Hydrodynamics and Scale-up of Anaerobic Stirred Digesters

The investigation presented in this work is aimed at providing a detailed characterization of the hydrodynamics in a digester of typical design, considering different scale-down criteria for the selection of the agitation conditions, with the final purpose of suggesting a methodology for aiding in reducing the energy demand of the digesters while optimizing the biogas production rates. A stirred tank of 40 litres having the same geometry of an industrial digester of 1500 m3 is investigated by means of experiments and simulations. A model fluid mimicking the rheological behaviour of the digester content stirred in a biogas production plant, which exhibits a pseudo-plastic behaviour, is adopted. The velocity field obtained from Particle Image Velocimetry and the results of Computational Fluid Dynamics simulations are discussed, focusing on well-known critical hydrodynamic features for the biogas production, namely low-velocity zones, velocity gradients and shear stresses. The detailed fluid dynamics analysis can contribute to improve the equipment design, to optimize the energy requirement and to avoid failure of the biogas production due to poor or improper mixing of the feedstock.