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Microbubble-aided water and wastewaterpurification: A review

Article in Reviews in Chemical Engineering December 2012

Impact Factor: 2.41 DOI: 10.1515/revce-2012-0007

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Snigdha Khuntia

Indian Institute of Technology Guwahati

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Indian Institute of Technology Guwahati

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DOI 10.1515/revce-2012-0007 Rev Chem Eng 2012; 28(4-6): 191221

Snigdha Khuntia, Subrata Kumar Majumder* and Pallab Ghosh

Microbubble-aided water and wastewaterpurification: a review

Abstract: Microbubble-based methods, in recent times,

have been widely used for purification of water and

wastewater. Microbubbles have several physicochemi-

cal properties, which make them eminently suitable for

wastewater treatment. In this review, these properties

have been analyzed in detail from the perspective of appli-

cation. Various types of microbubble generators and their

operation principles have been discussed. The transport

of gas into the aqueous phase has been explained, and the

correlations to predict the volumetric mass transfer coef-

ficient have been presented. Many practical applicationsusing ozone, oxygen and air microbubbles, some of which

are currently at various stages of commercialization, have

been presented. Other important uses of microbubbles

for wastewater treatment, namely, removal of fine solid

particulate matter and oil, have also been discussed. In

addition, directions for future research of microbubble

technology and their potential applications have been

identified.

Keywords: microbubble; microbubble generator; ozona-

tion; wastewater treatment.

*Corresponding author: Subrata Kumar Majumder,Department of

Chemical Engineering, Indian Institute of Technology Guwahati,

Guwahati-781039, Assam, India,

e-mail: [email protected]; [email protected]

Snigdha Khuntia :Department of Chemical Engineering, Indian

Institute of Technology Guwahati, Guwahati-781039, Assam, India

Pallab Ghosh:Department of Chemical Engineering, Indian Institute

of Technology Guwahati, Guwahati-781039, Assam, India

1 Introduction

The harmful effects of toxic chemicals in civilization are

well-known. It is not possible to eliminate all such chem-

icals due to several reasons, such as yield of secondary

compounds and an economic rate of production. Pres-

ervation of a safe and clean environment has become a

social problem, and water is particularly important from

this perspective (Hiroshi 2006). Amid deepening global

environmental problems, various efforts have been

taken for improving water quality (Akiko et al. 2005). In

recent years, microbubbles have received some impor-

tance among water purification technologies (Burns et al.

1997, Nakano et al. 2005, Matsuo et al. 2006, Usui 2006,

Yamasaki et al. 2009, 2010, Agarwal et al. 2010, Wen et al.

2011). Microbubbles of air, oxygen and ozone are being

extensively used in various water treatment applications.

Apart from water treatment, microbubbles have

found use in various other fields. In medical therapeutic

applications, microbubbles have been used for scanning

body organs, and also as a drug or gene carrier (Lindner2004, Tsutsui et al. 2004, Matsumoto et al. 2005, Hernot

and Klibanov 2008, Maliwal and Patidar 2008, Kurup

and Naik 2010, Dicker et al. 2011). Microbubbles have

been used for antibacterial activities under aerobic and

anaerobic conditions (Himuro et al. 2009). Microbubbles,

stabilized by various surfactants, are used for adsorp-

tion of protein from aqueous solution (Jauregi and Varley

1998). Petroleum-based as well as biological surfactants

have been used for stabilizing microbubbles (Kukizaki

and Baba 2008, Xu et al. 2011). Microbubbles have been

functionalized by surfactants, nanoparticles, pharmaceu-

ticals and bioactive molecules. Owing to their small size,

microbubbles are highly effective in industrial separation

processes, such as removal of volatile contaminants and

particulate matters present in the aqueous phase (Ahmed

and Jameson 1985). Flotation processes employing micro-

bubbles are useful for the removal of low-density particu-

late matter present in water (Terasaka and Shinpo 2007).

Microbubbles act as carriers of fine particles, which are

lifted up from the bottom of the column. The flotation

process can be dissolved-air flotation, dispersed-air flo-

tation or electroflotation, depending on the microbubble

generation technique (Ketkar et al. 1991, Liu et al. 2010a).Microbubbles have also found use in cleaning (Himuro

2007, Akuzawa et al. 2010), soil washing (Roy et al. 1992),

removal of oil from soil (Gotoh et al. 2006) and water

(Xiaohui et al. 2011), fermentation (Kaster et al. 1990, Ago

et al. 2005, Xu et al. 2011), marine fish farming (Tsutsumi

2010), horticulture (Park and Kurata 2009), food tech-

nology (Shen et al. 2008, Xu et al. 2008, Soli et al. 2010),

absorption of acid gas (e.g., CO2) by alkali (Akimov et al.

2011), and many more applications, which have been

summarized by Li (2006). Microbubbles can enhance the

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192 S. Khuntia et al.: Microbubble-aided water purification

growth rate of marine creatures, such as oysters and scal-

lops, by providing a large gas-liquid interfacial area that

facilitates interphase transfer of air.

Water treatment using microbubbles has recently

become a well-known technology for many industrial

applications due to its superior efficiency, compared

with conventional methods (Ohnari 1997, Jyoti and Pandit2001). Ozone microbubbles have been used for oxida-

tion, disinfection (Sumikura et al. 2007), decolorization

and deodorization (Shin et al. 1999). Ozone has a strong

oxidizing ability, and by utilizing this capability its use

in water purification or sewage water treatment is antici-

pated for sterilization, removal of color and odor, and

degradation of organic substances (Camel and Bermond

1998). In this review, various properties of microbubbles,

available technologies for generation of microbubbles and

the measurement of their size are discussed. In addition,

the major applications of microbubbles in the treatment ofwater and wastewater are discussed from the perspective

of future applications and research directions.

2 Structure and properties

of microbubbles

Microbubbles are tiny spherical bubbles with a diameter

of 50 m. They are different from ubiquitous commonbubbles (also known as macrobubbles), not only in terms

of size but also in terms of their physicochemical proper-

ties. These special physicochemical properties have made

microbubbles particularly useful in various water and

wastewater treatment applications (Ohnari et al. 2002,

Takahashi 2004, 2010, Tsuge 2010). These features are:

low rising velocity through water, surface having high

curvature, large gas-liquid interfacial area and electrically

charged gas-liquid interface. Many of the useful features

of microbubbles for wastewater treatment are associated

with these properties.

2.1 Size and shape

Microbubbles are spherical in shape. For a three-dimen-

sional object with a given volume, the area is minimal

when the object has a spherical shape, which is mathemat-

ically expressed by isoperimetric inequality (Osserman

1978). Therefore, gas-liquid interfacial energy is minimal

when the bubble is spherical. Macrobubbles, which are

commonly used in fermentors, gas-liquid reactors and

ore flotation equipment, have a diameter in the range of

25 mm. These bubbles are also called millibubbles. The

microbubbles have a diameter of 50 m (Takahashi

et al. 2007a). Bubbles, which have a diameter of


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S. Khuntia et al.: Microbubble-aided water purification 193

coalescence by various repulsive interfacial forces (e.g.,

electrostatic double layer, steric and hydration forces).

The stability of the surfactant-stabilized microbubbles

allows them to be pumped without collapse. Therefore,

such microbubbles can be produced in a small vessel and

then pumped to a much larger reactor.

Microbubbles, formed ultrasonically or by specialgenerators, have a wide size distribution. It is a challeng-

ing task to produce monodispersed microbubbles (Zhang

and Li 2010), which are important in medical applications

(Feshitan et al. 2009). Size distribution can be unimodal

or bimodal. A typical size distribution (Tsuge et al. 2009) is

shown in Figure 2. The microbubble generation technique

has a significant effect on the size of the microbubbles (Li

2006). Tsuge et al. (2009) have studied single pore nozzle

and rotating flow microbubble generators and compared

the size of the bubbles formed by them. With the rotat-

ing flow microbubble generator, the bubbles are shearedand crushed by the rotating flow, which results in smaller

microbubbles. In addition, at large outlet pressure of the

pump bubbles of smaller size are produced (with both

generators). This is due to the change in pressure at the

time of bubble formation, which is greater and results in

smaller bubbles. The presence of surface active impurities

and electrolytes can have a significant effect on microbub-

ble size. Walker et al. (2001) have reported that the size of

microbubbles decreased with increasing NaCl concentra-

tion up to 1 mol/dm3. It has been reported (Hofmeier et al.

1995) that surface elasticity plays an important role in the

formation of smaller bubbles in the presence of NaCl. Pure

liquids, which have small surface elasticity, give rise to

large bubbles through coalescence. In aqueous solutions,

12

9

Single pore nozzle

Rotating flow microbubble generator

6

Frequency(number%)

3

00 50 100

Bubble diameter (mm)

150 200

Figure 2 Size distributions of microbubbles generated by a single

pore nozzle and a rotating flow microbubble generator at 0.55 MPa

pressure (Tsuge et al. 2009).

where surface elasticity is larger, bubble size is reduced

through inhibition of coalescence.

2.1.1 Measurement of size of microbubbles

Several methods are available for measuring the sizeof microbubbles. Bubble diameter down to 1 m canbe measured by a photographic technique (Zhang et al.

2000, Kawahara et al. 2009). Laser light scattering is a

more advanced method for measuring the size of micro-

bubbles. In this method, a photomultiplier tube detects

the scattered light. The relationship between the intensity

of the scattered light and the bubble diameter varies with

the size of the bubbles. For nanobubbles, the Rayleigh

scattering theory may be applicable (if the diameter of

the bubbles is much smaller than the wavelength of the

light), and the scattered intensity is proportional to thesixth power of the bubble diameter (Pelssers et al. 1990).

For larger bubbles, for which the diameter is close to the

wavelength of light, the Mie scattering theory is used to

measure the bubble diameter (Kerker 1969). The scattered

light intensity varies in a complex manner with the dia-

meter of the bubble. For large microbubbles, the scattered

intensity varies with the square of the bubble diameter

(Glantschnig and Chen 1981). The laser scattering tech-

nique is used to measure bubble size distribution in the

range of 0.022000 m. The size and scattered intensity iscalibrated using standard particles (e.g., polystyrene latex

particles). When spherical bubbles are irradiated by laser,

a scatter pattern is obtained. However, bands of interfer-

ence can also be seen on a plane away from the focus. The

bubble diameter is proportional to the number of interfer-

ence bands (Tsuge 2010). Bubble diameters ranging from

10 m to 1 mm can be measured by this technique.Another method for the measurement of size of large

microbubbles is the pore electrical resistance method

(Tsuge 2010). When electrodes are placed in an electro-

lyte solution, which is partitioned with a separator having

small pores, and a voltage applied to the electrodes, elec-

trical resistance is determined by the resistance of the partwith the micropores. When microbubbles pass through

these pores, electrical resistance increases, and the

number of bubbles and their size can be determined from

the resistance. Bubble diameter in the range of 0.2600

m can be measured by this method. However, if the dis-tribution of bubble diameter has a range wider than this,

the pore size needs to be adjusted.

Guidi et al. (2010) have presented a technique com-

prising acoustic and optical methods to investigate

the response of isolated lipid-shelled microbubbles to




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low-pressure ultrasound tone bursts. These bursts induced

slow deflation of the microbubbles. Their experimental

set-up had a microscope connected to a camera acquiring

one frame per pulse transmitted by a single-element trans-

ducer. The behavior of each bubble was measured at mul-

tiple frequencies by cyclically changing the transmission

frequency over the range of 24 MHz. The bubble echoeswere captured by a second transducer and coherently

recorded. Microbubbles with radii larger than 3 m didnot experience any size reduction. Smaller bubbles slowly

deflated, generally until the radius reached a value around

1.4 m, independent of the initial microbubble size. Theresonant radius was evaluated from the echo amplitude

normalized with respect to the geometrical cross-section.

Hosokawa et al. (2009) have developed a technique

to measure the diameter of microbubbles by using phase

Doppler anemometry and an image processing method

(Burger and Burge 2008), which is based on the Sobelfilter (Schau 1980) and Hough transform (Yu et al. 2007).

The size distribution and the mean diameter of the micro-

bubbles were evaluated based on the diameters measured

by both methods. They have claimed that size distribution

can be accurately determined for a wide range of diam-

eters by these methods.

Dynamic light scattering (DLS) (Berne and Pecora

2000) is a sophisticated technique for the measurement

of diameter of MNBs and nanobubbles. This method

measures Brownian motion and relates this to the size of

the bubbles. The Brownian motion becomes slower with

the increase in the size of the bubble. The velocity of the

Brownian motion is defined by the translational diffusion

coefficient. The size of a bubble is calculated from the

translational diffusion coefficient by using the Stokes-

Einstein equation (Ghosh 2009a).

3 A b

RTd

N D=

(1)

The diameter measured in DLS is a value that refers to

how a particle diffuses within a liquid. Thus, it is referred

to as the hydrodynamic diameter. The diameter that isobtained by this technique is the diameter of a sphere

that has the same translational diffusion coefficient as

the bubble. The translational diffusion coefficient will

depend not only on the size of the bubble but also on any

surface structure, as well as the concentration and type of

ions in the medium. Factors that affect the diffusion speed

of particles are ionic strength of the medium (which influ-

ences the thickness of the electrostatic double layer) and

the surface structure (e.g., adsorbed long chain molecules

projected out into the liquid). Variation in temperature can

cause variation in viscosity and, hence, induce convection

currents, which would alter the random movement of the

molecules. Therefore, temperature must be very stable in

the DLS experiments.

An X-ray particle tracking velocimetry (PTV) tech-

nique has been developed by Lee and Kim (2005) to

simultaneously measure the size and velocity of micro-bubbles in a liquid without optical aberration. This

technique is based on a combination of in-line X-ray

holography and PTV. The X-ray PTV technique uses a con-

figuration similar to that of conventional optical imaging

techniques. These researchers generated microbubbles

with diameters in the range of 1060 m from a fine wireby electrical heating and used them as tracer particles.

The X-ray PTV technique simultaneously recorded the

size and velocity of the microbubbles moving upward in

an opaque tube of 2.7 mm inner diameter. Owing to the

different refractive indices of water and air, phase con-trast X-ray images showed the exact size and shape of

the microbubbles.

As microbubbles generally have a broad size distribu-

tion, the average diameter of the microbubbles is often

expressed in terms of the Sauter mean diameter (SMD), d32

(Kawahara et al. 2009, Liu et al. 2010b, Maeda et al. 2010),

which is defined as:

3

32 2

i i

i i

n dd

n d=

(2)

The bubble size distribution is often plotted as a

cumulative distribution in which a percentage of the total

volume of bubbles below a given size is plotted vs. bubble

size. 90d, 50dand 10d correspond to 90, 50 and 10 volume

percents on this cumulative size distribution curve. The

bubble size dispersion coefficient () is defined as (Kuki-

zaki and Goto 2006):

90 10

50

-d d

d=

(3)

DLS provides the z-average diameter of the bubbles.This is the mean diameter of the distribution based on the

intensity of scattered light, and this diameter is obtained

by the cumulants analysis of the correlation function. It

is not a mass or number average because it is calculated

from signal intensity. In DLS, this is the most important

and stable value that is produced. This is the size to be

used if a value is required for quality control purposes.

It will be comparable with other techniques if the dis-

tribution is unimodal, the bubbles are spherical and

monodispersed.




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would be only 1 s (Ljunggren and Eriksson 1997). Somescientists have raised questions on the applicability of the

Young-Laplace equation in the nanometer dimensions,

due to the departure of surface tension from the macro-

scopic value, because of the variation of thermodynamic

quantities as a consequence of the large curvature asso-

ciated with nanobubbles (Li Juan et al. 2008). The appli-

cability of the Young-Laplace equation to nanobubbles

has been validated in a few studies (Matsumoto 2008,

Matsumoto and Tanaka 2008). Molecular dynamics simu-

lations have shown that the density of a nanoscale nitro-

gen bubble in water can be as large as 270 kg m3 (Fang

and Hu 2006). The following equation has been developed

to predict the lifetime of nanobubbles (Li Juan et al. 2008):

( )

3

0 4

48 1-g

g l

Hd

D

=

(9)

Therefore, as the density of gas in the nanobubble

increases, the lifetime of the nanobubble can increase

dramatically. The gas in the nanobubbles might form a

new phase if its density is so large. Another important

characteristic of shrinking microbubbles is the hydrate

formation, which makes them useful in gas storage and

transportation (Takahashi et al. 2003). Gas hydrates

(also termed clathrates) are crystalline compounds

which occur when water forms a cage-like structure

around smaller guest molecules (Sloan 1998). According

to Henrys law, the amount of dissolved gas around the

shrinking bubble increases with increasing gas pressure

inside the bubble. The area surrounding a microbub-

ble changes its state to favor the nucleation of hydrate.

Some of the nuclei grow to hydrate films surrounding the

bubble, and the entire bubble could change to a hydrate

particle stabilized by the thick film. It has been reported

that the formation of hydrate may impart stability to the

nanobubbles for a prolonged time (of the order of months)

(Takahashi 2009).

2.4 Electrical properties

The surface of microbubbles in water is charged. Being

charged, microbubbles move towards the oppositely

charged electrode when an electric field is applied. This

surface charge can be measured in terms of potential,which is the potential at the plane of shear (Hiemenz and

Rajagopalan 1997, Ghosh 2009a). Although the location of

the plane of shear is not precisely known (and therefore

the value of potential may be different from the actualpotential at the surface), it is the only quantity that can be

experimentally measured. The potential is determined

Bursts (coalesces) atthe air-water interface

Rapidly rises toward

the water surface

Ultimately disappears

in water by dissolution

Gradually

shrinks in water

Stable formonths

MicrobubbleMacrobubble

Water surface (air-water interface)

Nanobubble

Figure 4 Difference between macrobubble, microbubble and nanobubble in terms of their behavior in water.




8/32


by measuring the electrophoretic mobility of the micro-

bubble and then applying the Smoluchowski equation

(Ghosh 2009a):

0

E

=

(10)

The charge at the gas-liquid interface plays a very

crucial role in the stability of the microbubbles against

coalescence with neighbor bubbles in the dispersion

(Ghosh 2009b, Srinivas and Ghosh 2011), and in the gen-

eration of free radicals (such as hydroxyl radicals, OH),

which are immensely important in the oxidation of unde-

sirable inorganic and organic compounds present in

wastewater (Takahashi et al. 2007a,b, Bando et al. 2008a,

Li et al. 2009a,b). The potential of air microbubbles indistilled water is negative. The value reported by Taka-

hashi (2005a) and Qu et al. (2009) are -35 mV and -57.93.9mV, respectively at pH=5.8. The latter authors had addeda very small amount of NaCl (i.e., 0.05 mol m 3) to the dis-

tilled water. Hasegawa et al. (2009) have generated ozone

microbubbles using two microbubble generators with

/3 and /6 radian slit angles. They observed very small

variations in the value of potential (i.e., -43 and -38 mV,respectively) for these two slit angles.

The potential does not significantly vary with bubblediameter (Takahashi 2005a, Hasegawa et al. 2009), which

is expected from the theory of electrostatic double layer.

However, when microbubbles shrink in water, the tran-

sient value of potential shows a significant variationwith time and, hence, with bubble diameter (Takahashi

2010). When the bubble shrinks, the ions at the air-

water interface are concentrated into a narrower region.

Therefore, they tend to diffuse into the bulk water from

the interface. However, the rate of diffusion of the ions is

slower than the rate of shrinkage of the bubble. Thus, the

ions cannot escape fast enough and become concentrated

near the interface, which is manifested by the increase in potential. It has been reported that the concentrated ions

at the bubble boundary prevent escape of the internal gas

and stabilize nanobubbles (Takahashi 2009, Tsuge 2010).When the microbubble collapses, an ionic field of very

high ion concentration is created that helps the forma-

tion of free radicals, which are the main reactive species

in the oxidative treatment of wastewater (Takahashi et al.

2007a). When an external stimulus, such as an ultrasonic

wave, is applied to water, microbubbles form by acoustic

cavitation. As the pressure inside the bubble is inversely

proportional to the bubble diameter, rapid shrinking

(pressure collapse) leads to a sharp increase in pressure.

When the rate of such pressure increase is sufficiently

40

0

-40

-80

Zetapotential(mV)

-120

2 7

pH

Experimental data

12

Figure 5 Variation ofpotentials of microbubbles with pH(Takahashi 2005a).

high, the temperature within the microbubble also rises

sharply because of adiabatic compression. As a result,

at the time of microbubble collapse, an area with a pres-

sure of a few thousand atmospheres and a temperature

of a few thousand degrees is created. This extreme reac-

tion site (termed hot spot) is a very small area but has

enough potential to force the decomposition of gas mole-cules in the microbubble, which creates the free radicals

(Takahashi 2010).

The potential strongly depends on the pH of themedium. Takahashi (2005a) has reported that the potential of microbubbles in distilled water is negative

above pH=4.5. In highly alkaline solutions (pH>10), the potential exceeds -100 mV. However, below pH=4.5,the potential is positive. These results are shown inFigure 5. The charge on the gas-water interface is devel-

oped due to the adsorption of OH and H+ ions. The

counterions are attracted towards the interface andform an electrostatic double layer. At high pH, where the

solution is alkaline, the OH ions adsorb at the air-water

interface rendering the microbubble negatively charged.

In acidic solution, there is an excess of the H+ ions over

the OH ions at the interface. The presence of inorganic

salts (e.g., NaCl and MgCl2) makes the potential less

negative, depending on their concentration (Takahashi

2005a, Srinivas and Ghosh 2011).

The potential of a microbubble significantly dependson the presence of surface active compounds (e.g., sur-

factants and alcohols) in water. These compounds have

a high concentration at the air-water interface even when

they are present in small quantities. Therefore, they easily

displace the OH and H+ ions from the interface, because

the surface activity of these ions is much smaller. The




9/32


cationic surfactants present in water render the poten-tial positive, whereas the anionic surfactants give rise to

negative potential (Yoon and Yordan 1986). Aliphaticalcohols, when present in small amounts, do not signifi-

cantly alter the potential of microbubbles (Elmahdyet al. 2008). Wastewater contains various surface active

compounds released from animals and plants. Therefore,the electric charge of microbubbles in industrial wastewa-

ter can vary widely.

It has been reported that free radicals are also gener-

ated from collapse of air, oxygen and ozone microbubbles

(Takahashi et al. 2007b). The shrinking rate of collapsing

microbubbles is much slower than that of ultrasound-

induced cavitation. Therefore, the shrinking speed of

collapsing microbubbles is not sufficient for generating

adiabatic compression. There is lack of sufficient data for

a detailed explanation of the appropriate mechanism of

generation of hydroxyl radicals from collapsing micro-bubbles. However, from many experiments, for example,

electron spin resonance spectrum and poly vinyl alcohol

(PVA) decomposition, Takahashi et al. (2007b) have given

a probable theory of generation of free radicals from col-

lapsing microbubbles. They have observed that there is

an increase in -potential with shrinkage of microbub-bles under highly acidic conditions. The low pH caused

a change in the -potential of microbubbles, which accel-erated the collapse speed due to the reduction in electro-

static repulsion between opposite sides of the bubble wall.

If the speed of collapse is high enough, due to adiabatic

compression, a hot spot will cause the generation of high

temperature and thus the free radical may be generated.

Takahashi et al. (2007b) observed that the acoustic cavita-

tion had a negligible effect on the decomposition of PVA,

whereas ozone microbubbles effectively decomposed

PVA. It implies that high temperature is not responsible

for increased generation of OH radicals by ozone micro-

bubbles under strongly acidic conditions. Therefore, they

concluded that during the collapse of the microbubble,

some excess ions become trapped at the air-water inter-

face. This high ion concentration at the surface of micro-

bubbles increases -potential. This extreme accumulationof ions at the site of the collapsed microbubbles is suffi-

cient for the generation of free radicals from air, oxygen or

ozone microbubbles.

3 Microbubble generators (MBGs)

Microbubbles have been generated by a wide variety

of methods (Sebba 1985, Michelsen and Sebba 1994,

Ohnari et al. 1999, Ohnari 2000, 2002, Sadatomi 2003,

Matsuyama et al. 2006, Sadatomi et al. 2007, Shakouchi

et al. 2007, Takahashi 2009, Ikeura et al. 2011, Terasaka

et al. 2011). The method of generation of microbubbles

has an effect on their properties and consequently on

the effectiveness of wastewater treatment. To illus-

trate, generation of fine MNBs of high number densityis required in wastewater treatment. Basically, there are

two types of MBGs, that is, gas-water circulation and

pressurization-followed-by-decompression. In the gas-

water circulation type of MBG, the gas is introduced into

the water vortex, and the gas bubbles thus formed are

converted into microbubbles by breaking the vortex. In

the pressurization-decompression type of MBG, a suffi-

cient amount of gas is dissolved in water under moder-

ately high pressure to form a supersaturated solution.

The solution is unstable and the gas escapes from water

generating a large number of microbubbles.The spiral-liquid-flow MBG (Ohnari et al. 1999) is

shown in Figure 6A. The pumped water is tangentially

introduced from a side hole into a cylinder. The spiral

flow of liquid forms a maelstrom-like cavity in the cylin-

der (Terasaka et al. 2011). The gas is sucked from an orifice

on the bottom and then spouts out with the liquid from

a hole situated at the top of the cylinder. The microbub-

bles are generated by the centrifugal force imparted by

the high-speed rotating liquid. The rotational speed lies

in the range of 18853770 rad s1. The gas flow rate/liquid

flow rate ratio is in the range of 1/7 to 1/15. One of the early

MBGs of this category, M2-LM, has been used for oyster

culture in Japan. The details of this generator have been

described in the work of Li (2006). By supplying the micro-

bubbles to the seawater around the oysters, their growth

rate was significantly increased. The range of bubble

diameter obtained from the M2-LM generator was 1050

m. However, the concentration of the microbubbles waslower than the pressurization-decompression type of gen-

erators. Li and Tsuge (2006a) connected this MBG to the

outlet of a centrifugal pump that can stably operate even

when the gas content is 710%. Because of the whirlpool

mixing effect and increased pressure, the air was pres-surized and dissolved in water. The air that could not dis-

solve was converted into microbubbles in the MBG. As the

microbubbles were formed in a spiral pattern, coalescence

of the bubbles after their formation could be prevented,

and the concentration of the bubbles increased.

In the venturi-type MBG (Figure 6B), a liquid stream

containing macrobubbles flows from the inlet of a venturi

tube. The two-phase flow is accelerated through the throat

of the venturi tube. The pressure changes rapidly and the

microbubbles are formed by reducing the macrobubbles




10/32


and/or by cavitation. The ejector-type MBG is shown in

Figure 6C. The liquid flow channels in the cylindrical gen-

erator are designed to shrink and stepwise enlarge. The

gas is self-sucked from the most reduced pressure point

and reduced to a number of microbubbles by cavitation.

In the MBG developed by Sadatomi et al. (2005), pressur-

ized water is introduced into a pipe with a spherical body

in the core. The water velocity around the body, especially

in a downstream region, becomes higher than the inlet

velocity, and thus the pressure there becomes lower. If thepressure becomes less than the atmospheric pressure, air

is automatically sucked into the water stream through a

number of small holes drilled on the pipe wall in the lower

pressure region. Because the water flow there is highly

turbulent, and as a result of the shear flow the air sucked

is broken into a large number of microbubbles.

A MBG method similar to those described above

is hydrodynamic cavitation (Senthilkumar and Pandit

1999, Gogate and Pandit 2000a, 2004a, Senthilkumar

et al. 2000, Sivakumar and Pandit 2002, Ambulgekar et

al. 2005, Wang et al. 2008, Wang and Zhang 2009, Gogate

2011, Saharan et al. 2012). With this method, cavitation is

generated by the flow of liquid through a simple geometry

(e.g., a venturi tube or an orifice plate) under controlled

conditions. When the pressure at the throat (vena con-

tracta) falls below the vapor pressure of the liquid, the

liquid flashes, generating a number of cavities. These

cavities subsequently collapse when the pressure recovers

downstream of the mechanical constriction. The collapse

of the cavitation bubbles initiates some physicochemicaleffects (e.g., production of shock waves, shear forces and

chemical reactions), resulting in the intensification of the

dispersion processes. Free radicals are generated by these

processes, which are utilized in the treatment of waste-

water. Several studies (Ambulgekar et al. 2005, Wang

et al. 2008, Wang and Zhang 2009, Saharan et al. 2012)

have reported the decomposition of pesticides and dyes

by using the hydrodynamic cavitation method.

The pressurization-decompression type of MBG is

shown in Figure 6D. The gas is dissolved in the liquid in

Microbubbles

Microbubbles

Throat

Gas water

Gas

Gas

PumpVent

Pressurized section

Decompressed section

Liquid

Liquid

Pump

AB

C

D

Spiral

liquid flow

Gas pillar

Gas

Microbubbles

Figure 6 Microbubble generators: (A) spiral liquid flow, (B) venturi, (C) ejector, and (D) pressurization-decompression (adapted fromTerasaka et al. 2011, with permission from the copyright holder, Elsevier Publishing).




11/32


a tank by pressurizing the gas-liquid mixture. When this

supersaturated liquid is flashed using a reducing valve,

microbubbles are generated. The size and number of the

microbubbles depend on the pressure and decompression

process. Terasaka et al. (2011) have made a comparative

study on the performance of the four types of MBGs shown

in Figure 6, in terms of the gas hold-up and the mass trans-fer of gas into the liquid phase. They reported that the spi-

ral-liquid-flow and pressurization-decompression types

of MBGs showed high gas hold-up. The former type also

showed high mass transfer coefficient of oxygen. MBGs

are much more efficient for transferring gas into the liquid

phase than the conventional gas distributors (e.g., perfo-

rated plate and constant-flow nozzle). However, the power

requirement of MBGs is higher because of the requirement

of the pump, which is not required by conventional gas

distributors. Ikeura et al. (2011) have reported that the

pressurization-decompression type of MBG is more effec-tive for the decomposition of fenitrothion (a pesticide)

by ozone. The main factor that contributed to this differ-

ence in performance was the smaller size of microbubbles

generated by this type of MBG, which could come into

contact with the pesticide (which had infiltrated into vege-

tables) more easily. The variation of the fluid flow pattern

from one MBG to another can lead to the difference in

microbubble size, broadness of the size distribution, and

also the shape of the microbubble (Nouri et al. 2008).

Some researchers have reported that electrical properties

of microbubbles can vary with the method of their genera-

tion (Hasegawa et al. 2008), which can have an important

influence on the treatment of wastewater.

Maeda et al. (2010) have studied the effect of liquid

volumetric flux and flow pattern in the decompression

nozzle on the mean diameter and number density of

microbubbles generated by a pressurization-decompres-

sion MBG. They observed that very few microbubbles were

generated when there was no cavitation. When the bubble

cavitation took place, the mean bubble diameter was low,

which did not depend on the liquid flux, and the bubble

number density increased with the flux. For sheet cavita-

tion, the mean bubble diameter increased with the liquidflux, whereas the number density remained invariant.

Bredwell and Worden (1998) have used a spinning-

disk MBG employing a high-speed electric motor. A disk

of 5 cm diameter and 1 cm thickness was spun at speeds

above 419 rad s1. Stationary baffles located within 5 mm of

the spinning disk created a localized high-shear zone. The

stainless steel disk and baffles were mounted in a fermen-

tation vessel of 6 dm3capacity. A nonionic surfactant (i.e.,

Tween 20) was used to stabilize the microbubbles. The

surfactant molecules adsorbed at the air-water interface

formed a monolayer. This layer played an important role

in the stability of the microbubbles (DArrigo 1984).

Several other methods have also been employed to

generate microbubbles. For example, porous glass mem-

branes have been used for the generation of microbubbles

(Fujikawa et al. 2003, Kukizaki et al. 2004, Kukizaki 2006,

Kukizaki and Goto 2006, 2007, Kukizaki 2009, Trushinet al. 2011). Kukizaki et al. (2004) have used a sintered

porous glass membrane with a mean pore size of 84 nm

to produce MNBs of 720 nm mean diameter. They added

a surfactant to prevent coalescence of the bubbles. They

observed that the mean diameter of the bubbles was pro-

portional to the mean pore diameter of the membrane.

The type of surfactant was found to be important in the

size of the microbubbles (Kukizaki and Baba 2008). Mitani

et al. (2005) have used a microporous diffuser system that

produced sub-micron ozone MNBs. The diffuser extended

concentrically throughout the length of a cylindricalreactor in order to increase the contact time between dis-

solved ozone and the pollutants present in water, as well

as to increase the interfacial area between the gas and

the liquid. Coalescence occurred in the top portion of the

reactor. Fine bubbles appeared throughout the length of

the reactor, thus creating a very large gas-liquid interfa-

cial area, which enhanced the mass transfer rate of ozone.

Zhang et al. (2001) have used a rotating porous plate at

the air intake unit of the device. Compressed air was taken

into the device through the rotating porous plate. The size

of the bubbles decreased with increasing rotational speed,

which increased the dissolution of air into the water.

Microbubbles of uniform diameter have been pro-

duced by exposing the bubbles formed by a needle-like

narrow tube to ultrasonic waves (Tsuge 2010). Micro-

bubbles were produced in a high-viscous liquid (e.g.,

silicone oil), and had a diameter of 10 m. Microbub-bles have also been produced by condensation of mixed

vapors (Terasaka et al. 2009). The advantage of this

method is that the power requirement is less, compared

with mechanical MBGs. Nitrogen microbubbles (with the

peak of the diameter distribution in the range of 2040

m) have been generated by blowing a mixture of nitro-gen and steam into water through a fine nozzle (Tsuge

2010). The steam condensed, but the nitrogen did not

condense. The size of the microbubbles changed depend-

ing on the gas composition, inside diameter of the nozzle

and vapor velocity. Tsuge et al. (2008) have performed

electrolysis of water and demonstrated the effect of stir-

ring (at low frequency) on the formation of microbubbles

of hydrogen and oxygen from the electrodes. The range of

diameter of the microbubbles was 15100 m. Shin et al.(1997, 1999) have generated microbubbles by electrostatic




12/32


spraying. They observed that the bubble size decreased

with decreasing Reynolds number and increasing Weber

number. Three different modes of bubble formation were

observed: a spraying mode at low airflow rate and high

applied voltage, a dripping mode at high flow rate and low

applied voltage, and a mixed spraying-dripping mode.

Walker et al. (2001) have reported that the electrostaticspraying method is suitable for generating microbubbles

in low-conductivity solutions, but not suitable at high-salt

concentrations due to the high electric current. Small-

pore gas diffusers are more suitable in these solutions.

The acoustic cavitation method is another technique

that generates MNBs. Although widely used in various

laboratory experiments, this method has hardly been

used in large-scale water treatment, due to high cost

and expertise required in diverse areas such as chemical

engineering, materials science and acoustics (Gogate and

Pandit 2000a). With this method, cavities are producedby passing sound waves, usually ultrasound (>16 kHz),through the liquid medium. The passage of ultrasound

through the aqueous medium generates cavities, pro-

motes their growth and their collapse. The entire process

of cavity generation, growth and collapse occurs over

a period of microseconds (Gogate and Pandit 2000b).

During the collapse of the bubble, a high pressure (e.g.,

several hundred times that of the atmospheric pressure)

and high temperature (e.g., several thousand Kelvin) are

generated (Leong et al. 2011). These extreme reaction

sites force decomposition of the gas molecules in the

bubble, which generates free radicals (e.g., OH). It has

been reported in the literature that ultrasound sonication

generates microbubbles that are smaller in diameter than

those generated mechanically, and the size distribution is

narrower. Several studies have reported the use of acoustic

methods for purification of wastewater (Dahl 1976, Suslick

et al. 1986, Teo et al. 2001, Sivakumar and Pandit 2002,

Sivakumar et al., 2002, Goel et al. 2004, Gogate and Pandit

2004a,b, Adewuji 2005, Sivasankar and Moholkar 2009,

Laxmi et al. 2010). In-depth reviews of wastewater treat-

ment by acoustic methods have been presented in these

studies. In this review article, details of the ultrasonicmethods of wastewater treatment have not been covered.

Riverforest Corporation (USA) has manufactured

different types of MBGs ranging from small-, medium-

and large-scale applications. The MBG model ASMK-III

is a pressurized system which can be used for lab-scale

study (microbubble output=6.7104m 3s 1). Other models,SMX-115 and SMX-155, are medium capacity MBGs with

microbubble flow rates of 8.3104m 3s 1and 0.001 m 3s 1,respectively. The swirling type Tornado MB nozzle is ope-

rated at a minimum pressure of 0.04 MPa and flow rates

in the range of 5104m 3 s 1and 0.001 m 3 s 1, which areused efficiently for small-scale applications. Different

types of MBGs with nozzles at high air-water ratio are also

available for large-scale applications. For special uses of

ozone, oxygen, CO2and hydrogen microbubbles, the MB

nozzle with the MB400 pressure chamber may be suit-

able to produce very thick and uniform microbubbles.Negatron Co. Ltd. (Korea) also manufactures many MBGs

for various applications with a wide range of micro-

bubble size (0.01


13/32


Eq. (12) shows that the mass transfer coefficient

increases with decreasing size of the microbubble. Waste-

water often contains surface active impurities. These

compounds quickly adsorb on the surface of the micro-

bubbles and immobilize the surface. The mass transferrate is reduced by this process. Apart from the reduction

in surface flow, surfactant molecules act as a physical

barrier for the gas molecules to pass through the inter-

face (Koide et al. 1976). In addition, the surfactant con-

centration may alter the thickness of the liquid shell of

the microbubbles, which reduces mass transfer rate. The

Frssling equation is valid for solid microbubbles (i.e.,

microbubbles whose surface is immobile and behave like

solid particles) (Motarjemi and Jameson 1978):

1 31 2

0.6l l

l

k d du

D D

= (13)

For circulating microbubbles, the Higbie equation is

applicable (Motarjemi and Jameson 1978):

1 2

2ld

Dk

t

=

(14)

The mass transfer coefficients predicted by Eqs. (13)

and (14) differ considerably. Kawahara et al. (2009) have

presented a comparison of kl obtained by these two equa-

tions. Experimental data on oxygen transfer in tap waterand saline water show an increase in the mass trans-

fer coefficient with increasing bubble diameter, which

is opposite to that predicted by these equations. They

attributed this to the bubble-induced turbulence, which

is proportional to the bubble diameter (Sato et al. 1981).

Kawahara et al. (2009) have presented modifications

of Eqs. (13) and (14). They have reported that a unique

relationship exists between kl

and the product of Sauter

mean diameter (d32

) and bubble rising velocity (u), which

is given by:

0.756

-10 327.46 10lk ud

D

=

(15)

The following equation, given by Calderbank and

Moo-Young (1961), is applicable to the bubbles, which

have a diameter


14/32


mass transfer coefficient (kl) and interfacial area per unit

volume (a) is used. This product, kla, is termed volumet-

ric mass transfer coefficient. The interfacial area per unit

volume is related to the fractional gas hold-up (g) and

Sauter mean diameter (d32

) as (Kawahara et al. 2009):

32

6 gad

=(19)

The quantities, klaand c*, are sometimes called clean

water parameters. If the initial concentration of the gas in

water (i.e., at t=0) is c0, then integration of Eq. (18) gives

the variation of concentration of the gas in the aqueous

phase:

*

*0

-ln -

-l

c ck at

c c

=

(20)

The volumetric mass transfer coefficient, kla, can be

determined from the slope of the plot of*

*0

-ln

-

c c

c c

vs.

time (Ago et al. 2005), or by a non-linear regression analy-

sis, as described by ASCE (2007). Several correlations for

the computation of klahave been reported in the literature

(Akita and Yoshida 1973, Vant Riet 1979, Koide et al. 1983,

Bredwell and Worden 1998, Nedeltchev et al. 2006a,b,

Kawahara et al. 2009). However, only a few studies have

reported the applicability of these correlations for predict-

ing the volumetric mass transfer coefficient in microbub-

ble systems. The available experimental data indicate

that these correlations are not accurate for the predictive

purpose.

The transfer of gas is expressed as the volumetric

transfer rate (VGTR) of gas (Chu et al. 2008a):

VGTR=kla(c*-c) (21)

Experimental data reported in the literature on the

microbubble systems indicate that high values of volu-

metric mass transfer coefficient can be obtained even

in the (mechanically) unagitated systems. For example,

Bredwell and Worden (1998) have reported klavalues foroxygen microbubbles (which had an average initial dia-

meter of 60 m) in the range of 0.060.5 s1. These valuesare considerably higher than those for mechanically

agitated, commercial-scale fermentors. They have demon-

strated the enhancement of mass transfer using micro-

bubbles for synthesis-gas fermentation. Butyribacterium

methylotrophicumwas grown in a continuous, stirred-tank

reactor. The kla value for microbubble sparging was six

times larger than the conventional gas sparging through

a 10-m stainless steel frit, which produced 0.51 mm

0.009

0.007

0.005

kla(

s-1)

0.003

0.001 0.001

0.004

0.007

VO

TR(kgm-3

s-1)

Air flow rate (dm3s-1)

0.013

0.010

0 0.005 0.010 0.015 0.020 0.0300.025

Figure 8 Variation ofklaand volumetric oxygen transfer rate (VOTR)

with air flow rate (Chu et al. 2008a).

diameter bubbles. The gas flow rate for the microbubbles

was only approximately one-half of that for conventional

sparging. Thus, the large specific interfacial area for the

microbubbles resulted in high values of kla. Similar results

have been reported by Kaster et al. (1990). Ago et al. (2005)

have measured klafor carbon dioxide microbubbles. They

observed an increase in the volumetric mass transfer coef-ficient by several times compared with the conventional

bubbling technique. Chu et al. (2007a) have studied mass

transfer of ozone in water by microbubbles. Compared

with an ordinary bubble contactor, the mass transfer effi-

ciency in the microbubble system was 1.62.7 times higher.

Increase in the flow of gas led to the increase in the volu-

metric mass transfer coefficient and the gas transfer rate,

as illustrated in Figure 8 (Chu et al. 2008a).

Nakano et al. (2005) have compared the volumetric

mass transfer coefficients of oxygen in water by the micro-

bubble and air-stone dissolution methods. The MBG pro-duced a much higher k

lathan the air-stone method. As the

gas flow rate was increased, the volumetric mass transfer

coefficient increased considerably. Li and Tsuge (2006b)

have reported that the volumetric mass transfer coefficient

for ozone in water increased with the increasing induced-

gas and water flow rates. Yasuda et al. (2010) have studied

mass transfer of ozone in water in an airlift bubble column

reactor with a draft tube. They observed that klaincreased

with increasing diameter of the draft tube.

5 Ozonation using microbubbles

Oxidation processes constitute a major step in the treat-

ment of wastewater. Ozone is applied to remove the




15/32


organic and inorganic compounds present in the waste-

water. Despite several advantages of using ozone, it has

a few disadvantages, which limit its application in water

treatment. The main drawbacks of ozone are its relatively

low solubility and stability in water. In addition, other

factors that have limited the use of ozone are the high cost

of production and only a partial oxidation of the organiccompounds present in water. The partial oxidation can

lead to the generation of carcinogenic products. However,

often, partial oxidation of the organic compounds leads to

biodegradable products (e.g., carboxylic acids, aldehydes

and ketones) (Glaze and Weinberg 1993), which can be

removed by adsorption on activated carbon. The reaction

of ozone with the pollutants in water is rather slow (Taki

et al. 2008), and the overall reaction rate can be affected

by both the reaction kinetics and mass transfer (Zhou and

Smith 2000). Several advanced ozonation and catalytic

ozonation processes have been attempted to render theuse of ozone commercially viable (Gunten 2003, Kasprzyk-

Hordern et al. 2003, Ikehata et al. 2008). The mechanism

of decomposition of ozone in water is presented in Table 1

(Kasprzyk-Hordern et al. 2003, Beltrn 2004).

To alleviate some of the drawbacks of conventional

ozonation processes mentioned above, microbubble-aided

ozonation has been successfully attempted by several sci-

entists. Many organic and inorganic compounds react with

ozone or hydroxyl radicals directly or indirectly (Gunten

2003). Catalysts are also used to enhance the free radical

generation from ozone. Free hydroxyl radicals are more

powerful than molecular ozone for oxidation. In addi-

tion, ozone microbubbles are also capable of generating

hydroxyl radicals under certain conditions as discussed in

Section 2.2. The significant increase in ion concentration

Reaction Rate constant

Initiation

O+OHHO

+O

(dmmols)

Propagation

HOO

+H+ .s

O

+H+HO

(dmmols) O

+O

O

+O

.(dmmols)

O+H+HO

.(dmmols)

HOO

+H+ .s

HOHO+O

.s

O+HOHO

4(dmmols)

HO4HO

+O

.s

Termination

HO4+HO

4H

O

+2O

(dmmols)

HO4+HO

3H

O

+O

+O

(dmmols)

Table 1 Mechanism of decomposition of ozone in pure water

(Kasprzyk-Hordern et al. 2003, Beltrn 2004).

around the shrinking gas-water interface helps generation

of free radicals (Takahashi et al. 2007a). In fact, in recent

times, a major application of microbubbles in wastewater

treatment involves ozonation. This involves decoloriza-

tion (e.g., removal of dyestuff), degradation of pesticides

and other harmful organic compounds, and removal of

odor (e.g., residual ammonia) from water/wastewater.Some of the applications of ozone microbubbles are speci-

fied in Table 2.

Air, oxygen and nitrogen microbubbles are also used

for decomposition of various compounds. Some ozone

resistant compounds, for example, PVA, dimethyl sul-

foxide (DMSO), can be decomposed using hydroxyl radi-

cals. Ozone microbubbles under strong acidic conditions

generate hydroxyl radicals, which can be effectively used

for decomposition of PVA and DMSO (Li et al. 2009c).

Takahashi et al. (2007a) have reported that air microbub-

bles also generate hydroxyl radicals. Li et al. (2009c) haveused air and ozone microbubbles for the study of decom-

position of DMSO. They concluded that the amount of free

radicals generated from air microbubbles is less than that

of ozone microbubbles. Li et al. (2009b) have reported

that phenol degradation can be effectively done by free

radicals generated from air, oxygen and nitrogen micro-

bubbles. They observed that for 2 h duration of phenol-

decomposition, the rate of decomposition by free radicals

generated from nitrogen, air and oxygen microbubbles

was increased by 36%, 59% and 83%, respectively.

5.1 Removal of color

Wastewaters released from the textile and dye manu-

facturing industries are heavily colored, a major part of

which is non-biodegradable in nature. Ozonation has

been used by various researchers for the removal of color

(Shu and Huang 1995, Wang et al. 2003). Microbubbles

enhance the efficiency of ozonation. Chu et al. (2007b)

have ozonated aqueous solutions of CI Reactive Black 5

(an azo dye of molecular formula C26

H21

N5O

19S

64Na) using

microbubbles. The average diameter of the microbubbleswas 58 m. They have compared the performance of themicrobubble system with a conventional bubble contac-

tor. The reaction rate was much higher in the microbubble

system. They obtained good efficiency of removal of color

and observed that all color was removed within 1.8 ks. A

test using terephthalic acid as the chemical probe implied

that more hydroxyl radicals were produced in the micro-

bubble system, which contributed to the degradation of

dye molecules. Some of the results from their study are

presented in Figure 9.




16/32


Categoryofimpurity

Type

ofeffluent

Impuritiesforrem

oval

Methodofmicrobubble

generation

Sizeof

microbubble(m)

Removal(%)

Time(ks)

References

So

lubleorgan

ics

Petro

leum

in

dustr

ial

wastewater

BTEX

Electrostaticspray

ing

.

Wa

lkereta

l.(

)

Chlorinate

dorgan

iccompoun

d

Un

de

rgroun

dwater

TCE

Airs

hearing

MBG

.

Na

kanoeta

l.(

)

Co

lor

Texti

lewastewater

COD

Sp

ira

lliqu

idflow

MBG

[O

]=.. g m, O

microbubble flow

rate=. cmsKobayashi

et al. ()

Pectobacterium

carotovorum subsp.

carotovorum

O . >

Table 3 Use of microbubbles in water disinfection.




20/32


microorganisms. Gas-liquid dispersions, such as foams,

have been used to increase the biodegradation of hydro-

carbons (Ripley et al. 2002). Air microbubbles are often

used to supply oxygen in aerobic biodegradation pro-

cesses. Microbubbles can deliver oxygen to rather inac-

cessible regions and are more efficient than conventional

millibubbles (Kutty et al. 2010). Fresh microbubblesreplace the oxygen-depleted bubbles and biodegrada-

tion continues effectively. Choi et al. (2009) and Park

et al. (2009) have employed a saponin-based microbubble

suspension to enhance aerobic biodegradation of phen-

anthrene. As microbubble suspension flowed through the

sand column, oxygen was delivered to the less permeable

regions. Burkholderia cepacia RPH1, a phenanthrene-

degrading bacterium, was transported in a suspended

form in the microbubble suspension. The bacterial cells

partially attached to the gas-water interface. A significant

increase in biodegradation efficiency was obtained byintroducing the microbubbles.

Some studies have been reported in the literature on

the enhancement in aerobic biodegradation of phenol

(Michelsen et al. 1984), p-xylene (Jenkins et al. 1993),

pentachlorophenol (Mulligan and Eftekhari 2003) and

TCE (Rothmel et al. 1998) using microbubbles. Michelsen

et al. (1988) have developed a microbubble-based biode-

gradation system for treating hazardous wastes. They have

built a test cell for in situbiodegradation of flowing ground

water by the air microbubbles in 5065% dispersion. The

microbubbles were stabilized by surfactants. Microbubble

dispersion proved to be superior to the sparged air-water

injections. Approximately 25% of oxygen present in the

microbubbles increased dissolved oxygen, as anaerobic

groundwater flowed through the treatment zone. Approxi-mately one-third of this oxygen was required to biodegrade

the surfactant used to stabilize the microbubbles. The use

of surfactant in preparing and stabilizing the microbub-

bles has a side effect that the surfactant itself can pollute

water. The modern commercial microbubble generators,

e.g., the ASMK-III of the Riverforest Corporation, do not

require a surfactant for generating microbubbles. Some of

the applications of air and oxygen microbubbles in water

disinfection are listed in Table 4.

SHARP Corporation (Japan) has developed a non-

dilution technology to remove 90% of nitrogen containedin wastewater discharged from its semiconductor facilities

(SHARP Corporation 2006). Their process involves the use

of MNBs of air (or oxygen-enriched air) to activate micro-

organisms. An improved microorganism culture increased

their concentration. The process developed by them is

schematically shown in Figure 12. By using activated

microorganisms, they have treated nitrogen contained in

wastewater without dilution.

Compound Microbe type Microbubble

type

Contact

time (ks)

Removal

(%)

References

Biodegradation p-Xylene Pseudomonas putida O . Jenkins et al. ()

Trichloroethylene ENV bacteria Air . Rothmel et al. ()

n-Hexadecane Acinetobacter junii O Ripley et al. ()

Pentachlorophenol Rhamnolipid Air Mulligan and Eftekhari ()

Phenanthrene Burkholderia cepacia RPH O Choi et al. ()

Municipal water

treatment

TOC Air Yamashita et al. ()

SS Air

TN Air .

TP Air

DOC Air .

Sulfate Desulfotomaculum nigrificans Air .

Nitrification TN Air Winarto et al. ()

Coagulation-

flotation

COD Air a Liu et al. (a,b)

Color Air a

Nitrate Air a

Ammonia Air a

Table 4 Use of air/oxygen microbubbles in water treatment.aRemoval efficiencies (in %) for the coagulation-microbubble-floatation process are higher than those of the coagulation-sedimentation

process.

COD, chemical oxygen demand; DOC, dissolved organic carbon; TN, total nitrogen; TOC, total organic carbon; TP, total phosphorous;

SS, suspended solid.




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Tasaki et al. (2009a) studied the degradation of methyl

orange using oxygen microbubbles (d=5.8 m). They per-formed photodegradation experiments with a BLB black

light blue lamp (365 nm), a UV-C germicidal lamp (254 nm)

and an ozone lamp (185+254 nm) both with and without

microbubbles. The oxygen microbubbles accelerated the

decolorization rate of methyl orange under 185+254 nm

irradiation. By contrast, the microbubbles under 365 and

254 nm irradiation were ineffective for decolorization.

They found that the pseudo-zero order decolorization

reaction constant in the microbubble system was almost

twice that in the conventional millibubble system. Total

organic carbon (TOC) reduction rate of methyl orange was

greatly enhanced by microbubbles under 185+254 nm irra-

diation. However, TOC reduction rate by nitrogen micro-

bubbles was much slower than that with 185+254 nm irra-

diation only.

Takahashi et al. (2007a) have investigated the decom-

position of phenol in aqueous solution using air micro-

bubbles, without using any dynamic stimulus such as

UV irradiation or ultrasonic wave. They added a small

amount of nitric acid to generate the hydroxyl radical

from collapsing microbubbles. Under acidic conditions,approximately 30% of phenol was decomposed in 10.8 ks.

The intermediate products of phenol decomposition, that

is, hydroquinone, benzoquinone, and formic and oxalic

acids, were detected. They have reported that hydro-

chloric and sulfuric acids were also effective in generating

hydroxyl radicals.

Li et al. (2009b) have reported a strong effect of pH on

phenol degradation. Using air microbubbles, 59% phenol

degraded after 7.2 ks at pH=2.3, whereas only 3% phenoldegraded at pH=4.5, after the same amount of time. The

Before treatment

1. Microorganisms activated

2. Microorganisms concentrated

Optimize the quality

and conditions of

microorganism culture~5 g dm-3

10 g dm-3

MNB

MNB

After microorganisms are

increased and activated

Microorganisms

Ammonia, etc.

Figure 12 Activation and increase in microorganism population by using air micro-nanobubbles for treatment of wastewater

(adapted from SHARP Corporation Japan 2006, with permission from the copyright holder, SHARP Corporation).

1.2

1.0

0.8

0.6

0.4

Phenolconcentr

ation(molm-3)

0.2

0 0

5

10

15

TOCremoval

efficiency(%)

20

25

0 2000 4000

Time (s)

6000

Phenol concentration profile

TOC removal efficiency profile

8000

Figure 13 Variation of phenol concentration and TOC removal

efficiency with time (Li et al. 2009b).

concentration profile of phenol and total organic carbon

removal efficiency are shown in Figure 13. With oxygen

microbubbles, phenol degradation efficiency was much

higher and a similar effect of pH was also observed with

oxygen microbubbles. With nitrogen microbubbles,

degradation efficiency was 36% only, at pH=2.3.Petroleum-based surfactants (e.g., alkylbenzene

sulfonates) are widely used as industrial detergents, emul-

sifiers and dispersing agents. Consequently, it is common to

find high concentrations of these surfactants in municipal

and industrial wastewaters, particularly from the washing

processes. Tasaki et al. (2009b) have employed a vacuum

UV method using MNBs (average diameter=720 nm)for the degradation of these surfactants. Sodium dodecyl

benzene sulfonates (SDBS) were used as the model




22/32


surfactant. Degradation experiments were conducted with

an ozone lamp, both with and without the MNBs. Their

results show that the oxidation rate of SDBS was signifi-

cantly enhanced by the oxygen MNBs. After 1 day of opera-

tion, 99.8% of SDBS was oxidized and 76.8% total organic

compound was removed in the integrated nanobubbles/

vacuum ultraviolet (VUV) system. The MNBs were foundto be more effective than the microbubbles (of 75.8 maverage diameter), due to the larger gas-liquid interfacial

area of the former system.

The effectiveness of air microbubbles in applications

such as decolorization can be enhanced by the use of UV

irradiation. The decolorization of methylene blue solution

has been studied by Shibata et al. (2011). UV irradiation

improved decolorization at all pH values of the solution.

The effect of UV irradiation on decolorization was most

intensive for the neutral solution due to the enhanced

radical generation through photochemical reaction.A high decomposition performance was obtained with the

venturi-type MBG in the presence of UV irradiation.

6.2 Removal of fine particles fromwastewater

Wastewater often contains a significant amount of finely

dispersed solid particles. Removal of these particles

can be effectively done by microbubble flotation. The

microbubble-based flotation method has been practiced

in the processing of fine minerals (Yoon et al. 1992, Yoon

1993). Air flotation is frequently used in water treatment

to remove algae from nutrient-rich water, or to treat water

of low turbidity at low temperature. Two types of air flota-

tion methods are used, that is, dissolved air flotation (DAF)

and induced air flotation (IAF). The DAF method is fairly

common for removing pollutants such as colloidal matters,

fine and ultrafine particles, precipitates, ions, microorgan-

isms, proteins, and dispersed and emulsified oils present

in water. Reviews on air flotation methods have been pre-

sented in the literature (Edzwald 1995, Rubio et al. 2002,

Rodrigues and Rubio 2007). Details of commercially avail-able microbubble-based flotation technologies for waste-

water treatment (e.g., those provided by Sionex, WesTech,

Aeromax Systems and Purac Engineering) have been pre-

sented by Rubio et al. (2002).

In the DAF method, wastewater is introduced into the

water treatment unit. The fine particles in wastewater are

flocculated in the pretreatment process. There are two

steps in the pretreatment process, that is, particle desta-

bilization and particle flocculation. Particle destabili-

zation takes place by adding a chemical coagulant in a

flash mixer, where the coagulant is dispersed uniformly

and quickly by a high-speed stirrer. The particle floccu-

lation and growth of aggregates occur during the slow

mixing stage in the flocculator. A part of the purified water

is recycled and saturated up to 80% with air at an elevated

pressure. The pressurized water is then decompressed via

injection nozzles in the flotation cell. Microbubbles, typi-cally of the size range of 4080 m, are formed in thisprocess. The bubbles readily adhere to the surface of the

floc particles forming bubble-particle agglomerates, and

rise to the liquid surface where a layer of sludge is formed.

The use of fine microbubbles improves the bubble-

particle collision efficiency, which is one of the most

important factors in the removal of particles. The amount

of air dissolved in water at a given pressure and tempera-

ture is rather low. The maximum value of air/water ratio is

approximately 1/17 to 1/13 (Li 2006).

The DAF method is more popular than the IAF method.However, the DAF method has a few disadvantages, such

as high electrical power requirement, complexity in the

system and higher service cost. In the IAF method, rela-

tively large bubbles with diameters of several hundreds

of micrometers are formed by mechanical agitation or

sparger air injection. This method has been occasionally

used in wastewater treatment. For example, oils and fats

from dairy and abattoir wastes and blue-green algae from

natural wastewaters and maturation ponds have been

removed by the IAF method using a confined plunging jet

flotation cell (Yan and Jameson 2004). Large bubbles lead

to poor collision efficiency, and high shear in the conven-

tional mechanical flotation cells can lead to the breakage

of fragile particles, thereby generating finer particles. The

conventional technique is to add a surfactant to reduce

the bubble size. However, addition of a surfactant leads to

various pollution problems and an additional cost for the

treatment of sludge. Li (2006) has developed a separated

IAF system in which the bubble generation zone was sepa-

rated from the flotation. The microbubbles generated in

this IAF system had diameters in the range of 20200 m,and the average diameter was 70 m. Li studied the

removal of fine kaolin dispersions in water.The surface charge of the bubbles and particles plays

an important role in separation. The potential, therefore,is an important parameter in DAF or IAF. Coagulants, e.g.,

alum [Al2(SO4)

3], are added to reduce electrostatic repul-

sion between the particles so that they can stick together

and create flocs. The microbubbles are negatively charged

(see Section 2.4) and thus also are the kaolin particles dis-

persed in water. The potential of kaolin becomes lessnegative with the addition of alum, approaches zero and

even becomes positive as the alum dosage is increased.




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Therefore, the collision between the negatively charged

microbubble and the particle becomes more fruitful as the

alum dosage is increased. Consequently, good removal

efficiency (8090%) can be achieved at an optimal alum

dosage. Too much addition of alum decreases the poten-tial of the floc particles (Han and Dockko 1999), which

reduces the particle removal efficiency.Terasaka and Shinpo (2007) have developed a micro-

bubble-based floatation (MBF) system for removal of

carbon particles (mean diameter=1 m) from wastewater.The spiral liquid flow MBG (M2-LM type) was attached

at the bottom of the floatation column. The mean diam-

eter of the microbubbles was 63 m. Various cationic,anionic and nonionic surfactants (i.e., cetyltrimethyl-

ammonium bromide, sodium dodecyl sulfate and Tween

20) were added, depending on charge requirements. The

carbon particles concentrated in the foam layer at the top

of the flotation column. The concentration of the carbonparticles in the foam reached a maximum value when the

concentration of surfactant was 5% of its critical micelleconcentration (CMC). By contrast, the concentration of

the carbon particles in liquid phase decreased exponen-

tially within an hour of operation. They also proposed a

model to design the microbubble flotation column for the

removal of suspended fine particles.

Terasaka et al. (2008) have used MBF to recover the

iron oxide fine particles (mean diameter=4.5 m) from asuspension containing dilute surfactants (i.e., Tween 20,

sodium dodecyl sulfate and cetyltrimethylammonium

bromide). The suspended metal oxide particles were

hardly removed from the suspension using other aerators

such as a single orifice and a glass ball filter. However,

the MBF technique achieved 90% recovery of the parti-

cles in just 60 min. The surfaces of the microbubbles and

the metal oxide particles were electrically charged. When

pH was controlled between 4.4 and 7.8, the surface of the

microbubbles was negatively charged and the surface of

the metal oxide particles was charged positively. There-

fore, the microbubbles and particles were attracted to

each other. At pH 5, the microbubbles were adsorbed

well on the metal oxide particles. In the foam layer at thetop of the flotation column, the particles were trapped

and then removed from the bulk liquid. The recoverabil-

ity depended on the degree of ionization and the concen-

tration of added surfactant. In their system, the suitable

concentration of the surfactant was only 12% of the CMC

to achieve the highest recoverability of the suspended par-

ticles. To understand the mechanism of the separation of

the metal oxide particles using MBF, a kinetic model was

proposed, which takes into account the adsorption and

release of the particles.

The size of the microbubbles plays an important

role in MBF efficiency. Small microbubbles are required

for efficiently removing colloidal impurities from waste-

water. Adsorption on the surface of the microbubbles is an

important parameter (Yoshida et al. 2008). Cassell et al.

(1975) have investigated the effects of the concentration of

frother (e.g., ethanol) and collector (e.g., lauric acid) uponbubble size, and upon the removal efficiency of humic

acid. Aluminum sulfate was employed as a coagulant.

All experiments were conducted at a pH of 7.5 in a 0.4 dm3

batch flotation cell. Ethanol produced a greater effect on

bubble size than lauric acid. A small addition of ethanol

reduced the bubble size drastically. They have reported

that microflotation removal efficiency rapidly decreased

when the bubble diameter exceeded 5560 m.

6.3 Removal of oil from wastewater

Produced water generated at the exploration and produc-

tion sites contains a large amount of oil. The oil compo-

nents may be either dissolved in the water or present as

a dispersed phase. The oil may have aliphatic, aromatic,

phenolic and fatty acid components. Several methods,

e.g., gravity separation (with or without corrugated-plate

interceptor), induced gas flotation (IGF), induced static

flotation (ISF), hydrocyclone and centrifuge, are used for

removing oil from wastewater. In the microbubble-based

flotation of oil from wastewater, the oil droplets adhere

to the surface of the bubbles, rise upward and collect at

the surface of water. On the surface, a frothy layer of oil

and gas is formed, which is skimmed off. Smaller micro-

bubbles are more effective in separating oil from water,

which results in a drier froth and a very low skim volume.

The water leaving the skim tank can have as little as

5 g m3oil (which typically correlates to a separation effi-

ciency >95%). A large number of microbubbles per unitvolume of the dispersion create a higher probability for the

oil droplets to contact the bubbles. The microbubbles also

provide a large total surface area for attachment. The MBF

process reduces the use of flocculants. The electrostaticcharge on the bubble and oil surfaces plays an important

role in the efficiency of the removal of oil. Flotation of the

oil is promoted by decreasing the electrostatic repulsion

between oil flocs and air bubbles (Gray et al. 1997). Rubio

et al. (2002) have presented a review of the existing tech-

nologies for separating oil from wastewater.

Leung (1988) has developed a technique for the use of

microbubbles in the conventional hot water process in

the primary flotation and settling step to recover bitumen.

In this process, streams of steam and air in admixture




24/32


were injected via a submerged nozzle into a flowing

aqueous stream. A fine dispersion of microbubbles (with a

diameter


25/32


used. This can greatly contribute to the achievement of a

secure and safe society.

The use of hydroxyl radicals generated from collaps-

ing microbubbles is a novel method for the treatment of

wastewater. Only a few studies have been reported to date

regarding the treatment of wastewater using hydroxyl

radicals generated from microbubbles. The mechanismof free radical generation is yet to be studied for ozone,

air, oxygen and nitrogen microbubbles. The use of ozone

in the water treatment process may yield some second-

ary and harmful compounds (Gunten and Hoign 1994).

These surplus problems in water treatment can be solved

by the use of hydroxyl radicals generated from collaps-

ing microbubbles. Use of catalysts for the generation of

hydroxyl radicals from microbubbles can also be a tool for

research in the water treatment process.

The effect of the container can be important on the

efficiency of the microbubble-aided processes because themicrobubbles adhere to the surface of the container (Yang

et al. 2003). When the bubble is far away from the wall,

its transport in the bulk liquid is affected by the external

forces such as buoyancy and fluid drag. As the bubble

approaches the wall within a distance comparable to its

size, displacement of the fluid between the bubble and the

collector leads to additional hydrodynamic drag on the

bubble. At even closer distances (


26/32


Nomenclature

a interfacial area per unit volume, m1

Ac

cross-sectional area of the column, m2

c concentration, mol m3

c* saturation concentration of the gas in water, mol m3

d diameter of microbubble, m

d0 initial diameter of nanobubble, m

d32

Sauter mean diameter, m

D diffusivity of gas in liquid, m2s 1

Db

translational diffusion coeffi cient of microbubble, m2s 1

E electrophoretic mobility, m2V 1s 1

g acceleration due to gravity, m s2

H Henrys law constant, Pa m3mol 1

kg

gas phase mass transfer coeffi cient, mol N1s 1

Kg

overall g

microbubble-aided water and wastewater purification

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