The rheological behavior is known to be of great interest in sludge treatment, but the material complexity induces strong measurement difficulties. The literature highlights two characteristics deeply impacting sludge rheological properties and their characterization: Thixotropic behavior and organic matter (Seyssiecq et al., 2003; Eshtiaghi et al., 2013), both being related to each other. Indeed, by looking at how thixotropy affects elastic modulus and yield stress with different mechanical histories at different organic matter ratios, it turns out that the higher the organic matter, the higher the thixotropic behavior.