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Keywords: sewage sludge, digestion, biogas, computational fluid dynamics, non-Newtonian fluid, turbulence, energy.

Summary

Renewable energy resources produce very little carbon or other greenhouse gases and consequently their development is an integral part of the UK Government's strategy for reducing carbon emissions. Currently, the UK water industry generates 493 GWh of renewable energy each year, equating to 14% of the total energy used to treat water and wastewater in 2006. Anaerobic digestion is the most widespread technology for the treatment of wastewater sludge, the by-product of wastewater treatment. This natural process uses bacteria to break down biodegradable material and produces a biogas rich in methane. The current drive to maximise energy recovery means that the biogas from anaerobic digestion is increasingly harnessed by means of combined heat and power technology. There exists, therefore, the need to optimise the performance of anaerobic digestion vessels in order to maximise energy recovery.

Computational fluid dynamics (CFD) is an accepted and well-used means of assessing and optimising process designs without necessarily incurring the expense of prototype development. Recent years have seen an increase in the application of CFD to water industry issues. However, the use of CFD to model the flow and mixing of sewage sludge has been limited.

In this paper, the development of a CFD model to simulate the mechanical mixing of sewage sludge at laboratory scale is reported. The paper considers the different methods by which the non-Newtonian nature of the sludge can be modelled (e.g. non-Newtonian power law, Herschel-Bulkley model) and also considers the means by which turbulence closure may be attained. The paper recommends a strategy for modelling mechanically mixed sewage sludge at laboratory scale. Generated flow patterns are analysed and conclusions drawn as to the effectiveness of mixing in the laboratory scale digester. Data relating biogas yield to different mixing regimes are reported and combined with CFD model results. From this, conclusions regarding velocity gradients and biogas yield are drawn.

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Front Page Browse CCP CSETS CTR IJRT Other Authors Search Purchase Guide FAQ Contact us	Civil-Comp Proceedings ISSN 1759-3433 CCP: 91 PROCEEDINGS OF THE TWELFTH INTERNATIONAL CONFERENCE ON CIVIL, STRUCTURAL AND ENVIRONMENTAL ENGINEERING COMPUTING Edited by: B.H.V. Topping, L.F. Costa Neves and R.C. Barros Paper 260 Computational Fluid Dynamics Modelling of the Mixing of Sewage Sludge in an Anaerobic Digester J. Bridgeman School of Civil Engineering, University of Birmingham, United Kingdom doi:10.4203/ccp.91.260 purchase the full-text of this paper Full Bibliographic Reference for this paper J. Bridgeman, "Computational Fluid Dynamics Modelling of the Mixing of Sewage Sludge in an Anaerobic Digester", in B.H.V. Topping, L.F. Costa Neves, R.C. Barros, (Editors), "Proceedings of the Twelfth International Conference on Civil, Structural and Environmental Engineering Computing", Civil-Comp Press, Stirlingshire, UK, Paper 260, 2009. doi:10.4203/ccp.91.260 Keywords: sewage sludge, digestion, biogas, computational fluid dynamics, non-Newtonian fluid, turbulence, energy. Summary Renewable energy resources produce very little carbon or other greenhouse gases and consequently their development is an integral part of the UK Government's strategy for reducing carbon emissions. Currently, the UK water industry generates 493 GWh of renewable energy each year, equating to 14% of the total energy used to treat water and wastewater in 2006. Anaerobic digestion is the most widespread technology for the treatment of wastewater sludge, the by-product of wastewater treatment. This natural process uses bacteria to break down biodegradable material and produces a biogas rich in methane. The current drive to maximise energy recovery means that the biogas from anaerobic digestion is increasingly harnessed by means of combined heat and power technology. There exists, therefore, the need to optimise the performance of anaerobic digestion vessels in order to maximise energy recovery. Computational fluid dynamics (CFD) is an accepted and well-used means of assessing and optimising process designs without necessarily incurring the expense of prototype development. Recent years have seen an increase in the application of CFD to water industry issues. However, the use of CFD to model the flow and mixing of sewage sludge has been limited. In this paper, the development of a CFD model to simulate the mechanical mixing of sewage sludge at laboratory scale is reported. The paper considers the different methods by which the non-Newtonian nature of the sludge can be modelled (e.g. non-Newtonian power law, Herschel-Bulkley model) and also considers the means by which turbulence closure may be attained. The paper recommends a strategy for modelling mechanically mixed sewage sludge at laboratory scale. Generated flow patterns are analysed and conclusions drawn as to the effectiveness of mixing in the laboratory scale digester. Data relating biogas yield to different mixing regimes are reported and combined with CFD model results. From this, conclusions regarding velocity gradients and biogas yield are drawn. purchase the full-text of this paper (price £20) go to the previous paper go to the next paper return to the table of contents return to the book description purchase this book (price £140 +P&P)
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