Measurement of Rheological Properties of Biogas Digester Content Using a Rotational Rheometer with a Ball Measuring System

The rheological characterization of digester content from biogas plants presents challenges due to its heterogeneous composition, particularly in systems processing high-fiber substrates. Understanding these properties is crucial for optimizing key operational processes such as pumping, stirring, and heat exchange, thereby improving overall plant efficiency. This manuscript presents a novel method for measuring the rheological properties of digester content without the need to first separate solid fractions. The method uses a rotational rheometer with a ball measuring system (BMS). Compared to conventional rheometric methods, this approach allows a more representative analysis by preserving its actual composition. Furthermore, the applicability of the proposed method was demonstrated using digester contents from a full-scale agricultural biogas plant. Samples were collected from the main digester (MF), post-digester (PF), and the digestate storage (DS). Results revealed a significant reduction in apparent viscosity (consistency factor, K) from approximately 250 Pa × s in the MF to 10 Pa × s in the DS, attributed to microbial degradation and ongoing disintegration processes. In contrast, the flow index (n) remained relatively stable, exhibiting only a slight increase from the MF to the FPS, with average values ranging between 0.250 and 0.300. The slight increase in the flow index suggests that the digestate exhibits reduced viscosity at higher shear rates, such as during mixing. Despite its advantages, the BMS method has some limitations, including potential sedimentation effects, particle-sphere interactions, and temperature sensitivity. However, averaging multiple flow curves improves measurement consistency and provides accurate data on the consistency factor (K) and flow index (n) of analyzed digestate. Overall, these findings affirm the BMS-based rheometer as a reliable tool for routine rheological assessments in biogas research.