Granular biofilms are dense spherical complex biological systems composed mainly of multi-microbial cells, water, and extracellular polymeric substances (EPS). They facilitate efficient purification and settling of activated sludge in wastewater treatment processes. The viscoelastic properties of these complex biofilm systems are important characteristics that control their growth and dictate how they respond to hydrodynamic forces and chemical stimuli. However, the viscoelastic properties of granular biofilms are poorly understood. In this paper, we study granular biofilms' viscoelastic properties using optical coherence elastography (OCE), a nondestructive method that integrates optical coherence tomography (OCT) with elastic wave propagation. While quantitative viscoelastic characterization of granular biofilms is challenging due to their heterogeneous properties, we show that elastic waves are suitable for this purpose. First, we employ guided elastic waves in a thin section of a granular biofilm to reveal a two-layered profile for the viscoelastic properties. Next, we utilize circumferential elastic waves that propagate near the surface of a non-sectioned spherical biofilm to quantify the layered system's viscoelastic properties. To the best of our knowledge, this work is the first quantitative study that characterizes the layered viscoelastic properties of granular biofilms. The measurement approach may provide a platform to study the interplay between the viscoelastic properties and other characteristics of granular biofilms such as the complex microbial system, morphology, and oxygen distribution.

颗粒生物膜是致密的球形复杂生物系统，主要由多种微生物细胞、水和细胞外聚合物（EPS）组成。它们有助于废水处理过程中活性污泥的有效净化和沉降。这些复杂生物膜系统的粘弹性是控制它们生长并决定它们如何响应水动力和化学刺激的重要特征。然而，人们对颗粒生物膜的粘弹性知之甚少。在本文中，我们使用光学相干弹性成像 (OCE) 研究颗粒生物膜的粘弹性，这是一种将光学相干断层扫描 (OCT) 与弹性波传播相结合的无损方法。虽然颗粒生物膜的定量粘弹性表征由于其异质性而具有挑战性，但我们表明弹性波适用于此目的。首先，我们在颗粒生物膜的薄截面中使用引导弹性波来揭示粘弹性特性的两层轮廓。接下来，我们利用在非切片球形生物膜表面附近传播的圆周弹性波来量化分层系统的粘弹性。据我们所知，这项工作是首次定量研究颗粒生物膜的层状粘弹性特性。该测量方法可以提供一个平台来研究粘弹性特性与颗粒生物膜的其他特性（例如复杂的微生物系统、形态、