Nanobubble Research

Substantial boosts in the low-energy nano-oxygenation of incoming process water were achieved at a municipal wastewater treatment plant (WWTP) upstream of activated sludge (AS) aeration lanes on a single-pass basis by means of an electric field nanobubble (NB) generation method (with unit residence times of the order of just 10–15 s). Both ambient air and O2 cylinders were used as gas sources. In both cases, it was found that the levels of dissolved oxygen (DO) were maintained far higher for much longer than those of conventionally aerated water in the AS lane—and at DO levels in the optimal operational WWTP oxygenation zone of about 2.5–3.5 mg/L. In the AS lanes themselves, there were also excellent conversions to nitrate from nitrite, owing to reactive oxygen species (ROS) and some improvements in BOD and E. coli profiles. Nanobubble-enhanced Dissolved Air Flotation (DAF) was found to be enhanced at shorter times for batch processes: settlement dynamics were slowed slightly initially upon contact with virgin NBs, although the overall time was not particularly affected, owing to faster settlement once the recruitment of micro- particulates took place around the NBs—actually making density-filtering ultimately more facile. The development of machine learning (ML) models predictive of NB populations was carried out in laboratory work with deionised water, in addition to WWTP influent water for a second class of field-oriented ML models based on a more narrow set of more easily and quickly measured data variables in the field, and correlations were found for a more facile prediction of important parameters, such as the NB generation rate and the particular dependent variable that is required to be correlated with the efficient and effective functioning of the nanobubble generator (NBG) for the task at hand—e.g., boosting dissolved oxygen (DO) or shifting Oxidative Reductive Potential (ORP).

In bulk liquid or on solid surfaces, nanobubbles (NBs) are gaseous domains at the nanoscale. They stand out due to their extended (meta)stability and great potential for use in practical settings. However, due to the high energy cost of bubble generation, maintenance issues, membrane bio-fouling, and the small actual population of NBs, significant advancements in nanobubble engineering through traditional mechanical generation approaches have been impeded thus far. With the introduction of the electric field approach to NB creation, which is based on electrostrictive NB generation from an incoming population of “electro- fragmented” meso-to micro bubbles (i.e., with bubble size broken down by the applied electric field), when properly engineered with a convective-flow turbulence profile, there have been noticeable improvements in solid-state operation and energy efficiency, even allowing for solar-powered deployment. Here, these innovative methods were applied to a selection of upstream and downstream activities in the oil–water–fuels nexus: advanc- ing core flood tests, oil–water separation, boosting the performance of produced-water treatment, and improving the thermodynamic cycle efficiency and carbon footprint of internal combustion engines. It was found that the application of electric field NBs results in a superior performance in these disparate operations from a variety of perspectives; for instance, ~20 and 7% drops in surface tension for CO₂- and air-NBs, respectively, a ~45% increase in core-flood yield for CO₂-NBs and 55% for oil–water separation efficiency for air-NBs, a rough doubling of magnesium- and calcium-carbonate formation in produced- water treatment via CO₂-NB addition, and air-NBs boosting diesel combustion efficiency by ~16%. This augurs well for NBs being a potent agent for sustainability in the oil and fuels sector (whether up-, mid-, or downstream), not least in terms of energy efficiency and environmental sustainability.

An integrated approach is sorely needed to treat biogas emanating from anaerobic digesters (AD) which is cost-effective, in terms of upgrade/purification to ~95–98% methane needed for pipeline injection. This is a very pressing environmental and waste-management problem. At present, biogas water-/solvent-washing operations require significant capital investment, with high operational and maintenance costs. In the present study, we deployed a facile and efficient novel nanobubble-formation approach using applied electric fields to boost biogas-enrichment operations: we achieve substantial methane enrichment via selective CO₂ and H₂S take-up in water in the form of nanobubbles. This enables an integrated waste-processing vision using cutting-edge engineering-science advances, and making anaerobic digestion a circular-economic and practical reality, that can be deployed at scale—initially developing at the small scale—and points the way for low-energy CO₂ capture in the form of nanobubbles by dint of the electric-field approach. In addition, we carried out nanobubble generation using various gases for water treatment for both up- and down-stream sludge-containing (waste)water, achieving meaningful operational successes in AD operations and organic-fertiliser production, respectively.

Nanobubbles (NBs) are gaseous domains at the nanoscale that can exist in bulk liquid or on solid surfaces. They are noteworthy for their high potential for real-world applications and their long (meta)stability. “Platform-wide” applications abound in medicine, wastewater treatment, hetero-coagulation, boundary-slip control in microfluidics, and nanoscopic cleaning. Here, we compare and contrast the industrial NB-generation performance of various types of commercial NB generators in both water-flow and submerged-in-water settings—in essence, comparing electric- field NB-generation approaches versus mechanical ones—finding that the former embodiments are superior from a variety of perspectives. It was found that the electric-field approach for NB generation surpasses traditional mechanical approaches for clean-water NB generation, especially when considering the energy running cost. In particular, more passive electric-field approaches are very operationally attractive for NB generation, where water and gas flow can be handled at little to no cost to the end operator, and/or submersible NB generators can be deployed, allowing for the use of photovoltaic approaches (with backup batteries for night-time and “low-sun” scenarios and air-/CO₂-pumping paraphernalia).

The advent of air nanobubbles (ANBs) has opened up a wide range of commercial applications spanning industries including wastewater treatment, food processing, biomedical engineering, and agriculture. The implementation of electric field-based air nanobubbles (EF-ANBs) irrigation presents a promising approach to enhance agricultural crop efficiency, concurrently promoting environmentally sustainable practices through reducing fertilizer usage. This study investigated the impact of EF-ANBs on the germination and overall growth of agricultural crops in soil. Results indicate a substantial enhancement in both germination rates and plant growth upon the application of EF-ANBs. Notably, the introduction of ANBs led to a significant enhancement in the germination rate of lettuce and basil, increasing from approximately 20% to 96% and from 16% to 53%, respectively over two days. Moreover, the presence of EF-ANBs facilitates superior hypocotyl elongation, exhibiting a 2.8- and a 1.6-fold increase in the elongation of lettuce and basil, respectively, over a six-day observation period. The enriched oxygen levels within the air nanobubbles expedite aerobic respiration, amplifying electron leakage from the electron transport chain (ETC) and resulting in heightened reactive oxygen species (ROS) production, playing a pivotal role in stimulating growth signaling. Furthermore, the application of EF-ANBs in irrigation surpasses the impact of traditional fertilizers, demonstrating a robust catalytic effect on the shoot, stem, and root length, as well as the leaf count of lettuce plants. Considering these parameters, a single fertilizer treatment (at various concentrations) during EF-ANBs administration, demonstrates superior plant growth compared to regular water combined with fertilizer. The findings underscore the synergistic interaction between aerobic respiration and the generation of ROS in promoting plant growth, particularly in the context of reduced fertilizer levels facilitated by the presence of EF-ANBs. This promising correlation holds significant potential in establishing more sustainability for ever-increasing environmentally conscious agriculture.

Micro- and nanobubbles are tiny gas bubbles that are smaller than 100 μm and 1 μm, respectively. This study investigated the impact of electric field ozone nanobubbles (EF-ONBs) on the purification of both deionised and aquaculture water bodies, finding that heightened reactive oxygen species (ROS) production and oxygen reduction potential (ORP) are correlated to a higher production of EF-ONBs. In particular, it was found that there were substantially reduced ultraviolet light requirements for aquaculture when using EF-ONBs to maintain aquaculture purification standards. It is clear that the approximately exponential decay is slowed down by almost ten times by EF-ONBs even without UV applied, and that it is still roughly six times longer than the ‘control’ case of standard O3 sparging in water (i.e., meso- and macro-bubbles with no meaningful level of dispersed-phase, bubble-mediated dissolution beyond the standard Henry’s law state—owing mostly to rapid Stokes’ law rising speeds). This has very positive implications for, inter alia, recirculation aeration systems featuring an ozonation cycle, as well as indoor agriculture under controlled-light environments and malting, where ozonation cycles are also often used or contemplated in process redesign strategies. Such promising results for EF-ONBs offer, inter alia, more sustainable aquaculture, water sterilisation, indoor farming, and malting.

The oxygenation of calorific fuels for use in internal combustion engines is an area of increasing research, not only in terms of thermodynamic-cycle efficiency, but also with a view towards realising reductions in emissions of environmentally harmful chemicals, e.g., CO and NO_x gases. In the present work, notable progress has been achieved in the combustion efficiency of petroleum-containing air nanobubbles in an internal combustion engine, with 14% improvement achieved for combustion time on a constant-load basis in comparison to the status quo of regular petroleum. This was done by single-pass passage of atmospheric air through a tubular pipe-flow nanobubble generator based on the principle of electrostrictive capture therefrom.

The bubble-based aeration of water is very energy-intensive, and there is often rapid loss of oxygen from the dispersed (bubble) phase in water, owing to larger bubble sizes in the micro-, meso- and macro-range and their associated instability. This “nano-oxygenation” of water, in contrast, by air nanobubbles offers longer-lived levels of dissolved oxygen (both in traditional- and “nano”-dissolved senses). The present research details how the use of both submersible and continuous-flow air-nanobubble generators based on an electrostriction-generation principle can boost levels of total (i.e., traditionally- and nano-) dissolved air on a sustained and long-time basis.

In this study, molecular-dynamics simulations were used to evaluate the nano-bubbles/droplets generated in water by CO₂ hydrate dissociation. Two different simulation sets of NVT (isothermal) and NPT (isobaric/isothermal) were conducted for different CO₂ concentrations and compared. The results could confirm that the CO₂ agglomeration behavior changed during the simulation and observed the coexistence of CO₂ nano-bubbles and nano-droplets in both systems. At first, the nano-bubbles could only be observed in the system, but some of them were turned into nano-droplets as the simulation proceeded. The NVT system is feasible to generate nano-bubbles/droplets, and the local CO₂ densities are higher for the NVT system, and it can also generate slightly larger CO₂ agglomeration. A higher concentration of CO₂ in the solution caused more nano-droplets in both sets, while a lower concentration may only produce nano-bubbles.

This study aims to provide critical knowledge by investigating the dynamics of CO₂ nanobubbles generated by a sustained external electric field. The evolution of the CO₂ nanobubble size, population as well as the zeta potential were analysed from DLS measurements. Following a transient decay in the first few hours after NB generation, it was found that the number of NBs stabilised within 3-4 days after generation to the order of 10⁸ per ml, whilst Ostwald ripening and expansion took place into the 100-150 nm diameter range over the same timescale. Zeta-potential measurements also indicated that this metastability was consistent with these bubble developments over the intermediate relaxation period of 3-4 days. We anticipate that the findings from this study will support the development of efficient and sustainable strategies for various technologies associated with CO₂ gas.

The CO₂-bubble-based carbonation of water using established methods is very energy-intensive with the status quo of mechanical bubble generation, and there is rapid loss of CO₂ from the dispersed (bubble) phase in water, due to larger bubbles sizes in the micro-, meso- and macro-range and their associated instability. Instead, the “nano-carbonation” of water, by CO₂ fine bubbles (including CO₂nanobubbles) provides for longer-lived levels of dissolved CO₂. The present research details how the use of both submersible and continuous-flow nanobubble generators can boost levels of dissolved CO₂ far above Henry’s-Law level, and tracks longevity and decay of the dissolved-CO₂ levels in the water. In particular, in the case of the submersible fine-bubble generators, both mechanical and novel electrostriction (electric-field) approaches are used, whilst in the case of pipe-based generators, hydrodynamic approaches are used for fine-bubble carbonation, with optional application of electric field to boost longer-time dissolved-CO₂ levels. The decay times for the dissolved CO₂ were assessed, which were a great deal longer – magnitudes – than without nano-carbonation, owing to the nanoscale CO₂ keeping the overall dissolved-CO₂ concentration higher with slower Fick’s-Law release.