Bubble rise velocity and bubble size in thickened waste activated sludge: Utilising electrical resistance tomography (ERT)
Graphical abstract
Introduction
Increasing volume of wastewater is becoming a crucial problem for the industries (Fatone et al., 2011). Among the available different treatment methods waste activated sludge treatment is the most widely used treatment (Seyssiecq et al., 2008). Aeration operation is known to be the heart of waste activated sludge process (Bailey et al., 2002). Aeration provides oxygen to bacteria for treating the wastewater. Oxygen is essential for the bacteria to allow biodegradation to take place. The supplied oxygen is utilised by bacteria in the wastewater to break down the organic matter to form carbon dioxide and water. Without the presence of sufficient oxygen, bacteria cannot biodegrade the incoming organic matter in a reasonable time (Henze and Henze, 1997). Thus adequate and evenly distributed oxygen supply is required in an aeration system for the rapid, economically viable and efficient treatment. However, the efficiency of the aeration depends on the hydrodynamics of the gas phase characteristic, i.e., gas holdup, bubble rise velocity and bubble size (Jamshidi and Mostoufi, 2017). Therefore measuring and understanding the gas phase properties like gas holdup and bubble rise velocity and bubble size in sludge is vital to increase the overall efficiency of the waste activated sludge process.
The number of studies has measured the gas phase characteristics and shown how rheology plays a crucial role in gas phase properties. Lind and Phillips (2010) estimated the viscous and viscoelastic properties of the fluid and reported that viscoelastic properties of fluid have a remarkable impact on bubble shape, size and rise velocity. The elasticity of fluid increases both the gas bubble collision and coalescence time because; elasticity decreases the size of the toroid wake behind a moving spherical bubble, thus making the detachment of the next bubble slower (Acharya and Ulbrecht, 1978; Dekée et al., 1986). However, most of these studies have been done on clear non-Newtonian model fluid as it is convenient to use the optical system of measurement. Furthermore, most of these fluids have a stable rheological behaviour, i.e., the rheology does not change with time (Bajón Fernández et al., 2015; Esmaeili et al., 2015; Fransolet et al., 2005).
Recently non-intrusive methods such as X-ray tomography, electrical resistance tomography (ERT), electrical impedance tomography (EIT) and electrical capacitance tomography (ECT) are being used for measuring the gas phase characteristics in opaque system such as sludge (Babaei et al., 2015a, b, Dziubiński et al., 2003; Fransolet et al., 2005; Jin et al., 2007; Warsito and Fan, 2001). The electrical resistance tomography (ERT) is a method that calculates the subsurface distribution of electrical resistivity from a large number of resistance measurements made from electrodes (Daily et al., 2004). It is a comparatively new imaging tool which is applied to the opaque system along with dynamic gas disengagement method to measure the gas phase characteristics in waste activated sludge (e.g. (Babaei et al., 2015a, b)). However, the concentration of sludge (WAS) used in the above mentioned studies was very low (0.07–1.5%). Other researchers (Fransolet et al., 2005; Jin et al., 2007; Khalili et al., 2018) have also successfully implemented electrical resistance tomography technique coupled with dynamic gas disengagement to measured bubble phase characteristics in the two-phase gas–liquid system and agitated systems of transparent liquids like Xanthan gum (1 g/l to 5 g/l). However, for economical treatment in the upcoming decentralised system, it is important to focus on high solids concentration of sludge. As the decentralised system capacity must relate to the local household and community and should not put excessive financial burden on the users (Capodaglio, 2017). Thus, operating in high solid concentration is imperative for decentralised sewage treatment plant. Moreover sludge being a complex rheological fluid, the gas phase characteristics varies significantly with the concentration.
Since, no study has been done on the measurement of gas characteristics at high solid concentration of sludge greater than 30 g/l, and no study has been reported on relationship between stress imposed by gas injection and bubble characteristics. The current study aims to investigate the gas phase characteristics in the concentrated waste activated sludge. Additionally, the role of viscoelastic properties on the gas phase characteristics was also analysed.
Section snippets
Sample preparation
Waste activated sludge was sampled from one of the waste water treatment plant in Victoria, Australia. The samples were stored at 4 °C for 30 days to reduce the microbial activity inside the sludge. This procedure helps with the stability of samples which results in reproducible data (Curvers et al., 2009). To prepare different concentration sludge samples, the sludge was thickened to higher concentration (6%) using centrifuge at 8000 rpm (i.e., at 12,200 g maximum relative centrifugal force) for
Rheological behaviour and modeling
Among the various models Power law, Bingham law, and Herschel–Bulkley; Herschel–Bulkley model Eq. (6). is the best fitting model to describe the sludge flow behaviour (Baudez and Coussot, 2001; Baudez et al., 2011; Eshtiaghi et al., 2013; Feng et al., 2016) for sludge above 3% solid concentrations. The three parameters, i.e. yield stress (, pa), the consistency index (K), Pa.sn), and the flow behaviour index (n) calculated for both the concentrations studied are reported in Table 1.
Gas holdup in the column
Conclusion
The impact of gas injection rate on bubble size at high solid concentration of waste activated sludge is explained. Gas holdup increases with increase in gas flow rate is shown. This increase in gas holdup is because stress imposed by gas injection modifies the sludge and increases the gas liquid contact. However, as the total solids concentration increases the gas holdup decreases due to coalescence.
For the first time an effective shear rate calculation for the Herschel–Bulkley fluid using HB
Acknowledgement
The Authors acknowledge South East Water support for providing sludge, to carry out the research, and RMIT University to provide Australian Government Research Training Program Scholarship for V. Bobade. The Authors also would like to thank Dr. Babu Iyer and Mike Allan for their technical support and facilities at RMIT, PC2 lab.
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