Contemporary application of microbubble technology in water treatment

With the current surge in global environmental challenges, there is a shift towards more eco-friendly and sustainable technologies. Microbubble (MB) technology has recently emerged as a viable option in water treatment. The increasing demand for water following the dramatic population and economic growth has led to more efforts directed towards wastewater treatment. Conventional water treatment technologies like flocculation, sedimentation, filtration, UV degradation have been shown to reduce water pollutants, but these technologies generally require a series of steps which increases the overall cost of the process (Rajasulochana & Preethy 2016). So, researchers for quite long have been in the pursuit of a more effective and cost-efficient technology.

Air, oxygen, and ozone microbubbles (MBs) have shown great potential for reducing the running cost and economising the wastewater treatment plants (WWTP) (Yamasaki et al. 2009). Micro- and nanobubbles have been reported to be effective in the processes of aeration, flotation and disinfection (Jyoti & Pandit 2001; Liu et al. 2016). Microbubbles are tiny bubbles with diameters in the range of 10–100 μm (Basso et al. 2018). The shared characteristics of these MBs, which make them unique is their longer residence time, slow buoyancy, self-pressuring effect, and large gas-liquid interfacial area as compared to the conventional macrobubbles (Hernandez-Alvarado et al. 2017).

The formation of bubbles in the water is thought to be a static or quasi-static progression followed up by dynamic regime of coalescence and break up. This overall process of bubble inception, its growth and collapse is called cavitation. The driving mechanism of cavitation is a reduction in the static pressure of liquid below its vapour pressure, leading to vapour/gas-filled cavities in the liquid. The cavitation is unlike the bubble formation in boiling where the process is primarily driven by temperature change.

The microbubble generator (MBG) is chosen depending upon the application requirement, whether it is in production on a laboratory scale, for research purposes, actual field trials or at an industrial scale. The capacity and concentration of MBs production required for the treatment and the cost of treatment also decides the MBG employed.

Based on source employed for cavitation, it is classified into the following types:

• 1.

  Hydrodynamic cavitation (HC) – This type of cavitation is induced as a result of local pressure and the velocity changes of the flowing liquid owing to the passage through restricted geometry of the system.

• 2.

  Acoustic cavitation – Cavitation is achieved through high intensity ultrasonic fields. The equipment and operational costs are higher for this type when scaled at the industry level.

• 3.

  Particle cavitation – Particle cavitation is caused by a beam of elementary particles rupturing the liquid flow.

• 4.

  Electrolysis cavitation – The application of electric current to the fluid to produce cavitation.

• 5.

  Optic cavitation – Such type of cavitation is produced as a result of interaction with high intensity light (laser), which breaks the continuum of liquid flow.

Out of these cavitation techniques, hydrodynamic and acoustic cavitation has been most widely applied in wastewater treatment (Mancuso et al. 2020). Hydrodynamic and acoustic cavitation has shown to bring about the desired physico-chemical changes in the bulk liquid (Temesgen et al. 2017). When it comes to pilot- and large-scale processing, HC technology remains the first choice for MB formation in water treatment owing to high energy consumption and limited cavitation zone in the acoustic cavitation technique (Badve et al. 2013).

In water treatment processes (WTPs), MB is generated by one of the below mentioned hydrodynamic process:

• Pressurised dissolution or decompression type generator – Gas is dissolved in the water stream by creating high pressure of about 304–405 KPa, i.e., super saturation of gas. Supersaturated gas being unstable escapes generating MBs.

• Cavitation by Venturi type generator – In this method, the MB is generated by the passage of air and water simultaneously through the Venturi tube. Pressurised fluid enters through the inlet and the velocity of the fluid increases at the cost of a decrease in static pressure in the narrow cross-section throat. This accelerated fluid generates cavitation and sucks in the gas creating microbubbles (Wilson et al. 2021). This type offers the benefit of lesser pump power, and compacted size.

• Orifice type generator – In this method, air is absorbed into the vacuum created by the movement of fluid through holes in the orifice plate. Unlike the Venturi, the orifice type generator generates intense cavitation conditions owing to immediate contraction and divergence sections.

• Ejector type generator – In this type of reactor, shrinking or step-by-step enlargement of the fluid channels occurs producing complex pressure profiles as the liquid flows. At the lowest pressure point, gas will be sucked in generating MBs from the shear of the turbulent liquid flow (Parmar & Majumder 2013).

Compared with acoustic cavitation, HC appears to be the more practical and economical way to generate MBs at pilot and industrial scale and at higher rates using a simple centrifugal pump along with a flow confiner like Venturi tube, orifice plate, and throttling valves (Wang et al. 2021). For more information on the MB generation methods and bubble characterisation techniques, one can refer to recent articles (Sakr et al. 2021).

Some studies have reported the generation of significant reactive oxygen species or free radicals by HC as the MBs collapse. For example, Khuntia et al. (2015) quantified the hydroxyl radicals produced from ozone MBs at different pH using p-chlorobenzoic acid radical probe. Their study revealed a higher generation of hydroxyl radicals at acidic pH than that in the alkaline medium. Zheng et al. (2015a, 2015b) ascribed the better degradation of biorefractory organic compounds by MB ozonation to the greater amount of unselective hydroxyl radicals produced using fluorescence detection. However, some other studies highlighted that the treatment process with MBs is most likely to be thermal based rather than free radical based (Bandala & Rodriguez-Narvaez 2019). There is debate over the mechanism behind the organic water pollutant degradation. Still, all these studies identified the feasibility of the HC in water treatment. Bandala & Rodriguez-Narvaez (2019) claimed that HC alone or coupled with other advanced oxidation techniques is a promising practice to remove organic contaminants like azo dye and antibiotics entering the water cycle. The limitation of the most advanced oxidation techniques to completely remove the complex organic matter in the water has led to the surge in exploring the effectiveness of HC or its coupling with other advanced techniques.

Previous reports have focussed on generation techniques, operational conditions employed, bubble size and distribution measurements, and characterisation techniques. However, limited efforts are directed towards studying the application gaps of MBs/NBs in water treatment technology. Few reports have accounted the possibility of scale up to industry level, but a comprehensive, up-to-date report on the existing knowledge gap in the application field is desired to explore the endless avenue of this technology. This article reviews and briefly discusses the various MB generation technologies employed and comprehends their applicability in the state-of-the-art WTPs.

Environment friendly biological treatment methods have been the preferred treatment of organic wastewater. The metabolism of microorganisms is used in biological treatments like bio-activated sludge, biofilm, and membrane bioreactors to degrade harmful chemicals. These processes are limited by the high cost incurred during aeration, sludge treatment and issues of membrane fouling. Aeration or oxygen supply in the conventional activated sludge system consumes 50–90% of total electricity of the WWTP and traditional aerators have extremely low oxygen transfer efficiency (Drewnowski et al. 2019). Microbubbles with long retention times and high gas mass-transfer efficiency are conducive to energy conservation and cost reduction in WWTP. Microbubbles make it possible to diffuse oxygen more effectively in aerobic wastewater treatment.

Microbubble technology is believed to support biological aerobic wastewater treatment owing to its high oxygen mass-transfer rates, but few studies have exposed the negative effects of microbubble technology on the mixed liquor properties of activated sludge during microbubble aeration. The high shear force generated during MB generation was found to breakdown sludge flocs, reducing the microbe population available for oxidation of organic matter (Liu et al. 2012a). To tackle this problem Budhijanto et al. (2015) proposed to combine MBGs with an attached growth aerobic system. The successful application of an MBG as the aerator with higher soluble chemical oxygen demand (SCOD) removal efficiency in the low gas flow rates is seen. They emphasised the careful design and selection of the MBG configuration and its relative position (if more than one MBG is used) in the reactor to avoid bubble coalescence. Lei et al. (2016) investigated the effect of microbubble aeration on the biofilm formation. The synthetic municipal wastewater was treated in a fixed bed biofilm reactor using microbubbles generated from the Shirasu porous glass (SPG) membrane system. Microbubble aeration led to 80% faster biofilm formation as compared to coarse bubble aeration. This enhancement was related to the improved attachment of the suspended microbes to the carrier surface by microbubbles. The SPG membrane area-based chemical oxygen demand (COD) removal capacity was found to be 6.88 kg COD/(m² d) with a COD removal efficiency as high as 91.7%.

Much research has also been carried out to study the effect of aeration by MBs on the degradation of organic matter, seed germination and growth. A visible difference in aerial vegetative plant growth is seen with MBs and macrobubble aeration (Park & Kurata 2009). MB aeration has also been known to boost the growth of white shrimps Litopenaeus vannamei and biofloc (Lim et al. 2021). Although small-scale MB experiments are suggestive of aeration intensification and its energy-saving capacity, however, it is intended to have further extended studies to replicate the results at larger scales and explore the efficiency of the process.

It is the most widely adopted means of removing contaminants of suspended oil, low density suspended solids and colloids in the wastewater treatment. The major steps leading to the separation are adsorption of gas bubbles on suspended particles forming bubble–particle aggregates. As formed aggregates are lighter, they rise to the water surface and can then be separated. Conventional air-flotation is limited by the bubble–particle interaction or collision and so can separate only a narrow particle size range. Several studies have shown that reducing the size of bubbles helped in increasing the overall efficiency of flotation separation by promoting bubble–particle collision and attachment (Moosai & Dawe 2003). Microbubble technology has been used to enhance the flotation process (Vion 2007). Experimental measurements have supported the application of smaller MBs to increase the mass-transfer coefficient in flotation and aeration process (Suwartha et al. 2020).

The separation of contaminants from oil-containing restaurant wastewater has been undertaken by the novel microbubble air flotation. It was reported that the highest oil removal efficiency was achieved when the microbubbles are of similar size as the oil droplets. The maximum removal efficiency of oil, COD and turbidity achieved with microbubble air flotation is 97.6%, 83.6% and 97.5%, respectively (Zheng et al. 2015a, 2015b). Liu et al. (2012b) reported a cost-effective and efficient ozone MB application in the coagulation-MB flotation process for the treatment of coke wastewater containing refractory organic compounds. Enhanced flotation degradation by ozone MB as compared to oxygen and air MB was attributed to its highest zeta potential and greater production of hydroxyl radicals (Liu et al. 2012b).

A superior microbial degradation and flotation separation has been suggested with microbubbles for wastewater treatment from the beverage industry. The study has supported the exploitation of bubble surface adsorption and flotation methods for the removal of high nitrogen-containing dissolved organic matter (DOM) from wastewater. As compared to macrobubbles, microbubbles have been shown to support the growth of aerobic bacteria and accelerate the degradation of DOM (Fukami et al. 2021).

Recently, an effective, and flotation simulation method has been proposed for the mixture optimisation between MBs and NBs in flotation arrangement. The laboratory-scale experimental study confirmed the deterioration effect of only NBs present to the water quality due to the long stagnation time of NB aggregate. However, the same study argued the improvement in removal efficiency of fine particles as small as 25 μm of nylon, polyvinyl chloride (PVC) and kaolin by hybrid bubbles (MB and NB). NBs help MBs to aggregate whereas MBs increase the removal efficiency of NB aggregates in the flotation process (Kim et al. 2020). It is envisaged that with the use of competitive microbubble technology in the flotation plants, the otherwise fraught field of flotation or physical separation is making a strong comeback as compared to large settling tanks set-up. The microbubble flotation technique is flourishing with clarification of lightly laden wastewater on the grounds of energy saving, compactness and ease of operation.

The hydroxyl ions accumulating on the microbubble gas–liquid interface is supposed to promote the self-decomposition of ozone and formation of (OH^.) hydroxyl radicals. This increased generation of OH^. with microbubbles contributed to enhanced degradation of organic contaminant like Atrazine (Liu et al. 2020). In a yet another study, 94% removal of diethyl phthalate (DEP) at pH of 7 and complete mineralisation was achieved at higher pH from ozone microbubbles. OH^. was believed to be the dominant reactive species responsible for oxidation of the DEP micropollutant as compared to the direct oxidation with molecular ozone (Jabesa & Ghosh 2016). Microbubble technology can enhance the bioremediation in ground water. The applicability of hydrogen microbubbles to enhance the process of hydrogenotrophic denitrification (HD) and removal of nitrogen without leaving behind residual carbon is portrayed. The hydrogen microbubble reactor performed better as compared to a millibubble reactor in the HD system, achieving as high as 99% nitrogen removal efficiency as compared to less than 10% removal efficiency in the latter. This technology also afforded to reduce the energy consumption by increasing the hydrogen utilisation efficiency to 50% for biological consumption and hydrogen effectiveness to reach 1.21 g-N/g-H₂ (Eamrat et al. 2020).

For complex effluents, a combination or hybrid treatment approaches have been utilised for achieving enhanced removal efficacy. A case study of real industrial effluent treatment with a combination of HC and oxidants is presented. The extent of reduction in COD value by the combination of HC and H₂O₂ (40%) was found to be significantly higher than the HC alone (7.9%) (Thanekar & Gogate 2019). Microbubble ozonation has been effective as a pre-treatment in a peat water treatment plant to remove carcinogenic disinfection by-products (DBPs) like trihalomethanes (THMs) and haloacetic acids (HAAs) formed during chlorination. The microbubble pre-ozonation treatment at pH 7 for 30 min decreased the concentration of THM4 to 33.73 ± 0.40 μg/L and that of HAA5 to 49.89 ± 0.09 μg/L acceptable as per USEPA standard (Qadafi et al. 2020).

The feasibility of microbubble ozonation and the presence of humic acids for abatement of pharmaceutical compound in feed water is systematically investigated. The degradation rate of the pharmaceutical compound was enhanced with the higher solubilisation rate of O₃ and OH^. and enhanced gas mass transfer, which is related to the smaller size of the microbubbles. The humic acid and temperature were found to have inhibitory and facilitatory effects, respectively, on the degradation efficiency (Lee et al. 2019).

A catalytic exhibition of microbubble ozonation for simulated printing and dyeing wastewater (methylene blue) is reported by Nkudede et al. (2021). A drastic increase in COD removal efficiency of up to 93.5% and fast decolourisation within 10 min time is recorded at high pH. To summarise, the implementation of microbubble technology is intended to be an efficient and eco-friendly approach to cut down on the chemical/oxidant dosage owing to excellent mass transfer as compared to conventional ozonation as respresented in Table 1.

Chlorine is one of the most used chemical disinfectants for treating drinking water. However, carcinogenic by-products of chlorine disinfection and its ineffectiveness in destroying hidden microorganism in bio-films are the major cause of concern. Apart from this, ultrasonication is known to be active in decomposing microorganisms by acoustic cavitation effect. The high energy shock waves produced by the gas bubble collapse form reactive oxygen species that can help in disintegrating the bacteria. But this technique is not of practical importance because of the allied cost factor. However, HC producing similar effects as acoustic cavitation can be a low-cost viable water treatment technology to be scaled up to an industrial level. Reflection from disinfection experimentation by air or ozone MB suggests faster E. coli inactivation kinetics, reduced reactor size and less ozone dose as compared to conventional ozonation systems (Sumikura et al. 2007). In yet another similar study, a novel ozonation system based on microbubble technology is suggested to overcome the lower utilisation efficiency associated with conventional ozonation disinfection. A reduction in operating cost was confirmed with the enhanced log reduction of Bacillus subtilis spores for the same ozone dosage by microbubble technology with high inlet ozone concentrations (Zhang et al. 2013).

Harmful algal blooms (HAB) consisting of algae and cyanobacteria because of their more frequent occurrence in water bodies like lakes and rivers are becoming an epidemic. The presence of these organisms in water results in decreases in dissolved oxygen content, penetration of light and the exchange of other gases which is harmful for the aquatic life. Certain chemical treatments are prevalent for remediation from HABs. However, these treatments disrupt the ecological balance of the environment. Again, some physical methods have also been employed in the past. But physical methods are ineffective against the toxins released as a result of algal cell damage. This sizeable challenge of removing algae as well as the generated toxins is very well taken up by the working miniatures called MBs (Seridou & Kalogerakis 2021). Hydrodynamic cavitation producing MBs and NBs is believed to be a feasible approach suggested for eliminating surface blooms and associated toxins. A pilot study on Lake Neatahwanta, New York, USA with field blooms suggested a 50% reduction in cyanobacteria chlorophyll after 72 h of microbubble treatment by HC. The higher decline percentage of 80% was reported with additional treatment with peroxides (Shaw 2020). A study by Thomas et al. (2019) showed the potential of MBs induced by small, inexpensive nozzles and water circulation systems to selectively destroy the harmful cyanobacterial bloom while leaving behind the beneficial algae that lacks gas vacuoles. This study demonstrated a dual phase treatment in which the strong turbulence/shear produced by HC damages the cyanobacterial membrane or cell wall of algae, while the free radicals generated in the MB process oxidised the cyanotoxins released from the disrupted cell wall. The efficacy of the treatment of algae was found to be dependent on inlet pressure of nozzles, treatment time, and type and concentration of algae. MB treatment was found to be more effective for removing vacuolated algae than vacuole-negative algae. This variability in treatment was attributed to the different cell structure and the presence of cellulose in cell walls and the absence of gas vacuoles, which is known to initiate the secondary production of free radicals (Thomas et al. 2019).

MBs have been utilised to treat both synthetic and real wastewater and have shown significant pollutant removal efficiency. Real wastewater is a complex mixture of different kinds of contaminants or pollutants such as hydrocarbons, pathogens, synthetic organic chemicals, toxic metals at the same time. The mixture of these contaminants has adverse effects on the treatment efficiency and complicates the treatment process. To ensure that improvements realised in pure water laboratory tests using MB system transverse across to more complex water matrices having organic matter, more experimental tests on real wastewater are to be carried out. For instance, air microbubbles have exhibited significant results in the treatment of dyeing wastewater. Compared to the conventional air bubbles, MBs have been shown to reduce the coagulant doses and treatment time and enhance the pre-treatment efficacy of coagulation–flotation. The removal efficiency of COD, colour and oil of 89, 72 and 99%, respectively, is achieved with coagulation microbubble flotation. After the coagulation microbubble flotation, the biodegradability index of wastewater also increased from 0.290 to 0.363 (Liu et al. 2010). In yet another study, the rate of decolourisation and organic reduction of practical textile wastewater was found to be much faster with MB ozonation. The COD removal efficiency with MB system was increased by 20% compared to the macrobubbles (Chu et al. 2008). An advanced treatment process with the novel combination of microbubble catalytic ozonation and biological process was successfully applied to treat real bio-treated coal chemical wastewater. The authors successfully demonstrated that catalytic microbubble ozonation results in 32.16% COD removal efficiency, consuming 1.38 mg of ozone per mg of influent COD removed with an ozone utilisation efficiency of 98%. Further biological treatment resulted in additional removal of biodegradable COD to 60.82% and release of inorganic nitrogen NH₃-N (Liu et al. 2018).

Another experiment examined the combination of MB and ultraviolet light irradiation to treat the river water containing refractory component from secondary effluent. This combination approach significantly improved the biodegradability of the water with increase in the amount of OH^• by almost 2–6 times compared with the conventional ozonation system. The enhanced degradation rate of refractory organics was seen with COD and UV, UV removal rates reaching as high as 37.50%, 81.15% and 94.74%, respectively (Gao et al. 2019).

There is an interesting back-and-forth relationship between treatment efficiency, cost and energy consumption in the selection of technology for large-scale application of microbubble technology. Each of these parameters is to be discussed in tandem in deciding the feasibility of the process. The reviewed data evidence a strong base for improvement through the implementation of microbubble system in ozonation, however, it must be emphasised that these data take into account the results of laboratory trials. The generation of ozone is portrayed as an energy demanding process with an energy requirement of 3.3–16 kWh/kg O₃ which covers 50–75% of the cost of total ozonation process.

Zhang et al. (2018) has reported a gas utilisation efficiency of 96% with 51 μm microbubble in a 32 cm deep tank. The enhanced gas utilisation suggests that a less amount of ozone is required to achieve an intended residual ozone concentration. So, microbubbles act to decrease the operational cost through decrease in the ozone input dose and minimized requirement of expensive off-gas destruction systems. Additionally, enhanced pollutant removal rates can offer opportunities for smaller dimensions of contact tanks and in this way reducing the capital investment and carbon footprint of the plant. Based on the minimum and maximum mass-transfer coefficient enhancement observed with microbubbles, Nam et al. (2021) and Zhang et al. (2018) have suggested the possibility of reducing the reactor volume by 16 and 81%, respectively, compared to systems using conventional-sized bubbles. However, no experimental work to date has normalised the reactor volume and tank depth when using microbubble systems.

Encouraging are findings of reduced reactor volume with microbubble generators in ozonation; and reducing the capital cost of ozonation and operational cost in areas including reduced chemical consumption and off-gas destruction makes the full-scale microbubble ozonation a feasible option. While there has been convincing data on enhanced gas utilisation and mass transfer using microbubbles, it has not been realised in practical situations as most of the experiments to date have utilised relatively shallow water depths compared with the commercial ozonation contactors with 3–7 m deep tanks.

Although the laboratory-scale set up does not give an accurate idea of energetics of upscaling, it can provide a suggestion on the probable trend. An economic efficiency study was carried out by Andinet et al. (2016) to assess the running cost–benefit of supplying air or oxygen microbubbles for aeration of 20 min. The air was found to be a more economical option at lower pressure conditions, whereas the cost of oxygen gas per unit dissolution decreased at higher pressure conditions. So, a more economical means of pressurised dissolution with a reduced gas flow rate was suggested to improve the natural water bottom area by oxygen MBs. An alleviation in membrane fouling is seen in the vacuum membrane distillation desalination process by MB aeration. Inclusion of MB aeration is merely contributing 2.8 ± 0.3% of the total energy consumption of the process at different pump pressures. MB aeration is shown to be evidently effective in improving the specific energy consumption, specifically while operating at increased pump pressures. Further investigation is recommended to yield a cleaner method of treatment without chemicals and to improve the specific energy consumption in all (Ye et al. 2019).

A low-cost, efficient mineralisation of total petroleum hydrocarbon (TPH) is achieved with microbubble ozonation by synergising oxidation and flotation phenomena. A 70.9% reduction in TPH was observed in 120 min of microbubble treatment with 0.27 gO₃/g total solid (TS). The total operation fee for this innovative process, including labour fee, power and additive consumption at the stated ozone dosage was calculated to be 3.44 × 10⁶ British pound sterling (GBP)/m³ TPH. This is much less when compared to other conventional oxidation processes (Sun et al. 2020). Again, a positive effect of HC in hybrid treatment is evident in a dye degradation experiment. The degradation of methyl orange dye reached as high as 90% by integrating HC with H₂O₂ and metal. The author reported that the energy required and hence the operating cost was 21 times smaller with this favourable integration of HC. The total operation cost to treat 1 m³ of wastewater containing 5 ppm of methyl orange was estimated to be 1707, 626 and 80 GBP/m³ for HC only, HC + H₂O₂, HC + H₂O₂ + metal system, respectively (Innocenzi et al. 2019).

Thanekar et al. (2020) compared the cavitation yield and operation cost of different types of treatments for degradation of dimethoate pollutant based on power consumption. The HC treatment alone was found to cost 8.5 GBP/m³ with a cavitation yield of 2.95 × 10⁻⁵ mg/J, whereas the combined treatment approach of HC + Fenton resulted in total operational cost of 1.2 GBP/m³ with a cavitation yield of 20.1 × 10⁻⁵ mg/J (Thanekar et al. 2020). In a similar study, the synergistic combination of HC and advanced oxidation processes (AOP) was applied for the treatment of industrial wastewater on a pilot scale (70 L capacity). Energy efficacy and cost analysis revealed a maximum COD removal efficiency of 63% in 180 min of tandem treatment estimating a total combined cost of 322.38 GBP/m³ for electricity and additive (Joshi & Gogate 2019).

An economical and suitable large-scale commercial operation of the HC/chlorine dioxide (ClO₂) composite process is proposed for the treatment of explosive-containing wastewater. The cost calculations and cavitation rate comparison suggest the reduced treatment cost of 8.77 GBP/m³ for HC/ClO₂ treatment as compared with 16.33 GBP/m³ for single cavitation along with an enhanced cavitation rate of 26.78 × 10⁻³ mg/J for the former (Wang et al. 2020). Table 2 shows the cavitation yield and operational costs of different treatment approaches based on energy requirement. The operational costs based on energy use data are converted to similar base units for comparative purpose. All costs were adjusted to the 2022 GBP. So, it could be concluded that combination treatment approaches help in cutting down the cost of the treatment.

Mukherjee et al. (2020) have reported an economically feasible process to recycle the grey water from a real-life kitchen stream. The HC treatment for 120 min resulted in a 25 and 15% reduction in COD and TOC value, respectively, which is thought to be higher than that produced using H₂O₂ and ozonation treatment. However, HC treatment efficiency was not enough to be adopted for real field study. A synergistic combination of HC with other AOPs like H₂O₂ and ozone increased % COD and TOC reduction to 98.25 and 76.26%, respectively which was acceptable for large-scale applications. Energy consumption and time needed for treatment is considered in tandem for technological integration and total cost in terms of operation cost and additive cost and is summarised in Table 3. All costs were adjusted to the 2022 GBP. Energetic and cost calculations support HC + H₂O₂ (5 g/L) as the most feasible treatment since the cost of this process is the least with comparable cavitation yield as HC + H₂O₂ + O₃ (Mukherjee et al. 2020).

A novel idea has been put forward by Ranade et al. (2021) to present a multi-scale modelling of HC devices for degradation of pollutants at four different scales (scaling of approximately 200 times of original capacity). The extended per-pass modelling described a decrease in degradation rate with increase in scale-up until a finite limiting value is reached.

Any specific conclusions cannot be drawn as to which hybrid technology is the best as such a judgement would be vague. A hybrid technology which is best suited for a particular type or nature of pollutant may not be as effective for an other type of pollutant. So, more efforts are to be directed to this end to further explore the sensible decision-making factors from the aspect of degradation performance, operating costs and energy savings in choosing the best hybrid technology to expand to industrial scale.

• Although much research and laboratory studies have shown the immense potential of microbubble technology in water treatment, the knowledge regarding the upscaling of methods and application of MB at the industrial level has been limited. More field size studies for extended periods are the need of the hour to evaluate the efficiency of the process and to obtain working knowledge and experience.

• Simulation studies on MB generation and reaction with contaminants are limited. Simulation aids in understanding the microbubble-assisted processes without carrying out the experiments. Simulations can be done before upscaling the process and can significantly cut the design and operation costs if used for optimisation of the process.

• Theoretical models have been developed to study the mass transfer and stability of MBs, but these models should be extended to comprehend the interaction of MBs with microorganisms and organic contaminants, taking into account the possible by-products of their reaction. Such studies would be beneficial for the optimal design of novel systems retrofitted with microbubble technology.

• The majority of the published literature has concentrated on mass-transfer efficiency of microbubble technology, but very less importance is being given to the heat transfer. MBGs during their course of operation are certainly generating heat (rise of the temperature of the medium), which could be prominent in several reaction mechanisms and can reduce the energetics of the treatment. This area can be further explored.

We identified the significant relevance of microbubbles in water treatment technology. The broad application of microbubble technology in water treatment is with reference to its enhanced gas solubility leading to efficient gas–liquid mass transfer and generation of oxidative free radicals. The application of microbubbles in disinfection entails from its dual phase treatment with mechanical shear produced during bubble collapse and oxidisation from free radical production. The microbubbles have been shown to selectively degrade harmful algae, leaving behind beneficial ones. The results of implementation of microbubble technology in aeration and ozonation overcoming the limitations of low gas utilisation efficiency and higher oxidant dosages of conventional methods have been predominantly encouraging. Many studies have stressed the role of microbubble technology in downsizing the treatment plant and reducing the operation costs of WWTPs along with increased contaminant removal efficacy. However, evidenced data from experiments correspond to the microbubble treatment in shallow water depths which may not be the same in the real scenario of WWTPs.

The cost and energy consumption analysis are indicative of the potency of microbubble technology combined with AOPs to replace the existing expensive processes in water treatment. However, the most effective treatment process and combination varies for different pollutants. More industrial-scale studies are recommended for bridging the gaps between laboratory-scale research and implementation at larger scales. A comprehensive assessment of degradation performance, energy efficiency and economic evaluation is to be carried out on real wastewater with different types and concentrations of pollutants to establish an optimised and distributed water treatment system. It is suggested that forthcoming studies should explore in detail the impact of treated water depth along with a reasonable and effective synergetic index of the microbubble/AOP hybrid system to establish a standardised approach for the comparison and clarification of best wastewater treatment strategy.

This project has received funding from the ‘THYME project’ programme.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.