied to it. The viscosity – or ‘thickness’ – of Newtonian fluids remains constant regardless of shear stress. On the other hand, non-Newtonian fluids have viscosities which vary with shear rate. These fluids can be either shear-thickening, where the viscosity increases with increased shearing, or shear-thinning, where the opposite is true. In the real world, most fluids can be considered non-Newtonian. Fluids can also possess a yield stress whereby flow can only be initiated once a limiting shear stress is applied. Furthermore, fluids can also possess viscoelastic properties – where their deformation behaviour can be described by a combination of ideally viscous liquid behaviour and ideally solid elastic behaviour.

Considering this, rheology plays an important role in all fluid-handling processes. Accordingly, sewage sludge rheology is an essential design parameter in wastewater treatment plants which determine the efficiency of sludge-handling systems. The hydrodynamic behaviour and rheological properties of sludge is significant to the optimization and design of sludge handling processes as noted by several researchers. Sludge rheological properties such as viscosity, yield stress and viscoelasticity are design parameters related to heat and mass transfer operations, mixing and power requirements, transport, dewatering and pumping. Thoughtful consideration of sludge’s rheology can go a long way to reducing operating costs in wastewater treatment plants, where sludge treatment comprises up to 50% of these costs.

Sewage sludge rheology is notably complex, especially at high sludge concentrations and numerous studies have been done to characterize its non-Newtonian behaviour. However, most of these studies only described sludge rheology at ambient to moderate temperatures and at lower range of sludge concentrations. A gap of information is encountered when considering sludge rheology in thermal pre-treatment processes. Although some work has been done, most rheology studies were limited to post-treatment sludge. Sludge’s rheology during actual process conditions remained largely unobserved.

Thermal pre-treatment is a branch of technologies which have found increased usage in sludge treatment processes. They involve the use of elevated pressures and temperatures, often above the boiling point of water, to achieve desirable physico-chemical changes in the sludge. Notably, thermal pre-treatment finds application in sludge anaerobic digestion where they help boost anaerobic digestion performance. One of the main advantages of thermal pre-treatment is an increase in biogas production from the anaerobic digestion of thermally-treated sludge. Besides that, thermally treated sludge is significantly reduced in viscosity, and is also much easier to dewater.

The lack of rheological information had been largely due to practical challenges associated with high-temperature and high-pressure measurement of the sludge rheology. However, recent commercial availability and advancements in rheometer technology have made some of these measurements possible. In our study, we performed in-situ rheological measurement of sludge’s flow behaviour at conditions mimicking a one-hour sludge thermal pre-treatment process at constant temperature. This was achieved by measuring the sludge in a pressure cell attachment which allowed the required high-pressure conditions. Since rheological measurements were performed in real-time, this allowed us to characterize the changes in the sludge rheology as thermal pre-treatment progressed, which had not previously been attempted.

Based on these measurements, results showed that sludge behaved as a non-Newtonian, shear-thinning fluid, exhibiting a yield stress. Figure 1(a) shows the shear stress-shear rate response of a sludge at 140 °C during thermal pre-treatment which was fitted to the Herschel-Bulkley rheological model. Sludge remained non-Newtonian despite being at high temperature conditions (< 150 °C). The yield stress and shear-thinning properties of sludge are both parameters which can affect the efficiency of mixing and heat-transfer performance. This means during thermal pre-treatment sludge should not simply be handled as a Newtonian fluid, if optimal performance is being considered.

It is well documented that thermal pre-treatment greatly reduces the viscosity of sludge. However, our results also showed the viscosity reduction was a time-dependent process which occurred progressively during thermal pre-treatment at constant temperature. As shown in Figure 1(b) the viscosity reduction followed a logarithmic trend with treatment duration whereby most of the viscosity drop occurred in the first 20 minutes of treatment. After that, the viscosity change was minimal. This observation was interesting as it reflected the behaviour of organic matter solubilization, which also follows a logarithmic trend with treatment duration. It further supports the idea that sludge’s rheology is partly due to the presence of biological flocs.

This is interesting as it highlights the potential for online monitoring of thermal pre-treatment processes via rheological measurements. We have shown in a previously published paper that a linear correlation existed between sludge’s rheological parameters, including yield stress and apparent viscosity, and its organic matter solubilisation (measured in terms of chemical oxygen demand). Compared to traditional methods for determination of sludge solubilization via chemical analysis, rheological measurements can be done much quicker and in real-time.

The rheological changes in sludge during thermal pre-treatment could be described using a combined model, based on logarithmic time-dependence and linear temperature relationship. These equations are useful for quick estimation of in-situ values of sludge’s rheological parameters in thermal pre-treatment, which is handy for process design, but can also provide an indication of sludge’s extent of solubilization.

Interestingly, rheological observations suggested that sludge concentration had minimal impact on the effectiveness of thermal pre-treatment. This was based on the observation that regardless of sludge concentration (7 – 13 wt%), the extent of viscosity and yield stress reduction were nearly equal at constant treatment time and temperatures. This suggests that plant operators are allowed a certain degree of flexibility when it comes to thermal pre-treatment of increasingly concentrated sludges, which is often desirable. Combined with the desirable viscosity reduction offered by thermal pre-treatment, this potentially means an increasingly concentrated sludge feed for anaerobic digesters, which translates to higher biogas production.

In our study, it was also found that the results from measuring the flow behaviour of sludge via oscillatory rheological methods were comparable to rotational rheological methods. Usually, oscillatory methods are reserved for measuring material viscoelasticity. However, this comparability with rotational measurement enables new possibilities for in situ rheological measurement and process monitoring as well as for overcoming practical difficulties in conducting experimental work, such as non-destructive sample measurement.

This study was headed by lead investigator Associate Professor Nicky Eshtiaghi and PhD researcher Kevin Hii, from the School of Engineering, RMIT University, Melbourne, Australia and the work is published in Water Research (DOI: 10.1016/j.watres.2019.03.039).