(43a) Application of Turbulent Taylor-Couette Flow to Study the Shear Strength of Biological Aggregates

Better understanding of the mechanical strength of microbial aggregates helps to improve the design and operation of the biological treatment systems. The existing methods for evaluating the shear strength of biological aggregates are based on subjecting the sample to a non-homogeneous turbulent flow condition and monitoring the reduction in the bio-aggregate size distribution. However, due to the non-uniform shear rate distribution in the flow field, these techniques provide only an approximation of the average strength. Therefore, the objective of the present study is to overcome this limitation by studying the breakage of biological aggregates under well-defined hydrodynamic conditions and to determine their shear strength distribution as a function of their size and age. A Couette flow cell consisting of two concentric cylinders with a 2 mm gap was developed to create a well-controlled shear conditions. The shear stress can be adjusted by changing the rotational speed of the inner cylinder using a variable drive. The shear strength distribution of bio-aggregates was estimated by measuring the changes in their size distribution after exposing the sample to various shear stress levels. Results obtained for different sizes fractions indicate that larger size fractions have a larger fraction of weak aggregates.