53    BUBBLE DYNAMICS AND AIR DISPERSION MECHANISMS
OF AIR FLOTATION PROCESS SYSTEMS
PART B: AIR DISPERSION
Milos Krofta, President
Lawrence K. Wang, Director
LENOX INSTITUTE FOR RESEARCH INC.
Lenox, Massachusetts 01240
INTRODUCTION
It is of great value in optimizing the engineering design and operating parameters of various
adsorptive bubble separation operations, such as dissolved air flotation and dispersed air flotation.
The purpose of this study is to investigate a new instrument for real time measurement of bubble
content and size distribution in a typical bubble flow reactor where air bubbles and bulk water are the
gas phase and liquid phase, respectively. This paper introduces the operation of a newly developed air
dispersion tester (or bubble generation tester), its theory, principles, operational procedures, analysis,
typical examples, and design applications for air flotation systems.
BUBBLE SEPARATION PROCESSES
Adsorptive bubble separation processes are used to concentrate or separate material which may be
molecular, colloidal or macroparticulate in size. The material is selectively adsorbed at the surfaces of
bubbles rising through the liquid, and the efficiency of the separation process depends partly on
differences in surface activity and, importantly, also on the number and size of gas bubbles.
Adsorptive bubble separation processes (such as dissolved air flotation and dispersed air flotation)
have many significant industrial applications including industrial effluent treatment, water purification, activated sludge thickening, oil-water separation, cellulose fiber concentration, etc.'"14 Recently
a 37.5 MGD flotation-filtration plant,14 and a 1.1 MGD flotation-filtration plant3,4 were installed in
Pittsfield, Massachusetts and Lenox, Massachusetts, respectively, for potable water production. A
dissolved air flotation clarifier was installed in 1983 for secondary clarification at an 18-MGD
activated sludge plant in Texas.'' Theoretical explorations and investigations of bubble dynamics and
air dispersion mechanisms become increasingly important to flotation design engineers.
The pressure, for generation of air bubbles is the major parameter controlling air solubility in an air
flotation unit and is an important factor in flotation operation. The total volume and size of air
bubbles produced on depressurization is proportional to the pressure of the process stream, the rate of
flow, and the pressure reducing mechanism. Large air bubbles produce a fast, turbulent rise rate
resulting in reduced air-solids contact time and bubble surface area. More efficient solids removal is
obtained with smaller air bubbles because of increased contact time and surface area.
An adequate air generation system for optimum results in an adsorptive bubble separation process
involving the use of dissolved air should satisfy the requirements of air volumetric flow rate, bubble
rising velocity, and power consumption. Usually the amount of generated air is about 0.5-3.0% of
water volume. This amount should be adjustable and measurable. The optimum rising velocity of air
bubbles is about 12 in/min. The bubble rising velocity should, however, not be below 5 in/min., and
not be over 20 in/min. (1 in = 2.54 cm). The air dispersion system must feature adjustment of the
bubble rising velocity, and also, the proportion of different rising velocities of the dispersed air
bubbles. The power consumption is an important economic factor. For a dissolved air flotation
system operated at full flow pressurization mode, the power consumption should be below 13 Hp/
m3/min (50 Hp/1000 GPM). For DAF operated at partial flow pressurization mode, the power
consumption should be below 7 Hp/m3/min (27 Hp/10O0 GPM). All the aforementioned parameters
need to be optimized.
44th Purdue Industrial Waste Conference Proceedings, © 1990 Lewis Publishers, Inc., Chelsea, Michigan 48118.
Printed in U.S.A.
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