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This study presents a numerical investigation of flow and rheological behaviour of activated sludge. These materials are usually driven by pumps in wastewater treatment plants. Because of the correct sizing of the pipeline systems, which is of great importance from the point of view of efficiency, the friction losses and loss coefficients of the components have to be known. These are well-known in the case of Newtonian fluids but few data are available if the rheological properties are non-Newtonian. Three non-Newtonian models (Ostwald, Bingham, Herschel-Bulkley) are investigated related to the friction factor of a straight pipe, the loss coefficients of an elbow and to the pressure drop on this element. For our study the rheological data were used from the literature, where the same sample origin was diluted or concentrated to achieve three different TSS (total suspended solids) contents for the same sludge (7.4 g/l; 6.2 g/l; 3.6 g/l). Moreover, modified Reynolds-number definitions are tested related to the non-Newtonian models in the case of the laminar, transition and turbulent regions.

This study presents an investigation on the flow of two non-Newtonian fluids. These materials can be found in industrial environment, such as pharmaceutical and food industries, and also in wastewater treatment. In industrial environment, these fluids are usually driven by pumps between two workstations in the system, which represents a significant proportion of the costs. In order to operate the system cost-efficiency and environment friendly accurate sizing is necessary, which requires data on the hydraulic resistance of the elements. In the case of Newtonian fluids, these parameters are well-known. However, the non-Newtonian fluids have a considerably narrower literature, so laboratory and numerical tests are desirable. In our work, the hydraulic losses of two real non-Newtonian fluids were studied which can be described with the power law rheological model. These studies included laboratory measurements and numerical simulations (Computational Fluid Dynamics, CFD), respectively. We investigated the friction factor of a straight pipe and loss coefficient of an elbow (R/D=2). The calculations were validated with our laboratory measurements and compared with the literature. Furthermore, the flow pattern in the pipe bend was also examined. The study presents the applicability and importance of the modification of the Reynolds number. Furthermore, the velocity profiles and the secondary flow structure in the elbow are also presented.

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