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The viscosity of activated sludge plays a major role on oxygen transfer and mass transport and consequently influences the hydrodynamic regime and the system performance of a membrane bioreactor (MBR). Yield stress is an important concept in rheology related to the mixing and power requirements in a stirred tank. This study examined the theological characteristics of activated sludge sampled in a pilot airlift MBR system for domestic wastewater treatment under ambient desert conditions, using a rotational rheometer (AR 2000, TA Instruments, USA) equipped with cone-plate geometry. Both static and dynamic yield stresses were observed at the transition point of 25 s(-1) of shear rate for an MLSS concentration range of 2.74-10.2 g/L. The static yield stress is a phenomenon that has not yet been evaluated for activated sludge. The mechanisms of the co-existence of both static and dynamic yield stresses and their engineering significance are discussed. In addition, a mathematical model to describe the theological properties of the sludge is proposed. The relationships between viscosity, MLSS, temperature and shear rate were obtained statistically. The activation energy for the viscosity of the sludge in the airlift sMBR was found to be 9.217 kJ mol(-1), and could be the cause of rapid fluctuation of transmembrane pressure with temperature variations. (C) 2009 Elsevier B.V. All rights reserved.