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- Herausgeber: John Wiley & Sons
- Kategorie: Fachliteratur
- Sprache: Englisch
Comprehensive resource exploring the basic principles of microbubbles including modeling and simulation, as well as applications across various industrial processes
Featuring in-depth case studies, Microbubbles delves into the science and engineering behind microbubbles, their unique properties, and the state-of-the-art techniques being utilized to unlock their full potential, with insight into their various industrial applications, such as in computational fluid dynamics (CFD) modeling, as well as statistical and numerical analyses of lab-scale and pilot-scale operations.
Written by a highly qualified author with significant research contributions to the field, this comprehensive resource discusses sample topics including:
- Fundamental concepts of mass transfer as well as reaction engineering and process design of microbubble-based systems
- Different types of microbubbles, including ozone, N2, air, and O2, and the scope of microbubble industrial scalability, with information on cost and energy estimation
- Intrinsic concepts of chemical and environmental engineering related to microbubbles and recent developments in the simulation of microbubble systems
- Latest breakthroughs in microbubble technology, encompassing their use in nanotechnology, pollution control and treatment, and environmental remediation
This book is an essential reference on the subject for researchers at the postgraduate, PhD, and postdoctoral levels, along with engineers and chemists working with water and wastewater treatment technology. Understanding the basics of mass transfer and solid operations is a prerequisite to reading.
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Veröffentlichungsjahr: 2024
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Table of Contents
Cover
Table of Contents
Title Page
Copyright
Preface
1 Overviews
1.1 Introduction
1.2 Generation Techniques of Microbubbles
1.3 Physical Properties of Microbubbles
1.4 Characterization of Microbubbles
Nomenclature
Greek Nomenclature
References
2 Mass Transfer of Microbubbles
2.1 Introduction
2.2 Fundamental Mass Transfer Theories Applicable to Microbubbles
2.3 Dissolution Behavior of Microbubbles
2.4 Effect of Parameters on
k
L
2.5 Dimensional Analysis of
k
L
a
2.6 Mass Transfer in Reactive System
Nomenaclature
Green Nomenclature
Dimensionless Numbers
References
3 Hydrodynamics of Microbubbles
3.1 Introduction
3.2 Hydrodynamic Parameters
3.3 Correlations of Void Fraction
3.4 Analysis of Flow Regimes
3.5 Factors Affecting the Bubble Size and Rise Velocity
3.6 Hydrodynamics of Microbubbles Depending on Microbubble Generators
3.7 Hydrodynamics of Microbubbles Flow in Pipes
3.8 The Residence Time of Microbubbles in the Column
Nomenclature
Greek Nomenclature
References
4 Radical Generation by Microbubbles
4.1 Introduction
4.2 Radical Generation in O
2
, N
2
, and Air Microbubbles
4.3 Radical Generation in O
3
Microbubbles
4.4 Application of Catalysts for Radical Generation
4.5 Influence of Parameters on Catalytic Processes
4.6 Quantification of Radicals
Nomenclature
References
5 Water Treatment and Related Applications of Microbubbles
5.1 Introduction
5.2 Preliminary and Primary Stages of Water Treatment
5.3 Secondary Effluent Treatment
5.4 Tertiary Treatment
5.5 Ozonation and Biological Process
5.6 Fiber, Pharmaceuticals, and Other Wastewaters
5.7 Integration of Microbubble with Membrane Process for Water Treatment
References
6 Other Industrial Applications of Microbubbles
6.1 Introduction
6.2 Flotation and Mineral Beneficiation
6.3 Cleaning and Degreasing
6.4 Scrubber
6.5 Biodiesel Production
6.6 Biogas Upgradation
6.7 Enhanced Oil Recovery by CO
2
-Microbubbles (MBs)
6.8 Microbubbles (MBs)-Membrane Systems
Nomenclature
Greek Nomenclature
References
7 CFD and Microbubbles
7.1 Introduction
7.2 CFD Simulation
Nomenclature
Greek Nomenclature
References
8 Cost Estimation and Energy Consumption
8.1 Introduction
8.2 Identification of Operating Parameters
8.3 Cost Estimation
8.4 Power Measurement
8.5 Experimental Method of Evaluation of the Performance of Microbubble Generators
8.6 Estimation of Energy Consumption for Lab-Scale Microbubble-Based System
8.7 Case Studies on Application of Microbubbles in Industrial Processes
8.8 Scale-up Models
8.9 Case studies on Scale-up projects of MB flotation from lab to industrial scale
Nomenclature
Greek Nomenclature
References
Index
End User License Agreement
List of Tables
Chapter 1
Table 1.1 Various factors affecting the size of MBs.
Table 1.2 Available literature on the techniques for MB size measurement and...
Table 1.3 Factors affecting the ζ potential of MBs.
Chapter 2
Table 2.1 Physical properties of gases used for MBs at 298 K (in water mediu...
Table 2.2 The relationship of type of reaction, Hatta number (
Ha
) and enhanc...
Chapter 3
Table 3.1 The recommended correlations of drag coefficients at different Re ...
Chapter 4
Table 4.1 Pharmaceutical compounds degradation using various catalytic O
3
+ ...
Table 4.2 Heterogeneous catalytic O
3
+ MB process for water treatment.
Chapter 5
Table 5.1 Application of Microbubbles (MBs) in the primary stage of wastewat...
Table 5.2 The characteristics of real effluents before and after MB treatmen...
Table 5.3 MB application in the secondary treatment process (work in 2010–20...
Table 5.4 The water parameters of the influent and effluents of wastewater t...
Table 5.5 MB application in the tertiary treatment process (work in 2010–202...
Chapter 6
Table 6.1 Some nontraditional applications of MBs to enhance the process eff...
Chapter 7
Table 7.1 Values of model coefficients for different
k
−
ε
models...
Chapter 8
Table 8.1 Steps involved in calculating the cost of MB system.
List of Illustrations
Chapter 1
Figure 1.1 A rotational flow-type MB generator.
Figure 1.2 Schematic of MB generator.
Figure 1.3 (a) Spiral type and (b) venturi-type MB generator.
Figure 1.4 A schematic difference projecting the size difference of various ...
Figure 1.5 Images of Microbubbles (MBs) captured by, (a) bubbles below 0.1 m...
Figure 1.6 Test apparatus for generating MBs and capturing.
Figure 1.7 (a) The optical device to capture the photograph of MBsand (b...
Figure 1.8 The experimental observations of MB size distribution with respec...
Figure 1.9 Surfactant‐coated MBs: (a) schematic structure of MBs and (b) pho...
Chapter 2
Figure 2.1 The dissolution of MBs in water (a) captured in a high-speed came...
Figure 2.2 (a) The effect of superficial gas velocity on the value of
k
L
a
an...
Figure 2.3 The rate of MB shrinkage affected by the saturation ratio.
Figure 2.4 The behavior of dissolution of MBs at different salt concentratio...
Figure 2.5 Comparing the experimental and model data to evaluate
k
L
a
in the ...
Chapter 3
Figure 3.1 The MBs rising speed in distilled water with electrical conductiv...
Figure 3.2 The illustration of experimental setup to measure the gas holdup ...
Figure 3.3 The gas holdup, ε
G
...
Figure 3.4 (a) Homogeneous flow pattern and heterogeneous flow pattern obser...
Figure 3.5 Identification of flow pattern using drift flux model, (a) in sin...
Figure 3.6 The observed gas holdup as a function of superficial gas velocity...
Figure 3.7 The values of RTDs in MB column for (a)
U
...
Chapter 4
Figure 4.1 The shrinkage of MBs and the corresponding
P
–
T
diagram proposed b...
Figure 4.2 (a) Effect of concentration of O
3
and (b) effect of MBs on the ra...
Figure 4.3 The experimental setup developed by Deng et al. for the catalytic...
Figure 4.4 The proposed pathway for the generation of OH
•
radicals in ...
Figure 4.5 The ESR spectrum to measure the DMPO-OH in the O
3
process with ma...
Figure 4.6 The ESR spectrum to measure the DMPO-OH in the O
3
process with MB...
Figure 4.7 (a) The O
3
concertation and exposure profile at pH 8, and (b) var...
Figure 4.8 The leaner fit to obtain the value of
R
ct
using Eq. (4.32).
Figure 4.9 The profile of O
3
, OH
•
radicals exposure, and
R
ct
as a func...
Chapter 5
Figure 5.1 The stages of water treatment and the scope of integration of MB-...
Figure 5.2 MBs used in the aeration of wastewater (a) a photograph of top vi...
Figure 5.3 The photographs of MB- and NB-based water treatment for water qua...
Figure 5.4 The catalytic UV-MB-based experimental setup.
Figure 5.5 The mechanism of collapse of MBs in the presence of UV irradiatio...
Figure 5.6 (a) List of pharmaceutical and personal care products found in se...
Figure 5.7 The wastewater treatment plant proposed by Hashimoto et al. (2021...
Figure 5.8 The application of O
3
-MBs for the treatment of a synthetic dye wa...
Figure 5.9 The ionization and formation pathways of intermediates during the...
Figure 5.10 The detailed degradation pathway of degradation of BG dye in the...
Figure 5.11 The integration of ozonation and biological processes for the tr...
Figure 5.12 A single setup for the O
3
-MB-UV-based bubble column.
Chapter 6
Figure 6.1 Rise of bubbles (a) in a conventional bubble column for macrobubb...
Figure 6.2 The profile of polydispersity index (PI) and bubble diameters use...
Figure 6.3 Effect of circulation velocity on the flotation rate constant of ...
Figure 6.4 Various small metallic parts to clean with MBs cleaning method....
Figure 6.5 Photographs captured in the degreasing process, showing the absor...
Figure 6.6 Degreasing of metal plates and steel pieces using MBs, captured b...
Figure 6.7 Design of experimental setup for the separation of metal ions fro...
Figure 6.8 Schematic flow diagram of CO
2
absorption and gas separation using...
Figure 6.9 (a) Effect of injection pressure for oil recovery using CO
2
MBs a...
Figure 6.10 The images of heavy oil recovery using CO
2
MBs, taken (a) after ...
Figure 6.11 The morphology of SPG membrane contactor for generating MBs.
Figure 6.12 Membrane pores controlling the MB diameter in the ceramic membra...
Figure 6.13 Matching the experimental and predicted data for the MBs average...
Chapter 7
Figure 7.1 Schematic diagram shows a volume “V” covering both dispersed and ...
Figure 7.2 Coupling of various models in typical multiphase flow problem.
Chapter 8
Figure 8.1 The role of salinity in the degradation of methyl orange dye usin...
Figure 8.2 The systematic steps involved in the separation of Mo fines from ...
Figure 8.3 Cost estimation of the solvent extraction process and MBs flotati...
Figure 8.4 Linear scale-up process in traditional hierarchy.
Figure 8.5 Scale-up strategy for industrial process.
Guide
Cover
Table of Contents
Title Page
Copyright
Preface
Begin Reading
Index
End User License Agreement
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Microbubbles
Engineering Aspects and Industrial Applications
Snigdha Khuntia
Ahmedabad University
Assistant Professor, School of Engineering and
Applied Science, Ahmedabad, India
Copyright © 2025 by John Wiley & Sons, Inc. All rights reserved, including rights for text and data mining and training of artificial technologies or similar technologies.
Published by John Wiley & Sons, Inc., Hoboken, New Jersey.Published simultaneously in Canada.
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Library of Congress Cataloging-in-Publication Data
Names: Khuntia, Snigdha, author.
Title: Microbubbles : engineering aspects and industrial applications / Snigdha Khuntia.
Description: Hoboken, New Jersey : Wiley, [2025] | Includes index.
Identifiers: LCCN 2024034342 (print) | LCCN 2024034343 (ebook) | ISBN 9781394249381 (hardback) | ISBN 9781394249404 (adobe pdf) | ISBN 9781394249398 (epub)
Subjects: LCSH: Microbubbles.
Classification: LCC QC161 .K47 2025 (print) | LCC QC161 (ebook) | DDC 530.4/275–dc23/eng20241010
LC record available at https://lccn.loc.gov/2024034342
LC ebook record available at https://lccn.loc.gov/2024034343
Cover Design: WileyCover Image: © onuma Inthapong/Getty Images
Preface
Bubbles are fascinating and mysterious. When they are tiny, they are even more interesting to work with. Ever since I came across the fascinating world of tiny bubbles, which are famous in the name of microbubbles and nanobubbles, I have been blown away with their capabilities in several applications. Humankind is working hard to make every process and every industry to meet the norms of sustainability goals, and searching for alternatives to save the planet earth from the spread of pollution. I am intrigued to know the capabilities of the bubbles to achieve these goals of sustainability.
There are two major application areas of micro and nanobubbles: drug delivery and medical applications, and environmental applications. Medical and drug delivery applications of microbubbles are very different from the environmental applications. While drug delivery systems mostly work with loading and carrying the drugs between multiple media, their generation methods are also very different than environmental applications. For water treatment, flotation, and biofuel production, the major concern is to meet the scale of production. As a chemical engineer, I always look forward for a system that can meet the supply and demand of industrial processes, at the same time achieving minimal waste generation.
The microbubbles drive me to connect all the basic fundamentals of fluid flow operation, mass transfer operation, along with the chemical reaction engineering, which are the backbones of any chemical engineer. In the era of artificial intelligence and machine learning, there are several processes that still need attention to enhance their efficiency by doing research on the fundamental principles. Ever since the microbubbles have been discovered, numerous works have been published on their stability in the therapeutic and drug delivery applications. Microbubble-based systems have more potential than the conventional processes used in any multiphase reaction or non-reaction-based systems. To harness this potential, one needs to understand the physics and hydrodynamics of these amazing tiny bubbles, which are also very useful in the reaction medium. Most of the chemical engineers deal with several kinds of reactors in which bubble column reactors are the most common. Till date, many industrial processes are using the conventional macrobubbles or also known as millibubbles, which could be easily replaced with microbubble systems and achieve greater efficiency in their processes.
Earlier, the generation of microbubbles was difficult and the machines or generators were not energy efficient or easily accessible. Many engineers and entrepreneurs have now established many companies that now design and fabricate custom-made bubble generators with variable size and capacity. I believe more work and attention must be given to this amazing technology, which has the potential to reduce waste produced in the process industries and several chemical manufacturing industries. The major challenge faced by the water treatment industry is the amount of waste produced through several steps, where microbubble-based systems can be easily implemented and the waste production can be reduced up to a major extent.
When a student or an early researcher wants to explore more about the microbubbles in the application of water treatment and related fields, it is very hard to find a book on this topic. A beginner might find it difficult to start with research articles, which mostly adhere to a problem statement in a narrow research field. For graduate students, this problem is very evident where they find it challenging to relate a research topic with the undergraduate courses. This book will try to fill the gap of fundamental principles of microbubbles and their application in the industrial processes, where the students and researchers could find it interesting to work more with microbubbles. Although all the recent works and developments of the microbubbles in several applications cannot be covered in a single book, the basic principles, properties, and applications in the pilot-scale and large-scale operations have been covered.
I sincerely want to thank my PhD supervisors Professor Pallab Ghosh and Professor Subrata Kumar Majumder for introducing me to microbubbles in the application of water treatment, from which I primarily got the encouragement to research more on microbubbles. There are several pioneers in the field of microbubbles who always inspire me to work more on this topic, and the author thanks all of them. The author wishes to thank Ahmedabad University for providing support and facility to carry out the research and generous support in writing this book. Many thanks to my research team and students who helped me to collect several research articles and also during the writing process. I also thank my coauthors Professor Manish Kumar Sinha and Professor Abhishek Yadav who were willing to contribute two chapters, which were very essential to complete the scope of this book.
Despite all our efforts, there might be several errors and misleading sentences that might be confusing for the readers. Therefore, I urge all the readers to contact me without hesitation to add more value to this book.
26 November 2024
Snigdha Khuntia
Ahmedabad UniversityAssistant ProfessorSchool of Engineering andApplied ScienceAhmedabad, India
1Overviews
1.1 Introduction
The pervasive threat of toxic chemicals in our society demands immediate attention. Despite the complexity of eliminating these hazardous substances, including the emergence of secondary compounds and economic constraints, safeguarding our environment remains paramount. With a particular emphasis on water quality as a linchpin of this endeavor (Hiroshi, 2006), the urgency to address this issue has never been more evident. As global environmental challenges escalate, a plethora of initiatives has arisen to bolster water purification efforts. Notably, microbubbles (MBs) have garnered increasing recognition within the sphere of water treatment technologies, signaling a promising avenue forward (Agarwal et al., 2010; Burns et al., 1997; Nakano et al., 2005; Wen et al., 2011).
MBs, tiny spherical bubbles filled with gas, exhibit remarkable individuality, resist aggregation, and typically range in size from 1 to 100 μm. Recent years have witnessed a notable upsurge in MB research, particularly in their application within various domains. These minute gas bubbles, when submerged in water, often measure less than 50 μm in diameter and are primarily composed of oxygen or air. Notably, MBs demonstrate an exceptional ability to remain suspended in water for extended periods. Gradually, the gas within them dissolves into the surrounding water, causing the bubbles to dissipate. MBs filled with air, oxygen, or ozone are extensively utilized across a diverse array of applications in the area of water treatment. Beyond that, MBs have been applied in numerous fields. For instance, in medical therapeutics, they serve as invaluable tools for organ imaging and as carriers for drugs or genes (Dicker et al., 2011; Kurup and Naik, 2010; Lindner, 2004). Moreover, MBs exhibit remarkable antibacterial properties, proving effective in both aerobic and anaerobic environments (Himuro et al., 2009). Additionally, various surfactant-stabilized MBs play a pivotal role in adsorbing proteins from water (Jauregi and Varley, 1998).
MBs have undergone functionalization with a variety of materials, spanning from surfactants and nanoparticles to pharmaceuticals and bioactive compounds. Their diminutive size endows MBs with exceptional efficiency in numerous industrial separation applications, notably in removing volatile contaminants and particulate matter (PM) from aqueous phases (Ahmed and Jameson, 1985). Both petroleum-based and biological surfactants have been employed to stabilize MBs, contributing to their versatility (Kukizaki and Baba, 2008; Xu et al., 2011). Flotation processes utilizing MBs hold particular significance in the separation of low-density PMs from water (Terasaka and Shinpo, 2007). In such processes, MBs serve as the carriers of PMs, lifting them from the column’s base. The selection of a specific flotation method, whether it involves dissolved air flotation (DAF), dispersed air flotation, or electro-flotation, hinges on the technique utilized for MB generation (Liu et al., 2010).
MBs exhibit versatility across a multitude of fields, showcasing their wide-ranging applications. They have been integral technologies in cleaning processes (Akuzawa et al., 2010), extraction of oil from both soil and water (Gotoh et al., 2006; Xiaohui et al., 2011), and soil washing procedures (Roy et al., 1992). Moreover, MBs have played pivotal roles in fermentation practices (Ago et al., 2005; Xu et al., 2011), marine fish farming (Tsutsumi, 2010), and horticulture (Park and Kurata, 2009). In the realm of food technology, they have been utilized for various purposes (Shen et al., 2008; Soli et al., 2010). Furthermore, MBs have been employed in acid gas absorption, such as CO2 using alkali substances (Akimov et al., 2011), and in the absorption of flue gases (Zeng et al., 2020), among numerous other applications summarized by Li (2006).
MBs exhibit an exceptional capacity to bolster the growth rate of marine organisms like oysters and scallops, facilitating efficient air transfer between phases through a substantial gas–liquid interfacial area. The adoption of MBs in water treatment has garnered substantial recognition across diverse industrial sectors, owing to their excellent efficacy as compared to the conventional methods (Jyoti and Pandit, 2001). In particular, ozone MBs have emerged as highly potent for oxidation, disinfection (Khuntia et al., 2012, 2013; Sumikura et al., 2007), decolorization of dyes and intermediates, and deodorization of water (Tasaki et al., 2009). Ozone, renowned for its robust oxidizing properties, proves invaluable in sterilization, color, and odor removal, as well as the degradation of organic substances in water purification and sewage treatment contexts (Camel and Bermond, 1998).
1.2 Generation Techniques of Microbubbles
The method chosen for MB generation plays a pivotal role in determining their properties and, consequently, their effectiveness in wastewater treatment. Achieving the production of fine MBs with a high density is essential for optimal wastewater treatment outcomes. Three primary types of microbubble generators (MBGs) find common use across various applications, and the types are gas–water circulation, pressurization–decompression, and no-liquid flow.
In gas–water circulation type, the required gas is introduced into a water vortex, where the turbulent flow transforms macrogas bubbles into MBs. Pressurization–decompression generators dissolve an ample amount of gas in water at moderate pressure. This process creates a supersaturated solution of gas in water. The gas escapes upon decompression from the water phase, generating a significant number of MBs. The third type, the no-liquid flow MBG, keeps the liquid static while only the gas is forced to mix, resulting in MB formation. Each of these generation methods is vital in customizing MB characteristics to fulfill the specific demands of wastewater treatment processes.
1.2.1 Types of Microbubble Generators
1.2.1.1 Gas–Water Circulation-Type Microbubble Generators
Rotational Liquid Flow Microbubble Generator (Spiral Flow Type) Ohnari (2000, 2002) led the pioneering development of the MBG, particularly the M2-LM type, engineered by Nanoplanet Research Institute Corporation. This cylindrical generator, measuring 50 mm in diameter and 100 mm in height, functions by introducing pressurized water from the bottom through a pump, initiating a rotational liquid flow within the cylinder. In agreement with Bernoulli’s theorem, the center of the cylinder experiences reduced pressure due to the gyratory liquid flow, prompting the intake of air. This process generates an air-swirling flow, resulting in the production of MBs near the cylinder outlet. The flow rate of liquid is approximately 12 L/min, and the rotational speed of the two-phase flow in the cylinder ranges from 300 to 600 rounds/s. The ratio of gas–liquid flow rate is maintained between 1/7 and 1/15.
The practical implementation of the M2-LM-type generator has been observed in the aquaculture sector of oysters in Hiroshima, where it produces MBs of 10 to 50 μm size. However, it is to be noted that the density of MBs generated by the M2-LM method is relatively lower compared to that achieved by the DAF method. In an alternative setup, a centrifugal pump by Nikuni Corporation Limited was combined with the rotating-flow MBG M2-M from Nanoplanet Research Institute Corporation. This system simultaneously draws in water and air using the pump, with air dissolving into the water before reaching the generator. The pressurized water then experiences decompression through the MBG, which operates at a high rotational velocity. A similar but new MBG that works on the similar principle of rotational liquid flow was invented by Sadatomi et al. (2005), illustrated in Figure 1.1. The schematic of the MBG is shown in Figure 1.2, where the pressurized water enters the pipe having a spherical body situated in the core. The MBG works on the principle of Bernoulli’s Theorem with conservation of energy and mass.
Venturi Type The venturi-type MBG operates with a liquid stream mostly water, entering the inlet of a venturi tube containing millibubbles. As the two-phase flow propagates through the throat, rapid pressure changes occur, leading to the formation of MBs through the reduction of millibubbles and/or cavitation. In certain cases, to prevent bubble coalescence, 50 mg/L of 3-pentanol is introduced, resulting in the predominant production of 100 μm MBs. A schematic of spiral and venturi-type MBGs has been shown in Figure 1.3.
Figure 1.1 A rotational flow-type MB generator.
Source: Reproduced with permission from Sadatomi et al. (2005)/ELSEVIER.
Figure 1.2 Schematic of MB generator.
Source: Sadatomi et al. (2005)/with permission of ELSEVIER.
Static Mixer Type The static mixer-type MBG has been developed by Fluid Engineering Institute Corp., which creates helical multiphase flow using guide vanes and a special mushroom-type projection or current cutter. This transforms it into MBs ranging in size from 5 to 50 μm, achieving a maximum production rate of 1500 L/min.
Ejector Type The ejector-type nozzle-based MBG operates by expelling liquid at high velocity, functioning as an aspirator, where gas is absorbed due to opposing pressure at the outlet. This process initiates turbulent mixing and shearing mechanisms, ultimately resulting in the production of MBs. The average size of these MBs is approximately 424 μm, however, a little bigger than the conventional MBs.
Dissolved Air Flotation Type DAF MBG operates by dissolving pressurized air, typically at around 3–4 atm, into water. This air-infused water is then discharged through the nozzle, generating MBs due to the supersaturation of air in the liquid. This process yields a high concentration of MBs, imparting a milky appearance to the liquid. The distribution of MB sizes typically exhibits two peaks, reflecting the complex interplay of factors involved in the generation process.
Figure 1.3 (a) Spiral type and (b) venturi-type MB generator.
Source: Terasaka et al. (2011) / with permission of ELSEVIER.
1.2.1.2 Pressurization–Decompression Type of Microbubble Generators
Gas dissolves by pressurizing into water within a tank to make the gas–liquid mixture. Upon flashing the supersaturated liquid through a reducing valve, MBs are generated, having size and quantity dependent on the decompression process. In a comparative study conducted by Terasaka et al. (2011), the performance of four MBGs was assessed, assessing gas holdup and gas mass transfer into the liquid phase. The spiral-liquid flow and the pressurization–decompression types of MBGs exhibited the highest gas holdup, with the former demonstrating a notable oxygen mass transfer coefficient. MBGs prove significantly more effective in transferring any gas into the liquid (mostly water) phase compared to commercial gas distributors such as perforated plates or constant-flow nozzles. However, the power demand for MBG is higher due to the additional pump attachment to create the pressure. Conventional gas distributors do not require this.
Ikeura et al. (2011) highlighted the effectiveness of the pressurization–decompression MBG in decomposing fenitrothion, a pesticide, using ozone. They attributed this superior performance to the tiny size of MBs generated by this generator variant, which facilitates enhanced contact of pesticides with water followed by the removal of excess pesticides in order to clean the vegetables. Variations in fluid flow patterns among different MBGs can result in differences in MB size, size distribution breadth, and MB shape. Moreover, some researchers have observed that the electrical properties of MBs may change depending on their generation method, which can significantly impact wastewater treatment outcomes.
1.2.1.3 No Liquid Flow-Type Microbubble Generators
Capillary Type Kukizaki and his research group (2009) have developed a novel sintered porous glass MBG fabricated from loamy soil, which is a capillary-type MBG. Along with MBs, nanobubbles (NBs) of diameter of 720 nm were generated from the sintered membrane, where the contactor has an average pore of size 84 nm. Various surface-active agents are often introduced to mitigate bubble coalescence.
Rotating Type In the rotating-type MBG, a high-speed rotation of a sintered material rotor within a surrounding stator automatically absorbs gas. Onoe et al. (2002) conducted reaction crystallization, yielding an average of 50 μm MBs. Notably, this generator eliminates the need for a liquid pump. Some of the available MBGs in this category are Fuki Works Corporation, Nomura Electronics Corporation, and Royal Electric Corporation, etc., which specialize in fabricating and selling rotating-type generators, mostly sold for the purification of lakes and other water bodies.
Ultrasound Type Yasuda et al. (2019) employed ultrasound irradiation on ultrapure water to create NBs of 90–100 nm in diameter. Achieving a high NB density was possible with the help of ultrasonication and irradiation time at a reduced ultrasonic frequency. In a study by Xu et al. (2008), common surfactants such as sodium dodecyl sulfate (SDS) and L-150 A (a mixture of ethanol 10% v/v, sucrose laurate ester 38% v/v, and water 52% v/v) were utilized for MB generation through sonication. Their findings revealed that using 1% SDS resulted in larger bubbles around 46 μm bubble. Additionally, the bubble density was 8.3 × 106/mL with SDS, while it increased to 2.5 × 107/mL with L-150 A. The impact of gas concentration on NB generation was also studied by Lee et al. (2020) through ultrasonic irradiation. They employed a 20 kHz ultrasonic horn booster, and they were subjected to ultrasonic irradiation for 10 minutes in three types of deionized water, such as undersaturated, saturated, and supersaturated. Their findings indicate that as the gas dissolution increased, the surface tension of water decreased, resulting in a reduced NB concentration.
Vapor Condensation Type In the vapor condensation-type MBG, nitrogen vapors and steam are mixed and blown into water from a nozzle. The steam condenses, yielding MBs, which is non-condensable nitrogen. The mean diameter of MBs ranges from 20 to 40 μm. The MB size is influenced by gas composition, diameter of the nozzle, and flow rate of mixed vapor.
Electrolysis Type MBs of O2 and N2 can be generated through electrolysis owing to redox reactions at the anode and cathode, respectively. Lucero et al. (2017) conducted studies to assess the impact of salt, current density, and distance between the electrodes on the rate of generation rate of MBs. The most dominating parameter was the current density that directly regulated the generation rate of MBs, with increasing current density leading to a more rapid bubble generation. The required energy for the electrolysis process is reduced by incorporating NaCl and reducing the electrode distance. In a study by Ulatowski et al. (2021), the effect of salt and duration of electrolysis on the mean MB diameter of N2 and O2 by electrolysis was investigated. Results indicated that the MB size increased with higher salt concentration, whereas only the hydrogen MBs were affected by the extension of electrolysis time and decreased in size with increased electrolysis time. Throughout the electrolysis process, the mean MB diameter initially decreased and attained an equilibrium size beyond an hour (Chandran et al., 2015).
1.2.2 Scale of Microbubble Generators
A variety of MBGs is now globally available, catering to diverse applications. In sectors such as wastewater treatment, fish culture, and related processes, there is a demand for plant-scale MBGs capable of meeting volumetric flow rate expectations while maintaining bubble diameter. Ozone is commonly used as the gas medium in MBs for many wastewater treatment processes, necessitating the MBG construction material to be corrosion-proof.
For laboratory-based studies, small pressurization–decompression and gas–water circulation types of MBG are prevalent. These generators can produce high-density MBs, with volumetric flow rates ranging from 5 × 10−4 m3/s onwards. Nikuni, based in Japan, stands out as a leading manufacturer of various MBGs, supplying units worldwide. Their products find applications in laboratories, spas, and DAFs. The KTM model, specifically designed for DAF, boasts a water flow rate ranging from 22 L/min to 967 L/min. Another Japanese company, Hinode Sangyo Corporation Ltd., is also known for manufacturing pressurized MB systems. The company aims to offer a contemporary MB-based solution for wastewater treatment, with a focus on avoiding sludge formation in conventional processes.
Distinguished by its departure from the conventional MB generation methods, Aklova Microgas technology relies on direct MB induction instead of the principle of gas dissolution. This innovation results in energy-efficient systems that function with fewer components. Consequently, Aklova MicroGasTM-powered plants boast easy installation, operation, and maintenance, featuring the smallest possible footprint. They claim to be the most convenient and highly energy-efficient MBG on the market for the DAF and gas transfer applications for O3, O2, and CO2 gases. MicroGasTM offers a gas flow rate range from 1–20 NL/min per unit for Series S, dedicated to laboratory and pilot-scale applications. Meanwhile, Series XL is designed for plant-scale applications with a gas flow rate requirement of 10–75 NL/min.
Among the widely encountered MBGs is the DAF type, readily available on e-commerce websites. Shanghai Edon Mechanical & Electrical Equipment Co., Ltd. specializes in manufacturing the coupling-type micro-nanobubble (MNB) generator, catering to a diverse range of flow rates and applications. For applications often found in laboratory studies, YLEC Consultants have patented the unique MB-generating device named Carmin. The Carmin generator initiates gas MBs production by requiring a high-water inlet pressure of water flow for creating a cavitation pocket. Subsequently, gas flow in controlled fashion is injected into the vapor cavity, breaking up into an MB cloud of very uniform size. The typical water supply flow rate ranges from 1–26 L/min, and the injected air flow rate varies from 5–180 mL/min. One additional Japan-based company, OK Engineering Co., Ltd., specializes in providing nozzle-type MBGs designed for use in seawater without the risk of clogging. These nozzles can generate both MBs and NBs at a discharge rate ranging from 6 L/min to 25 L/min.
In the realms of cutting-edge technological innovations, MB technology has found widespread application in highly developed nations such as Japan, Korea, and China. Across the expanse of Japan, MB-generating systems have been made available through local manufacturers, including esteemed entities such as Aura Tech Company Limited, Shigen Kaihatsu Company Limited, and Nanoplanet Company Limited (Terasaka et al., 2011). While these developed MBs showcase remarkable technological prowess worldwide, their adoption faces hurdles beyond technological prowess. In contrast to their prevalence in advanced nations, the global dissemination of MB technology encounters challenges. Despite its technological advancements, it remains a relatively costly venture, fraught with logistical intricacies in transportation to developing nations. Its direct implementation proves intricate and, in some cases, deemed unfeasible, further complicated by the scarcity of spare parts. Indonesia, a nation known for its vibrant tapestry of islands, presents an intriguing case study. The application of MB technology for water treatment remains nascent, with its widespread adoption yet to transpire. Within the Indonesian landscape, MB technology has primarily found its niche in specific domains, notably in fishery industries and industrial labs of small scale (Deendarlianto et al., 2015; Majid et al., 2016). Remarkably, Indonesia lacks a common sparger manufacturing industry, a notable gap attributed to the absence of comprehensive research initiatives within the nation’s borders. The relatively high operational costs of MB systems, stemming from the amalgamation of water pumps and air compressors (Terasaka et al., 2011), have deterred widespread implementation. However, the indisputable efficiency of MB technology, transcending its high energy consumption and cost of operation, beckons toward a future, where its transformative potential can be harnessed on a grand scale (Fan et al., 2018).
1.3 Physical Properties of Microbubbles
The fundamental distinction among macrobubbles, MBs, and NBs primarily hinges on their respective sizes. Macrobubbles typically exceed a diameter of 100 μm, while NBs are defined as those measuring less than 10 μm. Although there is no universally standardized size criterion, this size range is widely accepted worldwide. Over time, MBs gradually get smaller in size and eventually collapse due to prolonged stagnation. This process results in dissolution of the internal gases into the bulk water. In contrast, NBs have the remarkable ability to persist for extended periods, often months, without rupturing (Takahashi, 2009). Research has unveiled that the interface of NBs is marked by their robust hydrogen bonds, very similar to ice and gas hydrates. This unique property leads to decrease in diffusivity of NBs, contributing to their capacity to maintain a stable kinetic balance even in the presence of high internal pressure. For a visual representation, Figure 1.4 provides a clear schematic depicting the difference between macrobubbles, MBs, and NBs.
1.3.1 Physics of Microbubbles
Understanding the disparities between MBs and NBs based on their physical attributes and stability is of utmost importance. The MBs, while dispersed in the water, give a milky appearance and then ascend to the surface slowly. Various techniques, including atomic force microscopy (AFM), have demonstrated the presence of NBs at the interface made of liquid–solid phases, which revealed that NBs at the interface take on the appearance of spherical caps with a height of 10 nm and a diameter of approximately 100 nm (Agrawal et al., 2005; Switkes and Ruberti, 2004; Zhang et al., 2006a). It has also been observed that the NBs can be fused together to create a larger bubble using the tip of AFM. Initially, it was believed that high surface tension of NBs was the reason behind this and assumed that the gas would quickly dissipate from them within microseconds of formation (Matsumoto and Tanaka, 2008). However, under the right conditions, NBs can be freely formed and maintain their stability for extended periods. The longevity of NBs is attributed to the lower interfacial curvature compared to initially anticipated, owing to a higher contact angle (Zhang et al., 2006b). Moreover, the formation of NBs in water containing small organic molecules, such as alpha-cyclodextrin, ethanol, tetrahydrofuran, and urea, has also been documented (Jin et al., 2007).
Figure 1.4 A schematic difference projecting the size difference of various types of bubbles.
Source: Takahashi et al. (2007); reused with permission from the copyright holder, ACS Publishing.
Bubble rising velocity and surface charge are the pivotal factors for comprehending the characteristics of both MBs and NBs. MBs were observed to migrate toward oppositely charged electrodes while observed in an electrophoresis cell. The surface charge, commonly known as zeta (ζ) potential, of each MB can be ascertained from its movement speed, which is directly linked to the magnitude of the ζ potential (Graciaa et al., 2000; Takahashi, 2005; Yoon and Yordan, 1986). In fact, the prolonged stagnation of MBs helps to determine the ζ potential readily in distilled water, which typically measures around −35 mV. It has been elucidated that MBs carry a negative charge over a broad range of pH values. It is worth noting that both OH− and H+ ions affect the charging mechanism of MBs at the gas–water interface (Takahashi, 2005). Furthermore, the ζ potential remains constant under similar water conditions and independent of the MB size, representing a consistent quantity of electrical charge per unit area. Nonetheless, an increase in the ζ potential is observed as MBs shrink during their collapse. A decrease in MB size below the surface of water leads to elevated internal pressure within the MBs, a pressure directly proportional to the diameter of bubble. The famous Young–Laplace equation describes the relationship between pressure (P), radius (r), and surface tension (σ) for a curved interface, commonly expressed as:
This equation is used in various fields, including physics and engineering, to understand and calculate pressure differences across curved surfaces, such as bubbles and droplets.
In accordance with Henry’s law, as gas pressure increases, the quantity of dissolved gas contained in a shrinking bubble also increases. Research has demonstrated that the region surrounding an MB undergoes a shift in its state within a pressure–temperature diagram, promoting hydrate nucleation (Sloan, 1998). This behavior stands as one of the distinctive attributes of MBs.
1.3.2 Microbubble Size and Measurement Techniques
MBs inherently assume a spherical shape. In the realm of three-dimensional objects at a constant volume, the surface area is minimized while the object attains a spherical form, a principle aptly described by the isoperimetric inequality (Osserman, 1978
