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Rheological Characterisation of Thermally Hydrolysed Waste Activated Sludge

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Rheological Characterisation of Thermally Hydrolysed Waste Activated Sludge RHEOLOGICAL CHARACTERISATION OF THERMALLY HYDROLYSED WASTE ACTIVATED SLUDGE A thesis submitted in fulfilment of the requirements for the degree of Doctor of Philosophy Kevin Hii B. Eng. (Chemical), RMIT University School of Engineering College of Science, Engineering and Health RMIT University May 2019 DECLARATION I certify that except where due acknowledgement has been made, the work is that of the author alone; the work has not been submitted previously, in whole or in part, to qualify for any other academic award; the content of the thesis is the result of work which has been carried out since the official commencement date of the approved research program; any editorial work, paid or unpaid, carried out by a third party is acknowledged; and, ethics procedures and guidelines have been followed. I acknowledge the support I have received for my research through the provision of an Australian Government Research Training Program Scholarship. Kevin Hii April 2019 I ACKNOWLEDGEMENT I would like to express sincere gratitude to my senior supervisor Associate Prof. Nicky Eshtiaghi for her dedicated support and encouragement throughout my PhD candidature, going above and beyond in providing guidance and mentorship. Additionally, I would like to thank the rest of my supervisory team, Professor Rajarathinam Parthasarathy, Dr. Saeid Baroutian and Dr. Daniel Gapes for their insight and advice throughout the research project. Special thanks go to the administrative staff from the School of Engineering and technical staff in the Rheology and Materials Processing laboratory, including Dr. Muthu Pannirselvam and Dr. Babu Iyer, for ensuring a safe and healthy journey throughout my candidature. Also, I would like to thank RMIT University for support through APA scholarship. Furthermore, I would like to thank my fellow PhD colleagues, including Tharaka Bandara, Xuran Du, Md Sakinul Islam, Ehsan Farno, Flora Markis, Veena Bobade, Samira Miryahyaei, Navid Moghadam and Tanmoy Das for their help and encouragement. Additional thanks go to Patrick Morison, Gurmanpreet Kaler, and Kubra Isguder for providing me the opportunity to support and supervise their journey into the world of research, but more importantly for their assistance in data collection and experimental work. Mostly, I give thanks to the Universe for boundless Grace; the pioneers who have come before; and my family and friends for their continued support and unconditional love. II PUBLICATIONS JOURNAL PAPERS 1. Hii, K., Baroutian, S., Parthasarathy, R., Gapes, D. J., & Eshtiaghi, N. (2014). “A review of wet air oxidation and thermal hydrolysis technologies in sludge treatment.” Bioresource technology, 155, 289-299 2. Hii, K., Baroutian, S., Parthasarathy, R., Gapes, D. J., & Eshtiaghi, N. (2017). “Rheological measurements as a tool for monitoring the performance of high pressure and high temperature treatment of sewage sludge.” Water Research, 114, 254-263 3. Hii, K., Farno, E., Baroutian, S., Parthasarathy, R., Eshtiaghi, N., (2019). “Rheological characterization of thermal hydrolysed waste activated sludge.” Water Research, 156, 445- 455. PEER-REVIEWED CONFERENCE PAPERS: 1. Hii, K., Parthasarathy, R., Baroutian, S., Gapes, D. J., Eshtiaghi, N., “Hydrothermal Processing of Sludge – A Review”, Chemeca, 29Sept.-2nd Oct., Brisbane, Australia 2013 2. Hii K., Parthasarathy, R., Baroutian, S., Gapes, D. J., Eshtiaghi, N., “Changes in waste activated sludge viscosity during thermal treatment”, Chemeca Conference 2016, Paper no. 3403844. CONFERENCE PRESENTATIONS 1. Hii, K., Parthasarathy, R., Baroutian, S., Gapes, D. J., Eshtiaghi, N., “Viscosity of Waste Activated Sludge Under High Temperature and High Pressure Conditions”, CHEMECA 2014 Conference, Perth, Australia. 2. Hii, K., Parthasarathy, R., Baroutian, S., Gapes, D. J., Eshtiaghi, N., “Viscosity change of thickened waste activated sludge during thermal hydrolysis”, WETT 2015 Water Research Symposium, RMIT University, Melbourne, Australia. 3. Hii K., Parthasarathy, R., Baroutian, S., Gapes, D. J., Eshtiaghi, N., “Changes in waste activated sludge viscosity during thermal treatment”, Chemeca Conference 2016, Adelaide, Australia. III 4. Hii K., Parthasarathy, R., Baroutian, S., Gapes, D. J., Eshtiaghi, N., “Thermal hydrolysis of secondary sewage sludge and viscosity decrease”, 13th IWA Specialized Conference on Small Water and Wastewater Systems, 2016, Athens, Greece. 5. Hii, K., Parthasarathy, R., Baroutian, S., Gapes, D. J., Eshtiaghi, N., “Viscosity reduction of waste activated sludge during thermal hydrolysis”, WETT 2016 Water Research Symposium, RMIT University, Melbourne, Australia. 6. Hii K., Parthasarathy, R., Baroutian, S., Gapes, D. J., Eshtiaghi, N., “Viscous behaviour of thickened waste activated sludge during thermal hydrolysis processes”, Chemeca Conference 2017, Melbourne, Australia IV NOMENCLATURE COD chemical oxygen demand (mg/L) E FKV model spring constant (Pa) α E1 FKV model fractional modulus (Pa.s ) E2 FKV model elastic modulus (Pa). Eα Mittag-Leffler function of order α G’ storage modulus (Pa) G” loss modulus (Pa) k consistency index (Pa.sn) n power index for Herschel-Bulkley model rsCOD released Sx shift factor in the x-axis for master curve Sy shift factor in the y-axis for master curve T temperature (°C) TS total solids concentration (wt%) t time (s). 0 parameters of untreated sludge at 25 °C 7 parameters of 7 wt% sludge. f parameters of thermally-treated sludge at 25 °C i denotes values measured in situ, at elevated conditions α FKV model fitting constant (-) γ̇ shear rate (s-1) γ strain (-) η(γ̇) steady-shear viscosity (Pa.s) η*(ω) complex viscosity (Pa.s) η’(ω) dynamic viscosity (Pa.s) η∞ high-shear viscosity (Pa.s) σ shear stress (Pa) σc yield stress (Pa). τ respondence time (s) ω angular velocity (rad/s) Γ dimensionless shear rate, Γ = (η/ σc).γ̇ (-) V SUMMARY 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 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. 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 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 steady- shear data which are traditionally obtained via rotational measurement. 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 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. 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. The solubilisation of sludge organics also followed a logarithmic time-dependent
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