Experimental and numerical investigation of the hydrodynamics of mixed anaerobic digester
Balcha, Daniel Asrat
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A review of the literature indicates that the understanding of the mixing phenomena in anaerobic digesters is limited and the ability to measure digester mixing characteristics is lacking. Moreover, rheological characteristics of the sludge are largely ignored. The need for a more thorough understanding of fundamental mixing relationships and the ability to measure these relationships in the anaerobic digester is recognized. To this end, investigations using experimental and numerical methods to visualize flow patterns and quantify mixing that impact biogas yields is reported. Results from this research identifies optimum mixing regimes for digesters depending on their sludge rheology, operational regimes, digester dimensions and mixing systems, and also produces an industrial guide to improve digester design. The results show that in order to accurately predict the optimum mixing regime for a particular anaerobic digester, it is necessary to determine to what extent biogas output is dependent upon, and can be influenced by flow patterns in an anaerobic digester. These flows are in turn determined by physical characteristics of the digester, the feed regime, the mixing system and the rheological properties of the sludge. Mixing regimes are tailored to produce flow patterns that could potentially increase biogas production and/or reduce mixing energy input of the digester. An all-inclusive anaerobic digester model is developed and validated using advanced experimental techniques that can be used to optimize the digestion process in terms of physical and biological parameters. In either case, the model provides an ongoing management of anaerobic digester process to improve overall energy yield from the digester and reduce its environmental impact.