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| Using Mixing in Sludge Digesters to Maximise Energy Output | ||||||||||||
Although a well-designed sewage sludge digester has in the past been synonymous with a well-mixed digester, the associated energy and resource costs are significant. The water industry now seeks the minimum degree of mixing to minimise cost and environmental impact without compromising biogas output. The limited evidence base suggests that continual mixing might not be necessary for maximising sludge conversion to biogas. It also suggests that not all parts of a digester need to be mixed equally, with unmixed strata at the base of the digester demonstrating methane producing activity 1.5 times of that in mixed zones. In order to predict confidently the optimum mixing regime for a particular sludge digester, we need to determine to what extent biogas output depends on, and can be influenced by, flow patterns in a digester; flow patterns which are in turn determined by physical parameters of the digestion vessels, inflow mode, mixing systems and sludge rheology. Thus, mixing regimes for sludge digesters could be tailored to enable flow patterns that would potentially increase biogas production and/or reduce mixing energy input; in both cases improving the overall energy yield from that digester and reducing its environmental impact. The overall aim of this project is to identify mixing regimes and associated flow patterns that optimise biological activity and hence biogas output, while minimising energy input, for anaerobic sludge digesters. This will be achieved through an analysis of the relationship between biogas yield and mixing in laboratory-scale digesters for a range of physical sludge characteristics (e.g. viscosity, yield stress, solids content) and energy input characteristics (e.g. mixing speed and frequency, impeller size, number and position). The student will also develop a computational fluid dynamics (CFD) model of the laboratory digesters to visualise flow patterns and quantify turbulence effects that impact on biogas yield. The experimental and numerical results will then be used in combination to identify optimum mixing regimes for digesters depending on their sludge rheology, operational regimes, tank dimensions and mixing systems. Funding Notes To qualify for the full scholarship, you must be a UK national, or a UK permanent resident, or an EU national, who has been resident in the UK for more than three years. You should have a good degree (1st Class, 2.1 or MSc) in civil or chemical engineering, applied mathematics or a related physical science. Some experience in wastewater treatment or CFD is preferred but not essential. |
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