Rheological properties of digestate from agricultural biogas plants: An overview of measurement techniques and influencing factors

The main objective of this work is a comparison of measurement techniques that have been modified allowing for rheological measurements of agricultural digestate without a required pretreatment. This includes detailed information on the theoretical physical background, experimental set-up and for mixing rheometers also the calibration procedures, namely the Metzner-Otto concept/Rieger-Novak method and the Couette analogy. Furthermore, the advantages and disadvantages of rotational rheometers, torsion viscometers, mixing rheometers, pipe viscometers and the so-called ball measuring system are stated. As a result, pipe viscometers are found to be applicable for field experiments, while mixing rheometers were found to be the most suitable choice for lab-scale experiments. Within this work, a recommendation for a possible configuration for a mixing rheometer is provided. Moreover, this article gives an overview of research works dealing with the rheological behavior of digestate from agricultural biogas plants and first identified influencing factors. Investigated samples in the featured research works all exhibit a non-Newtonian shear-thinning flow behavior and the power-law model was applied the most. The most common identified influencing factors include temperature, total solid content, feedstock, particle size and anaerobic digestion, amongst others. However, observed correlations between these factors and the flow behavior and viscosity, respectively, to date are rather qualitative then quantitative, and extensive, systematic studies have yet to be carried out in order to establish quantitative relationships in between influencing factors. Based on the survey of relevant literature, recommendations for the measurement procedure and documentation of rheological measurements of digestate are provided.