Gas mixing in anaerobic digestion

Dapelo, Davide (2016). Gas mixing in anaerobic digestion. University of Birmingham. Ph.D.

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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: Thesis (Doctorates > Ph.D.)
Award Type: Doctorates > Ph.D.
College/Faculty: Colleges (2008 onwards) > College of Engineering & Physical Sciences
School or Department: School of Engineering, Department of Civil Engineering
Funders: European Commission
Subjects: Q Science > QC Physics
Q Science > QD Chemistry
T Technology > TP Chemical technology


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