DEVELOPMENT OF A FLUIDIC OSCILLATOR-DRIVEN FLOTATION SYSTEM

Treatment of liquid effluents is a serious challenge owing to the high stability and colloidal nature of the particles. In many applications, microbubbles (5 bars and consequently consuming ~90% of the total energy required in water purification plants. Other approaches in generating microbubbles for separation are not without challenges. One example is dispersed air flotation, which generates bubbles several orders of magnitude larger than the bubble exit pore and consequently unsuitable for flotation of these colloidal particles. These two concerns have been addressed in this research with the designing and development of a microbubble diffuser driven by a fluidic oscillator to facilitate microbubble generation suitable for flotation as well as investigating its performance for flotation applications. This fluidic oscillator converts continuous air supply into oscillatory flow with a regular frequency to generate bubbles of the scale of the exit pore. Bubble characterisation results showed that average bubble size generated under oscillatory air flow state from a 50 µm pore membrane was 86 µm, ~ twice the size of the diffuser pore size of 38 µm. In contrast, continuous airflow at the same rate through the same diffusers yielded an average bubble size of 1059 µm, 28 times larger than the pore size. In the first application, fluidic oscillator generated microbubbles were investigated for the separation of emulsified oil using Aluminium sulphate as the coagulant. The effect of surfactant concentration on oil droplet size was investigated. It was found that oil droplet size varied inversely proportional to surfactant concentration. In addition, it was found that the oil removal efficiency also depends on the surfactant concentration. The maximum oil removal efficiency by Microflotation was found to be 91% under lowest surfactant concentration tested (0.3 wt%) whilst at highest surfactant concentration used (10 wt%); lowest recovery efficiency (19.4%) was recorded. In the second application, the separation of algal cells under fluidic oscillator generated microbubbles was investigated by varying metallic coagulant types, concentration and pH. Best performances were recorded at the highest coagulant dose (150 mg/L) applied under acidic conditions (pH 5). Amongst the three metallic coagulants studied, ferric chloride yielded the overall best result of 99.2% under the optimum conditions followed closely by ferric sulphate (98.1%) and aluminium sulphate with 95.2%. The third application investigated the performance of Microflotation for the recovery of yeast cells from their growth medium at different pH levels, flocculant dose and varying bubble sizes. In this study, the food-grade-constituent- Chitosan was used as the flocculant. Results reaching 99% cell recovery were obtained under various conditions examined. Bubble size profiling showed an increase in average bubble size with diffuser pore size. Also, cell recovery efficiency was a function of both bubble size and particle size (cell size). For smaller particles (<50 μm), relatively smaller bubbles (<80 μm) were found to be more effective for recovery, otherwise, relatively larger bubbles (80-150 μm) proved to be efficient in recovering larger particles (particle size: ~250 μm). Acidic and neutral pHs were effective in separation as hydrophobic particles were formed. As pH tends towards alkalinity, flocs become more hydrophilic, leading to low recovery from the aqueous solution. In addition, separation efficiency was dependent on flocculant dose as increase in concentration improved flocculation and consequently, yeast recovery. However, above a critical concentration, overdosing occurred and inadvertently, recovery efficiency decreased. The results compare well with conventional dissolved air flotation (DAF) benchmarks, but has a highly turbulent flow, whereas Microflotation is laminar with several orders of magnitude lower energy density

Submicron gas bubbles in water

Gas bubbles smaller than 1 micrometre in water, commonly referred to as nanobubbles, is a growing field of research and innovation. Applications range from medical imaging and drug delivery to mining industry and environmental remediation. Despite much activity, important questions remain – which are the mechanisms that allow small gas bubbles to be stable against dissolution and are stable nanobubbles really as common and easily generated as is often claimed?This work demonstrates that several common nanobubble generation methods can generate particle agglomerates or oil droplets which can be mistaken for bubbles, whereas stable nano- and microbubbles are less easy to generate than commonly believed. The results further suggest that stable bubbles are normally stable due to a shell of surface-active organic compounds, whereas other proposed stability mechanisms are less likely. An unexpected finding was that sorbitan surfactant stabilized air nanobubbles can form long-lived bubble agglomerates.Holographic Nanoparticle Tracking Analysis (H-NTA) is demonstrated as a powerful new method to detect and differentiate between bubbles and particles in the same dispersion. As H-NTA determines the refractive index of tracked objects, bubbles will differ very significantly from solid particles or oil droplets. The method also enables detection of different populations of particles, agglomerates and oil droplets in the same dispersion

高濃度ヒドロキシルラジカルの製造技術及び生成したフリーラジカルの応用

本論文では、酸化力・殺菌力が強く、数分間存続する高濃度ヒドロキシルラジカル水の製造方法の開発及びヒドロキシルラジカルの基礎的研究成果について述べたものである。北九州市立大

The role of surface wettability on bubble formation in air-water systems.

The production of microbubbles is rapidly becoming of considerable global importance with many industries taking advantage of the increased mass transfer rates the bubbles can attain. Many factors have interrelated roles during bubble formation, with effects such as gas flow rate, liquid viscosity, pore size and pore orientation all imparting considerable influence during the formation process. Many of these features have been examined in detail and are relatively well understood. However, the role of surface wettability and the interactions at the gas-liquid-solid triple interface have for the most part been neglected, and it is the role of this wettability that is examined herein. Utilising the well-studied wet chemistry surface modification techniques of silanes and thiols, many substrates have been modified and the wettability tested. Contact angle goniometry has been utilised to assess the wetting characteristics of each substrate, and the role of surface roughness has been discussed in relation to both the static Young’s contact angle and the advancing and receding angles. Modified porous plates have been used to generate bubbles, with controlled single pore, multiple controlled pore, and multiple randomised pore systems being investigated. A steady flow of air was bubbled into distilled water through the various diffuser plates. It has been observed a contact angle of 90° is of vital importance, with a significant increase of bubble size above the 90° angle, defined as the hydrophobic wetting region. On the contrary, bubble size is greatly reduced below this angle, in the region defined as the hydrophilic region. The effect is seen to increase as the density of pores increases when the plate from which they are emitted is relatively smooth. Upon roughening, the effect is seen to diminish, and mechanisms for this process have been postulated. It is thought that the surface topography disrupts the modifying layers and also physically restricts the growing bubble, preventing the growth of the bubbles emitted from a hydrophobic surface. Attempts have been made to support this hypothesis both qualitatively and quantitatively. The fluidic oscillator of Zimmerman and Tesar has been examined, with numerous physical features being investigated. The oscillator was then added to the system to investigate the effect of wettability under substantial oscillation. It has been shown that the bubble size emitted from hydrophobic surfaces is significantly reduced when compared to the steady flow system. The effect is believed to be due to the ‘suction’ component of the oscillatory flow created by the oscillator. It has been seen via high speed photography that the growth rate of the growing bubble slows significantly as the flow begins to switch, before a reduction in size is seen as the gas is removed from the bubble. The opposing forces of buoyancy and suction act to elongate the bubble neck causing break off at a significantly reduced size. Although the diffuser plate is often observed to oscillate like the skin of a drum, this is not the predominant cause of the size reduction. Further experiments have been conducted using a synthetic actuator jet to create a pulsed air flow with only a positive component. Bubble size is not affected in this case, despite frequency sweeps being employed

Colloidal behavior of nanobubbles and their application in enhancing plant growth: mechanisms of nanobubble interactions with microbial and soil species

Climate change has resulted in increasing uncertainties of water resources and disturbance on agricultural activities. For example, the shortage of water resources, land erosion and pollution from runoff significantly affect agricultural sustainability. This dissertation research focuses on the fundamental studies of nanobubble (NB) water and explores the benefits for irrigation to enhance plant germination and growth. Unlike bulk bubbles, NBs exhibit prolonged stability in water and possess large surface areas that facilitate efficient mass transfer and potential tailored reactions (e.g., disinfection). However, the enhancement mechanisms for NBs on seed germination and plant growth remain elusive. This research first evaluated the membrane bubbling method to produce NBs in water and provided insights into the optimization of bubble water with desirable quality such as high bubble concentrations and small bubble sizes. The results demonstrate that the ceramic membranes with a hydrophilic surface and hydrophobic pores produced greater levels of NBs with small sizes compared to the pristine or surface hydrophobized membranes. Additionally, this study discovered that dissolution kinetics of oxygen NBs are strongly influenced by the initial bubble size and the dissolution could lead to shrinkage or expansion of bubbles in water. Smaller NBs exhibit a faster increase in DO, while larger NBs can result in higher equilibrium dissolved oxygen (DO) levels. Oxygen NBs significantly enhanced the oxygen transfer efficiency compared to microbubble aeration, exhibiting a remarkable increase of up to 300%, as well as a mass transfer coefficient of 21.05 h-1. Lastly, this study provides compelling evidence that NBs have a positive impact on seed germination and plant growth through changing various soil properties such as soil pH, oxygen content, redox potential and nutrient release, enzymatic activities and microbial communities. For example, oxygen NBs significantly boosted peroxidase activity in tomato leaves, with an impressive increase of 100%-1000%. The composition and structure of rhizosphere microbial communities in early tomato plants were found to be influenced by irrigation frequency, NB concentration, and the specific types of NBs used. Through discovering and characterizing these intriguing nanoscale phenomena and processes, this research aims to deliver new insight into novel sustainable agricultural practices using NB water that may increase agricultural production and reduce water and chemical fertilizer uses

Self-Propelled Micro/Nanomotors (MNMs) and Their Applications

The majority of the micro/nanomotors use the precious noble metal platinum for propulsion. However, platinum suffers from high-cost, scarcity, and possibility of deactivation in various media. In this thesis, we explored the MnO2 based materials for the fabrication of the high-performance and low-cost micro/nanomotors. These newly developed MnO2 based micromotors show great potential for replacing Pt and will greatly improves the applications of micro/nanomotors for biomedical science and environmental remediations areas

BIOFILM DEFOULING USING MICROBUBBLES GENERATED BY FLUIDIC OSCILLATIONS

The physical separation offered by membrane filters such as Reverse Osmosis (RO), Microfiltration (UF), Ultrafiltration (UF), and Nanofiltration (NF) has reduced the operating cost of such processes compared to distillation and chemical extraction. The advantages of the membrane such as high selectivity, high capacity, feasibility and cost effectiveness make them very good alternatives in separation industries especially cleaning technologies. Membranes, however, are easily fouled. Since the methods developed to defoul a membrane such as ultrasonic and chemical backflushing are always damaging to the membrane, this study is to explore the potential of microbubbles to restore the membrane to its operational condition. Microbubble clouds generated using fluidic oscillation produce non-coalescent bubbles, smaller and more uniform in size. Fluidic oscillation generated microbubbles are influenced by adjusting flow rate and oscillation frequency in conjunction with the diffuser pore size. The size of the microbubble produced is ranging from 30μm to 500μm at the lowest flow rate of air. The effect for cleaning purposes of microbubble injection with and without fluidic oscillation is explored by examination using Scanning Electron Microscopy (SEM), Total Suspended Solid (TSS) and system operational pressure drop (TMP). The smaller microbubble means higher surface contact area to remove the biofilm on the membrane filter. To further validate the effect of microbubbles on detaching and cleaning, FO generated microbubbles were sparged on biofilm (Chlamydomonas algae and HeLa cells) cultured on microscope slide surface. The detachment rates were compared by observing the density of algae and cells removed from the surface using lux meter and cell counting method. It is found that microbubbles generated using Higher Oscillation frequency of Fluidic Oscillator (HOFO) has a higher detachment and defouling rate. The highest defouling rate recorded for MF filter was 9.53mbar/min using HOFO, followed by 6.22mbar/min of microbubbles generated using Lower Oscillation frequency of Fluidic Oscillator (LOFO). Similar trends were observed in algae and cell detachment, the highest oscillation frequency of 335Hz has the highest detachment rate of 1.775lx/min and 1.7 ́104 cell/ml respectively. For MF systems, microbubbles generated using Higher Oscillation Frequency Oscillator (HOFO), increased the defouling rate by 64%. Similar observation recorded where HOFO increased detachment rate of Chlamydomonas algae and HeLa cells by 42% and 95% respectively