37    THE EFFECTS OF A MICROBUBBLE DISPERSION ON THE
PERFORMANCE OF A POTW DISSOLVED AIR
FLOTATION SYSTEM
Brian A. O'Palko, Senior Staff Engineer
Donald L. Michelsen, Professor
Department of Chemical Engineering
Virginia Polytechnic Institute and State University
Blacksburg, Virginia 24061-0211
INTRODUCTION
A microbubble dispersion (colloidal gas aphrons) can be described as a 10% to 70% dispersion of
50 ± 40 micron-sized diameter air bubbles in water. These fine microbubbles were first described by
Dr. Felix Sebba in 1971, and given the name microfoam due to the minute size of the bubbles. The
microbubbles were originally thought to be a gas emulsion system.1 The name was later changed when
they were shown in fact to be a dispersion of gas in liquid showing some colloidal properties.-
A microbubble dispersion differs from ordinary gas bubbles in that they contain a distinctive shell
layer around the bubbles created by a small concentration of a water soluble surfactant solution. This
shell layer is formed by the surfactant molecules positioning themselves at the air-water interface. This
thin film surrounding the bubbles is different from both the gas and bulk water phase and has
considerable mechanical strength, thereby giving the microbubbles surprising stability.-
To date, the following analytical measurements have been standardized to characterize
microbubbles:'
1. Quality: percent air in Ihe total dispersion typically with a 40%lo 70% range.
2. Stability (H): is the percent rise in one minute of the clear phase in a standard 250 mL
graduated; that is in 250 mL graduate in one minute (H')/mL of rise of the final
coalesced solution in percent. Typical range is 8 to 80.
3. Size Distribution: based on microscopic photography followed by image analysis.
4. Viscosity of Microbubble Solution: based on the flow capillary viscometer with semiquantitative analysis possible using a Haake viscosimeter.
5. Surface Tension: of surfactant solution used for CGA generation.
The measurements have become increasingly standardized in our testing. Both the quality and the
stability can be secured by taking a weighted standard 250 mL graduate and filling it with the
microbubbles to the 250 mL mark, weighing graduate and microbubbles from which the quality and
final height of coalesced microbubbles can be calculated. Then after one minute the H' (mL reading
on the graduate) is recorded from which the H, percent rise of coalesced microbubbles in one minute
can be determined. The H is really a normalized adjustment for the H'.
The microbubble properties of the typical CGA characterized above are as follows:3,4
1. Wet foam with spherical microbubbles if the quality is less than 65% to 70%.
2. Stability (reduced coalescence) is maintained with gentle stirring.
3. Limited coalescence; air in the small bubbles diffuse and disappear into larger bubbles.
4. Limited void air if generated properly; all microbubbles.
5. Microbubbles injected into saturated soil matrices tend lo adhere to the matrix and are
retained for periods of months.
6. CGA have diameters 50 ± 40 microns, compared to 50 to 150 microns for dissolved air
bubbles (10% quality), and 1000 to 3000 micron for most sparged systems. Once dispersed in liquid system rise velocity is proportional to diameter square. Thus good
retention is possible in systems with gentle mixing.
7. CGA have large surface area with area inversely proportional to the diameter in some
47th Purdue Industrial Waste Conference Proceedings, 1992 Lewis Publishers, Inc., Chelsea, Michigan 48118.
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