Application of Microbubbles Generated
by Fluidic Oscillation in the Anaerobic
Digestion Process
By
Mahmood Khazzal Hummadi Al-Mashhadani
A PhD thesis submitted toThe University of Sheffield
Chemical and Biological Engineering
February 2013
Abstract
Recently significant attention has been paid globally to renewable energy as an alternative to
fossil fuels, which are responsible for climate change and economic crises. Although various
sources of clean energy exist, the amount generated from these sources has not reached levels
capable of meeting the world's energy needs. Biogas produced from the digestion of organic
material in the absence of oxygen is one of the most promising possible alternatives for use in
generating electrical and heat energy or fuel for vehicles. In addition, it is considered a cheap
alternative compared with biofuels produced from other biological processes. However,
anaerobic digestion (means of producing biogas) faces many challenges, including low
production levels and operational problems.
The current research is a serious attempt to address these problems through the exploitation
of possible ways to increase production of methane and reduce the operation and
maintenance costs. The study suggests the use of a sparging system in anaerobic digesters to
convert unspontaneous reactions to spontaneous reactions by reducing the partial pressure of
intermediate gases; thus, the reactions become thermodynamically favourable and provide
impetus for a higher level of production. In order to increase the momentum, mass and heat
transfer rates, microbubbles generated by fluidic oscillation was used in a gaslift bioreactor.
The application of microbbubles in airlift bioreactors has been experimented with studying
the kinetics of carbon dioxide transfer mechanisms. The efficiency of CO2 removal from the
liquid as a step toward upgrading biogas has been increased up to 29% by microbubble
sparging (550μm) compared with fine bubble sparging (1300μm). The design and simulation
of a gaslift bioreactor with microbubbles generated by fluidic oscillation is discussed in the
present research. The simulation study was carried out by using computational fluidic
dynamics. The effect of bubble size, location of the diffuser, and dead zone on the efficiency
of the bioreactor has also been investigated. The simulation results show that the use of
microbubbles not only increased the surface area to volume ratio, but also increased mixing
efficiency through increasing the velocity of the circulation liquid around the draft tube.
Moreover, the simulation data demonstrated that when the diameter of the microbubbles
exceeded 200 μm, the “downcomer” region, which is equivalent to more than half of the
overall volume of the airlift bioreactor, is free from gas bubbles. The designed gaslift
bioreactor was also used for processing the digested sludge from acidic gases (carbon dioxide
and hydrogen sulphide), which have a pejorative effect on the environment when they are
eventually released, as well as causing operational difficulties through creating cavitation
phenomena and the accompanying pump load. The results obtained from the experiments
show that the removal of acidic gases in an airlift digester is significantly greater with
microbubble technology than with a conventional digester. Application of a sparging system
in anaerobic digestion to break down the organic matter under mesophilic conditions was
investigated in the present thesis. The study involved the use of different gases: nitrogen,
nitrogen with carbon dioxide, undiluted biogas, diluted biogas by carbon dioxide, and finally
pure carbon dioxide. The experiments were carried out with and without fluidic oscillation in
three identical lab-scale gaslift anaerobic digesters fed by kitchen waste under different
operational conditions. The results show that using pure nitrogen as a sparged gas in
anaerobic digestion, with or without microbubble technology, leads to a reduction in methane
production. The study concluded that the nitrogen sparging system causes the stripping of all
intermediate gases, such as carbon dioxide and hydrogen, which are necessary for other
bacteria to produce acetate and methane. Although the carbon dioxide was compensated for
the second part of experiments, this action did not lead to an increase in methane production.
Enhancement in methane production was observed when the undiluted and diluted biogas
was recycled in the anaerobic digestion. The results show that 10-14% more methane was
produced from the gaslift digester than was observed in the conventional anaerobic digester.
In the final part the gaslift digester was sparged with pure carbon dioxide. The results show
that using pure carbon dioxide led to the production of double the amount of methane than
was produced by the conventional digester. The sparging system used in the study also
incorporated a new heating system for anaerobic digestion that used direct-contact
evaporation process. The study used Central Composite Rotatable Design to create an
equation based on three parameters (liquid level, flow rate and heating time). This equation
was obtained by taking the five values for the individual parameters in each experiment while
the percentage of evaporation, water temperature and humidity were measured as objective
variables. The results show that liquid level has a significant effect on the liquid temperature
due to the existence of competition between the latent heat transferred and the sensible heat
in the direct contact system. The conversion of heat supplied to sensible heat was observed
with an increase in liquid level, which can be converted to latent heat when the liquid level is
decreased.
Finally, and according to the above results, the present thesis proposes the integration of
anaerobic digestion unit, combined heat and power (CHP unit), removal of acid gases unit
and upgrading of biogas unit. This integration, along with microbubbles generated by a
fluidic oscillator, represents an effective way to increase the efficiency of biogas production
and reduce the operational difficulties.