Predicting the rheological behaviour of sludge is essential in the design and optimisation of various unit operations of waste water treatment, most notably anaerobic digestion whereby the efficient mixing of sludge feed produces biogas and digested sludge. In this paper, the rheological behaviour of primary sludge (2.8%, 3.7%, 5.5%, 6.8% and 8.2% TS) and secondary sludge (2.8%, 4.0%, 5.0%, 6.5% and 9.2% TS) has been investigated. At low stress, below the yield stress, sludge behaved as a visco-elastic solid, whereby primary sludge yielded abruptly whilst secondary sludge flowed smoothly to steady state. In the steady state, both sludges behaved as shear thinning, yield stress fluids with primary sludge exhibiting highly thixotropic behaviour. The apparent viscosity, yield stress and fluid consistency of both primary and secondary sludge increase with increasing total solids concentration and followed the Herschel-Bulkley model. A master curve was developed based on the dimensionless form of the Herschel-Bulkley model allowing the rheology of primary and secondary sludge at any concentration to be determined. (C) 2014 Elsevier B.V. All rights reserved.