Hydrothermal liquefaction (HTL) is a thermochemical process that can convert organic materials in sewage sludge into crude-like oil. Knowledge of the rheology of sewage sludge as a feedstock is important for the design of a HTL Biorefinery as it provides essential information for pumping, pipeline design and heat transfer processes. This study aims at elucidating the rheological behaviour of sewage sludge slurries. Rheological parameters of sludge obtained from different stages in Wastewater Treatment Plants were estimated and used to calculate the pumping power required for slurry transportation in pipelines as part of the front-end engineering design stage for a biorefinery. Sludge slurries were determined to be Non-Newtonian with shear thickening characteristics. Statistically, the Herschel-Bulkley rheological model best fitted the experimental data with a yield stress. The apparent viscosity of municipal sewage sludge slurries measured within a shear rate range from 10 s(-1) to 300 s(-1) were determined to be between 0.008-0.05 Pa.s for stockpiled biosolids, 0.04-0.11 Pa.s for sludge without polymer dosage and 0.1-0.2 Pa.s for dewatered sludge with polymer. Power requirement for pumping sludge slurries were in the order: Stockpiled biosolid < Sludge without polymer dosage < Sludge with polymer dosage. (C) 2020 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.