title: Predicting the apparent viscosity and yield stress of digested and secondary sludge mixtures.
abstract: The legal banning of conventional sludge disposal methods such as landfill has led to a global movement towards achieving a sustainable sludge management strategy. Reusing sludge for energy production (biogas production) through the anaerobic digestion of sludge can provide a sustainable solution. However, for the optimum performance of digesters with minimal use of energy input, operating conditions must be regulated in accordance with the rheological characteristics of the sludge. If it is assumed that only secondary sludge enters the anaerobic digesters, an impact of variations to the solids concentration and volume fraction of each sludge type must be investigated to understand how the apparent viscosity and yield stress of the secondary and digested sludge mixture inside the digesters changes. In this study, five different total solids concentration of secondary and digested sludge were mixed at different digested sludge volume fractions ranging from 0 to 1. It was found that if secondary sludge was mixed with digested sludge at the same total solids concentration, the apparent viscosity and the yield stress of the mixture increased exponentially by increasing the volume fraction of digested sludge. However, if secondary sludge was added to digested sludge with a different solids concentration, the apparent viscosity and yield stress of the resulting mixed sludge was controlled by the concentrated sludge regardless of its type. Semi - empirical correlations were proposed to predict the apparent viscosity and yield stress of the mixed digested and secondary sludge. A master curve was also developed to predict the flow behaviour of sludge mixtures regardless of the total solid concentration and volume fraction of each sludge type within the studied solids concentration range of 1.4 and 7%TS. This model can be used for digesters optimization and design by predicting the rheology of sludge mixture inside digester.
authors.affiliation: Chemical Engineering, School of Engineering, RMIT University, Victoria, Australia. Electronic address: nicky.eshtiaghi@rmit.edu.au.
authors.forename: Nicky
authors.initials: N
authors.lastname: Eshtiaghi
authors.affiliation: Chemical Engineering, School of Engineering, RMIT University, Victoria, Australia.
authors.forename: Flora
authors.initials: F
authors.lastname: Markis
authors.affiliation: Chemical Engineering, School of Engineering, RMIT University, Victoria, Australia.
authors.forename: Dwen
authors.initials: D
authors.lastname: Zain
authors.affiliation: Chemical Engineering, School of Engineering, RMIT University, Victoria, Australia.
authors.forename: Kiet Hung
authors.initials: KH
authors.lastname: Mai
mesh_terms: D000693:Anaerobiosis; D019149:Bioreactors; D008962:Models, Theoretical; D012212:Rheology; D012722:Sewage; D014783:Viscosity; D014865:Waste Disposal, Fluid
chemical_list: D012722:Sewage
keywords: Apparent viscosity; Digested sludge; Mixtures; Secondary sludge; Surface charges; Yield stress