Evaluation of a Transparent Analog Fluid of Digested Sludge: Xanthan Gum Aqueous Solution

To verify the accuracy and reliability of the computational fluid dynamics simulation of sludge anaerobic digester, fluid flow visualization experiment is a reliable approach. However, it cannot be used for anaerobic digester due to the opaque nature of sludge. Xanthan gum (XG) aqueous solution was selected as the analog fluid of the digested sludge (DS) due to its good optical clarity and good stability. The rheological properties of DS and the XG aqueous solution were measured by applying a rotational viscometer and the measuring results qualitatively and quantitatively compared by graphics and mathematical statistics analysis. The experimental results revealed the rheological behaviour of XG aqueous solution was similar to DS. They were both typical pseudo-plastic non-Newtonian fluid and showed the same trend in the rheological curves with similar curvatures. Carreau model (R2=0.9578) was proposed to describe the rheological behaviour and calculate the rheological parameters of the analog fluid. The limiting viscosity of the XG aqueous solution linearly increased with the increases of the concentration within the concentration range of 0.20g/L∼1.60g/L. According to the linear relationship, 0.50g/L, 1.00g/L and 1.50g/L XG aqueous solution were selected to compare with DS. The result of Welch's t-test demonstrated that there is no significant difference in the rheological parameters of DS and the 1.00g/L XG aqueous solution at the significant level of α=0.05, which indicated the feasibility of choosing the 1.00g/L XG aqueous solution as transparent analog fluid of DS, and build a foundation for visualization experiment in anaerobic digester.