Different types of sludge (primary, secondary and digested) undergo mixing throughout the waste water treatment process. As such, its flow behaviour prior to and after mixing is altered dramatically. Although the flow behaviour of activated and digested sludge has been extensively studied, there is little knowledge on the flow behaviour of primary sludge as well as mixtures of the different types of sludge. This paper investigates the impact of different sludge types variation in the mixture on the flow behaviour of the final blend. First, the flow behaviour of primary, secondary and digested sludge was studied over a wide total solids concentration (2.8 to 9.0%TS). Next, the flow behaviour of mixtures of different types of sludge were studied over a wide volume fraction (0-1) and total solids concentration. The flow curves and apparent viscosity curves of primary, secondary and digested sludge at a similar total solids concentration (5, and 4.2% TS respectively) as well as mixtures of primary and secondary sludge (5%TS) and mixtures of primary, secondary and digested sludge (4.2%TS) are presented. The flow curves and apparent viscosity curves of mixtures of different types of sludge demonstrate that the yield stress and apparent viscosity of the mixture depends on the particle interactions between the flocs. The flow curves were modelled using a dimensionless form of the Herschel bulkley model such that a master curve was developed for different types of sludge mixtures. In turn, the flow behaviour of any sludge could be predicted from the master curve.
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