title: Rheological measurements as a tool for monitoring the performance of high pressure and high temperature treatment of sewage sludge.
abstract: Hydrothermal processing plays a significant role in sewage sludge treatment. However, the rheological behaviour of sludge during these processes is not fully understood. A better understanding of the sludge rheology under hydrothermal processing conditions can help improve process efficiency. Moreover, sludge rheology is easier to measure than chemical analyses. If a relationship could be established, it provides a possibility of using rheological measurement as a basis for monitoring the performance of hydrothermal processing. The rheological changes in thickened waste activated sludge (7 wt%) was investigated using a pressure cell-equipped rheometer during 60-min thermal hydrolysis (TH) at various temperatures (80-145 °C) and constant pressure (5 bar). Changes in the soluble chemical oxygen demand (COD) were measured using a separate reactor with a similar operating condition. The sludge behaved as a shear-thinning fluid and could be described by the Herschel-Bulkley model. At constant temperature, the yield stress and high-shear (600 s-1) viscosity of sludge decreased logarithmically over 60 min. At constant time, the yield stress and the high-shear viscosity decreased linearly with increasing TH temperature and these values was much less than corresponding properties after treatment and cooling down to 25 °C. The soluble COD of sludge also increased logarithmically over 60 min at constant temperature, and increased linearly with increasing temperature at constant time. Furthermore, the yield stress and high-shear viscosity reduction showed a linear correlation with the increase in soluble COD.
authors.affiliation: Chemical and Environmental Engineering, School of Engineering, RMIT University, 3001, Melbourne, Australia.
authors.forename: Kevin
authors.initials: K
authors.lastname: Hii
authors.affiliation: Chemical and Environmental Engineering, School of Engineering, RMIT University, 3001, Melbourne, Australia.
authors.forename: Raj
authors.initials: R
authors.lastname: Parthasarathy
authors.affiliation: Department of Chemical & Material Engineering, The University of Auckland, Auckland, New Zealand.
authors.forename: Saeid
authors.initials: S
authors.lastname: Baroutian
authors.affiliation: SCION, 49 Sala Street, Rotorua, 3010, New Zealand.
authors.forename: Daniel J
authors.initials: DJ
authors.lastname: Gapes
authors.affiliation: Chemical and Environmental Engineering, School of Engineering, RMIT University, 3001, Melbourne, Australia. Electronic address: nicky.eshtiaghi@rmit.edu.au.
authors.forename: Nicky
authors.initials: N
authors.lastname: Eshtiaghi
mesh_terms: D008962:Models, Theoretical; D012212:Rheology; D012722:Sewage / Q000737:chemistry*; D013696:Temperature; D014783:Viscosity
chemical_list: D012722:Sewage
keywords: Apparent viscosity; Sewage sludge rheology; Thermal hydrolysis; Waste activated sludge; Yield stress