Hydrodynamics of a model stirred anaerobic digester

The aim of this work is to identify the hydrodynamic characteristics of a model bioreactor, whose geometrical configuration is adopted at industrial scale for the biomethanation of agricultural scraps. The laboratory scale model bioreactor is a flat bottomed, unbaffled, cylindrical tank of small aspect ratio, centrally stirred by two Lightnin A310 axial impellers. It results from the scale-down of a full-scale design, adopting the geometrical similarity criterion. The Particle Image Velocimetry (PIV) technique is adopted in order to gain a detailed description of the single phase velocity field. The results collected at various impeller rotational speeds and two fill ratios are presented and the effect of the different operating conditions on the model digester hydrodynamics are identified. The experimental characterization allows to assess the appropriateness of the design to achieve an appropriate liquid flow pattern, which ultimately affects the solid feedstock suspension and distribution all over the vessel volume. Finally, possible geometrical modifications for improvements of the overall mixing operation are suggested.