This study examined the rheological characteristics of biological treatment sludges originating from secondary stage and sludge treatment unit of a municipal wastewater treatment plant (WWTP), using a rotational viscometer equipped with an ultralow adapter or small cylindrical-shaped spindle. A total of four mathematical approaches were used to investigate the fitting of the dependence of the shear stress to the shear rate. A generalized multi-statistical model was proposed to describe the rheological properties of the sludge. The overall results indicated that Ostwald equations have stronger prediction capabilities of the apparent viscosity of treatment sludges than other rheological modeling approaches and the proposed model. Although the proposed model structure did not exhibit a desirable performance, it may have an application potential to calculate the apparent viscosity for a particular shear rate and solid content, which can be measured easily in WWTPs. Water Environ. Res., 82, 782 (2010).