Rheological characterization of thermal hydrolysed waste activated sludge

Highlights

• Equations provided to predict in-situ TH rheological properties from ambient data.

• Rate and extent of TH solubilization are independent of sludge concentration.

• FKV model described viscoelastic properties, revealing weaker structure after TH.

• Viscoelastic data can estimate steady-shear viscosity of thickened sludge.Yield stress was approximated via Cox-Merz shift factors and dynamic measurements.

Abstract

Rheological properties are important in the design and operation of sludge-handling process. Despite this, the rheology of sludge in thermal hydrolysis processes (TH) is not well described.

In-situ measurements were performed to characterize the flow behaviour of various concentrations (7–13 wt%) of waste activated sludge (WAS) at TH conditions. Equations were presented for predicting in-situ rheological parameters (high-shear viscosity, η_∞,i, consistency index, k_i, and yield stress, σ_c,i) under various treatment conditions, which are useful for design of process units. The equations enable convenient estimation of in-situ properties based on ambient rheological measurements. Results suggested that the proportion of sludge solubilization and its rate were unaffected by varying sludge concentration.

Thermally treated sludge still exhibited gel-like, viscoelastic characteristics similar to untreated sludge; however, the storage (G′) and loss (G”) moduli decreased with higher treatment temperatures. Frequency and creep responses were described by a fractional derivatives Kelvin-Voigt (FKV) model, which showed increasing viscous characteristics of treated sludge. These equations can be utilised in CFD models. Results obtained from oscillatory measurements can also approximate steady-shear behaviour by comparing dynamic viscosity, η′(ω), and steady-shear viscosity, η(γ̇), whose values were very similar. This enables convenient estimation of steady-shear behaviour of sludge from oscillatory measurements, which is found to be a non-destructive technique for measuring flow behaviour of highly concentrated sludge. Yield stress can also be predicted from the product of modified Cox-Merz shift factors and storage modulus (G’). Practical engineering implications of the rheological observations were discussed.

Introduction

Thermal hydrolysis (TH) is a processing technique involving usage of high temperatures (100–200 °C) to achieve desirable physical or chemical changes in wastewater sludge. One major application is in anaerobic digestion, where TH is used as pre-treatment to overcome the rate-limiting hydrolysis step, showing favourable results and successful industrial-scale application (Barber, 2016; Carrère et al., 2010; Sapkaite et al., 2017; Zhen et al., 2017). In these processes, particulate organic matter is solubilized through application of heat for a defined period. This disintegrates cellular material to release organic compounds, which improves microorganism access to them, thus enhancing anaerobic digestion performance (Ariunbaatar et al., 2014; Suárez-Iglesias et al., 2017). Besides that, TH leads to advantageous rheological enhancements. For example, the viscosity of thermally-treated sludge is greatly reduced, which improves the efficiency of pumping, mixing, heating, digester loading, and sludge dewaterability (Farno et al., 2017; Morgan-Sagastume et al., 2011; Pérez-Elvira et al., 2008; Pérez-Elvira and Fdz-Polanco, 2012; Zhou et al., 2013). Rheology plays an important role in the design and operation of sludge handling systems (Dentel, 1997; Eshtiaghi et al., 2013). Despite extensive research on sludge TH, detailed studies related to its rheology are scarce (Barber, 2016). Accordingly, detailed characterization of sludge flow behaviour during TH and its viscoelastic properties is of interest and can lead to better implementation of TH processes.

Waste activated sludge (WAS) is the main sludge-type handled in TH processes; its rheology at ambient conditions has been well researched (Eshtiaghi et al., 2013; Ratkovich et al., 2013). WAS rheology is generally accepted to behave as a non-Newtonian, shear-thinning fluid, commonly described by the Herschel-Bulkley model. It exhibits thixotropic properties (Guibaud et al., 2004) and many studies identified the presence of yield stress (Farno et al., 2015; Markis et al., 2014; Ratkovich et al., 2013). Besides, WAS exhibits viscoelasticity, meaning that it initially shows solid-like behaviour under stress but liquid-like behaviour upon breakdown of floc structure. Furthermore, the rheological properties of WAS become more significant at high solids concentrations.

A few studies described the flow behaviour of WAS after TH. Herschel-Bulkley flow behaviour has been reported (170 °C, 60 min TH, for 5.4–18.7 wt% WAS) (Feng et al., 2014a) but Newtonian behaviour was also reported, such as Feng et al. (2014b) (170 °C, 60 min TH, for 5.4 wt% WAS) and Urrea et al. (2015) (160–200 °C TH of 2.3 wt% WAS). However, these studies were mainly concerned with post-thermally treated sludge measured at ambient conditions, which can deviate by up to 80% compared to in-situ measurements (Hii et al., 2017). In-situ characterization of WAS flow behaviour has been shown by Farno et al. (2016a), who reported the apparent viscosity and Herschel-Bulkley parameters were dependent on treatment duration and temperature. However, their study was limited to low temperature thermal processes (50–80 °C, 1 h). Recently (Hii et al., 2017), characterized in-situ WAS flow behaviour (80–145 °C, 1 h TH), but was limited to one concentration of sludge. Due to viscosity reduction, higher concentrations of feed sludge are desirable in TH, allowing higher organic loading rates in anaerobic digesters (Morgan-Sagastume et al., 2011). Then, its impact on TH rheology must be considered.

For untreated sludge, the effect of solids concentration on sludge viscosity and consistency index has been shown following an exponential function (Guibaud et al., 2004; Markis et al., 2014; Sanin, 2002; Tixier et al., 2003) or power-law model (Lotito et al., 1997; Markis et al., 2014). Whereas yield stress follows exponential (Guibaud et al., 2004; Mori et al., 2006) or power-law relationship (Forster, 2002; Lotito et al., 1997; Markis et al., 2014). For thermally treated WAS, an exponential relationship has been described for apparent viscosity and consistency index (Feng et al., 2014a). In another study, yield stress has been described following exponential relationship with concentration, whereas the consistency index and flow index followed a polynomial function of third and second order, respectively (Urrea et al., 2015). However, it has not been shown whether the same relationship exists during TH.

Viscoelastic properties have been reported for thermally treated WAS. Generally, the storage (G′) and loss moduli (G″) reduced as a result of TH, and G’>G” (Farno et al., 2016a; Feng et al., 2014a; Zhang et al., 2017), although some studies also suggest G”>G’ at low concentrations (<0.8 wt%) (Feng et al., 2014a, 2014b). However, Feng et al. (2014a) and Feng et al. (2014b) did not examine the impact of varying treatment temperature whereas Farno et al. (2016b) was related to low temperature (50–80 °C) thermal sludge processing. Zhang et al. (2017) characterized the viscoelastic properties of WAS (14.2 and 18.2 wt%) after low (60–90 °C) and high temperature (120–180 °C) TH. A Kay-Bernstein-Kearsley-Zappa (KBKZ) model described the viscoelastic properties. However, their study was concerned with polyacrylamide (PAM) conditioned sludge, which alters flocculation and network structure. Furthermore, Farno et al. (2018) have shown that a fractional derivatives Kelvin-Voigt model was more representative for sludge viscoelasticity but was limited to lower thermal treatment temperatures (<80 °C) and sludge concentrations (<6.1 wt%).

The current study expands the in-situ rheological characterization of WAS during TH for various sludge concentrations, evaluating the impact of sludge concentration, temperature and treatment time. Empirical equations are derived describing changes in the apparent viscosity, yield stress, and consistency index during TH at various sludge concentrations. Furthermore, the viscoelastic properties of the raw and thermally-treated WAS are investigated. The results from viscoelastic measurements are fitted to a fractional derivates Kelvin-Voigt model, which has not been attempted before for high-temperature thermally treated sludge. Finally, the adaptability of dynamic measurements to obtaining steady shear data is evaluated, assessing its potential for overcoming the practical challenges related to the steady shear flow measurement of highly concentred sludge.