Sustainable sludge management is becoming a major issue for wastewater treatment plants due to increasing urban populations and tightening environmental regulations for conventional sludge disposal methods. To address this problem, a good understanding of sludge behaviour is vital to improve and optimize the current state of wastewater treatment operations. This paper provides a review of the recent experimental works in order for researchers to be able to develop a reliable characterization technique for measuring the important properties of sludge such as viscosity, yield stress, thixotropy, and viscoelasticity and to better understand the impact of solids concentrations, temperature, and water content on these properties. In this context, choosing the appropriate rheological model and rheometer is also important.

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Abstract. -From both NMR and conventional rheometrical data we show that a foam cannot flow steadily below a critical, apparent shear rate and a critical shear stress. At low velocities the shear localizes in a layer of thickness decreasing with the apparent shear rate. When this thickness becomes smaller than a critical value hc (about 25 bubble diameters) the continuum assumption is no longer valid and the apparent behavior in this "discrete" regime differs from the rheological behavior of the foam in the "continuum" regime (for a sheared thickness larger than hc).Concentrated emulsions, suspensions, foams or polymeric gels, of common use in civil engineering, food, cosmetic or pharmaceutical industries, are pasty materials capable to flow only when a sufficiently large stress has been applied to them, a feature associated to their "soft jammed" structure. Since these materials might constitute a new state of matter reminiscent of the glassy state, they are actively studied in physics [1]. Although the solid-liquid transition was initially assumed to be continuous, i.e. the viscosity was considered to progressively tend to infinity as the shear stress decreases towards the yield stress, various recent experimental results with pasty materials have suggested that an abrupt transition occurs at a critical shear rate [2], which leads to shear-banding at low flow rates [3]. Recent theoretical works or numerical simulations [4] providing rheophysical approaches of the behavior of softjammed systems tend to confirm the generic character of such results. Besides, an analogous effect has long been observed with wormlike micellar solutions [5,6], but in that case the different flowing regions were shown to be composed of different phases [7]. For foams, shearbanding [8,9] and fracture [10] under specific conditions at low velocities, or localization of rearrangement processes beyond a critical deformation [11] under dynamic tests, were recently observed. Nevertheless, it is generally considered [10,12] that flows under other conditions are homogeneous and that these materials behave as simple yielding fluids, i.e. with flow rate continuously decreasing to zero as the applied stress decreases to the yield stress. Here we provide a complete set of macroscopic (conventional rheometry) and local (MRI) flow data for a foam. It is shown that foam flows systematically develop shear-banding at low imposed velocities and that the corresponding sheared layer does not exhibit a consistent rheological behavior when the sheared thickness is smaller than about 25 bubble diameters.c EDP Sciences

Producing biogas energy from the anaerobic digestion of wastewater sludge is one of the most challenging tasks facing engineers, because they are dealing with vast quantities of fundamentally scientifically poorly understood and unpredictable materials; while digesters need constant flow properties to operate efficiently. An accurate estimate of sludge rheological properties is required for the design and efficient operation of digestion, including mixing and pumping. In this paper, we have determined the rheological behaviour of digested sludge at different concentrations, and highlighted common features. At low shear stress, digested sludge behaves as a linear viscoelastic solid, but shear banding can occur and modify the apparent behaviour. At very high shear stress, the behaviour fits well to the Bingham model. Finally, we show that the rheological behaviour of digested sludge is qualitatively the same at different solids concentrations, and depends only on the yield stress and Bingham viscosity, both parameters being closely linked to the solids concentration.

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