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Microbial granular biofilms are spherical, multi-layered aggregates composed of communities of bacterial cells encased in a complex matrix of hydrated extracellular polymeric substances (EPS). While granular aggregates are increasingly used for applications in industrial and municipal wastewater treatment, their underlying mechanical properties are poorly understood. The challenges of viscoelastic characterization for these structures are due to their spherical geometry, spatially heterogeneous properties, and their delicate nature. In this study, we report a model-based approach for nondestructive characterization of viscoelastic properties (shear modulus and shear viscosity) of alginate spheres with different concentrations, which was motivated by our measurements in granular biofilms. The characterization technique relies on experimental measurements of circumferential elastic wave speeds as a function of …