Abstract

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 presentation, we will first depict how to perform good and reproducible measurements, then, we will focus on the rheological or the flow properties with the double objective of a better understanding of digested sludge behaviour and of a looking for a mimic material which could help to model industrial processes.

This means we will determine the rheological behaviour of digested sludge at different concentrations, and will highlight common features. We will show that the rheological behaviour of digested sludge is qualitatively the same at different solids concentrations, and depends only on the yield stress and high shear viscosity, both parameters being closely linked to the solids concentration.

We will also show that digested sludge is thermally dependent material: yield stress and high shear viscosity decrease with temperature and we will underline that the rheological behaviour is irreversibly altered by the thermal history. This result will suggest that the usual laws used to describe the thermal evolution of the rheological behaviour of fluids are no longer valid with anaerobic digested sludge and that thermal history have to be taken into account in rheological characterisation.

Then, focusing on the solid characteristics, we will demonstrate that digested sludge exhibits strong similarities with soft glassy materials and emulsions, which could be used as mimic materials.

Last, we will open some new insights to define sludge characteristics which could be seen as the fingerprint of the material and linked to the rheological properties. We will mainly focus on dielectric measurements and water activity, two parameters which can easily and quickly be determined.

About the speaker

Dr Jean-Christophe Baudez (PhD and Accreditation to Supervise Research – HDR in French) is a professional researcher at Irstea, the (French) National Research Institute of Science and Technology for Environment and Agriculture, formerly known as Cemagref, a French public research institute. Dr Baudez has done pioneering work on the measurement description and application of the solid/liquid transitions of thickened sewage sludge and he is now involved on technological research to apply fundamental results to industrial processes. He also developed other additional fields of research, such a dielectric properties of materials to link micro structure to macro structure.