Laboratory studies performed using a rotational viscometer with two coaxial cylinders were aimed to define the rheological properties of sludges coming from typical mechanical-biological treatment plants for municipal sewage, and to develop a model describing the rheological behaviour of the sludges. The concentration of solid parts in the investigated sludges ranged from 2.21 to 6.56%. The approximation of the true flow curves determined on the basis of experimental apparent flow curves was made according to the equation provided by Krieger, Elrod, Maron and Švec. To describe rheological characteristics the 3-parameter Herschel-Bulkley model was chosen. The study established the relationship between the rheological parameters of the model (τ₀, k, n) and the mass concentration (C_s). The critical concentration (C_s,gr) for the sludges was found to be 0.75%. The results of the research enable the dimensioning of the activated sludge pumping system of a sewage treatment plant.

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