(578d) Computational Fluid Dynamics Investigation of Mechanical Mixing in Anaerobic Bioreactors

Methane from anaerobic digestion of organic matter is a source of biorenewable energy. Mixing in bioreactors helps maintain uniform total solids (TS) distribution and maximizes the direct contact between active biomass and raw sludge. This study presents a computational fluid dynamic (CFD) technique that evaluates the mechanical mixing performance quantified by blending time with respect to power input in bioreactors. In the model development, the digestate was assumed to be a non-Newtonian power-law fluid that is dependent on the TS level, and mixing was achieved by a standard pitched blade turbine (PBT) or a modified PBT (m-PBT) impeller. The mixing experiments were conducted using xanthan gum, in which the axial and radial velocities obtained with particle image velocimetry measurements were used to examine the CFD results. The power and flow numbers of an impeller were verified with the laboratory specifications. A general procedure for each run includes two stages: (1) using a multiple reference frame approach to predict steady-state turbulent mixing, and (2) solving a transient scalar transport equation to obtain the blending time based on the converged flow fields. An optimum mixing design was proposed based on the lowest power input required to homogenize the substrate in the bioreactors.