Fluid dynamic analysis of non-Newtonian flow behavior of municipal sludge simulant in anaerobic digesters using submerged, recirculating jets

The current study aims at analyzing unsteady jet mixing of non-Newtonian fluids and the cavern structure in order to reduce the volume of stagnant zone in anaerobic digester tanks. The study was conducted by integrating computational analysis of the system into the experimental analysis. Accordingly, a highly viscous xanthan gum solution that mimics sludge with high solids concentrations was used to simulate municipal wastewater. Mixing was provided by a submerged, downwards-pointing and recirculating jet. Besides, a Computational Fluid Dynamics (CFD) model was developed to investigate different flow regimes, cavern formations, cavern growths, cavern volumes and the effects of rheological properties and specific power input on them. The variation in the xanthan gum rheology due to the digestion process was considered using the power law model. CFD was accomplished to investigate the hidden details of the system and experimental tests were used for validating CFD results. It was found that the bilaterally symmetrical structure of the cavern depended on the rheological properties and position of the outlet point, which directly affected the volume fraction of active/inactive zones. Accordingly, three different flow regimes for the cavern structure and the reduction trend of the inactive zone were recognized. Comparison of three different power inputs also demonstrated that reduction of inactive zones was not significantly affected by the power input beyond the optimum power input. Another key parameter was found to be the position of the outlet. It was found that active materials made more than half to 80% of the outlet while only half of the digester was filled with active materials.