(744b) A Fundamental Investigation of Single Bubble and Bubble Swarm Behavior With Respect to Fouling Prevention in Membrane Systems

A fundamental investigation of single
bubble and bubble swarm behavior with respect to fouling prevention in membrane
systems

L Böhm¹, M Kraume¹

¹Chair
of Chemical and Process Engineering, Technische Universität Berlin, Germany

FH6-1, Fraunhoferstrasse 33-36, 10587 Berlin, Germany. Tel: +493031472791, email: lutz.boehm@tu-berlin.de

Aim

Aeration is a common measure to detach
deposition layers from membranes. A detailed view at commercially available
flat sheet membrane modules reveals that although aeration is a main cost
factor in the operation of membrane bioreactors (MBR), no optimal design and
operation with respect to the cleaning efficiency was found yet. Different gap
distances between the membrane cushions, different bubble sizes, additional mechanical
cleaning with the help of granulates, pulsating aeration etc. are applied. This
investigation's focus is gaining a fundamental understanding about the behavior
of single bubbles and bubble swarms in such systems. The complexity of the
behavior of a bubble swarm in an MBR is rather high. The fundamental approach
of investigating the single bubble and bubble swarm behavior in one rectangular
channel will help getting a deeper insight into the bubble dynamics in such
systems. Especially the investigations with model solutions imitating the
rheological behavior of activated sludge can deepen the understanding of the
cleaning process.

Figure 1. Schematic
of the rectangular channel (a), front view of the flow field near a 5 mm
bubble in a 5 mm channel in stagnant water (b) and in non-Newtonian
Xanthan solution

Methods

For this purpose, an experimental
apparatus was built which represents one of the channels with a rectangular
cross section area between two flat sheet membrane cushions [1]. Acrylic glass
is used to make optical measurements possible (Fig.1a). The fully automated rig
is used to investigate single bubbles rising in the channel with the help of
the electrodiffusion method for the determination of
the generated shear stress on the walls, with a high speed camera for the
determination of the bubble dynamics and with particle image velocimetry for the determination of flow field parameters
in the liquid surrounding the bubble. In these investigations, the gap distance
between the plates (3-7 mm), the single bubble size (3-9 mm), the
superimposed liquid velocity (0-0.2 m/s) and the rheology of the liquid
(Newtonian, non-Newtonian) were varied. For the investigation of the bubble
swarm, in this list of parameters, the single bubble size is replaced by the
aeration rate and aerator type.

Results

Exemplary flow fields near a single
bubble are shown in Fig.1b) for the rise in water and c) for the rise in
Xanthan solution. The concentration of Xanthan is chosen in a way, so that the
shear thinning behavior of the solution is according to the one of activated
sludge in MBRs [3]. The flow fields show the strong influence of the rheology
of the continuous phase on the rise behavior. In case of the rise in water, the
typical vortices which detach periodically from both sides of the bubble are
visible. This leads to an oscillating movement of the bubble. Due to the
confinement by the walls, especially for the bubbles equal or larger in size in
comparison to the gap distance, only a zigzag-movement in the x-y-plane is
found. For bubbles smaller than the gap distance, a zigzag-movement in the
y-z-plane is appearing as well. Depending on the bubble size, in unconfined
geometries the bubble in the size range investigated here would rise in a
zigzag or helical motion. In case of the rise in the Xanthan solution, an
almost symmetric flow field with respect to the vertical center line is found.
Therefore, if appearing at all, only minor oscillations were found in any
direction. The very particular flow field in the non-Newtonian liquid was
similarly found for bubbles rising in unconfined geometries [2]. Near the
bubble, the liquid is pushed around the bubble's edges on both sides. In the
wake of the bubble, a triangular area with an upward flow on the edges of the triangular
and a downward flow in the center of the triangular was found. This is due to
the lower viscosity of the liquid in the high shear zone induced by the bubble
movement. This effect will be visible for bubble swarms as well. In the
investigation of the bubble swarm with the high speed camera, the bubble
dynamics, bubble rise velocities und rising path are compared to the single
bubble results and literature values e.g. from Yamanoi
and Kageyama [4].

Conclusion

This fundamental investigation illustrates
the influence of the liquid rheology in MBRs on the bubble dynamics and
therefore the cleaning effect. With shear thinning liquids, e.g. the
oscillation parallel to the membrane will be dampened and therefore the
membrane area that is influenced by the bubble will be reduced. Without
oscillation normal to membrane, the generated shear stress will be reduced as
the liquid film thickness between bubble and wall increases in comparison to
the film thickness in water. The investigation of the bubble swarm will show if
the effects found for single bubbles can be confirmed.

References

[1]       L. Böhm, A. Drews, and M. Kraume. Bubble induced shear stress in
flat sheet membrane systems - serial examination of single bubble experiments
with the electrodiffusion method. J. Membr. Sci.,
437:131?140, 2013.

[2]       D. Funfschilling
and H. Z. Li. Flow of non-Newtonian fluids around bubbles: PIV
measurements and birefringence visualisation. Chem. Eng. Sci., 56(3):1137 ?
1141, 2001.

[3]       S. Rosenberger,
K. Kubin, and M. Kraume.
Rheology of activated sludge in
membrane bioreactors. Eng. Life Sci., 2(9):269?275, 2002.

[4]       I. Yamanoi
and K. Kageyama. Evaluation of
bubble flow properties between flat sheet membranes in membrane bioreactor.
J. Membr. Sci.,
360(1-2):102?108, 2010.