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Gas mixing in anaerobic digestion

Dapelo, Davide (2016)
Ph.D. thesis, University of Birmingham.

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Abstract

Mesophilic anaerobic digestion is one of the most used and successful technologies to treat the sludges resulting from wastewater treatment. However, traditional approaches to digester design are firmly rooted in empiricism and rule of thumb rather than science. Mixing is an energy-intensive operation, and therefore the need to lower the wastewater process carbon footprint requires searching how to lower the input mixing energy without compromising–and indeed enhancing–biogas production. In particular, the literature on gas mixing is still particularly poor.

For the first time, an Euler-Lagrangian CFD model was developed for gas mixing in anaerobic digestion. The model was validated against laboratory experiments with PIV and PEPT techniques.

Full-scale simulations reproducing a real digester were performed with the validated model, and different scenarios were reproduced. Shear rate distribution was used as a parameter to assess the most appropriate value of input mixing power. The simulations also low-viscosity flow patterns for the first time. This phenomenon is intrinsically linked to the non-Newtonian nature of sludge, and leads to short-circuited mixing.

Switching biogas injection between two different nozzle series was found to be an effective strategy to mitigate the issue of the low-viscosity flow patterns. Final recommendations on input mixing power and switching time were given to improve mixing efficiency in the full-scale design taken into consideration.

A journal paper published in Water Research and a conference paper presented at the Fifteenth International Conference on Civil, Structural and Environmental Engineering Computing (Civil-Comp) were produced. Two other papers are currently in preparation.

Type of Work:Ph.D. thesis.
Supervisor(s):Bridgeman, John and Sterling, Mark
School/Faculty:Colleges (2008 onwards) > College of Engineering & Physical Sciences
Department:School of Civil Engineering
Subjects:QC Physics
QD Chemistry
TP Chemical technology
Institution:University of Birmingham
ID Code:6879
This unpublished thesis/dissertation is copyright of the author and/or third parties. The intellectual property rights of the author or third parties in respect of this work are as defined by The Copyright Designs and Patents Act 1988 or as modified by any successor legislation. Any use made of information contained in this thesis/dissertation must be in accordance with that legislation and must be properly acknowledged. Further distribution or reproduction in any format is prohibited without the permission of the copyright holder.
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