Browsing by Author "John, Alexander"

Now showing 1 - 4 of 4
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    Application of microbubbles to ozonation for drinking water treatment.
    (Cranfield University, 2022-11) John, Alexander; Jarvis, Peter; Carra ruiz, Irene; Jefferson, Bruce
    Ozonation is a widely used water treatment process that is used to oxidise contaminants as well as disinfect water. Conventional ozone contactors have a large energy requirement and deep tanks to ensure adequate mass transfer. As a result, the delivery of ozone into water is an energy intensive and expensive process. The use of microbubbles in water treatment is a new technology that has been shown to significantly improve gas-liquid contacting processes. Microbubbles have diameters ranging from 1 – 100 µm, whereas conventional bubbles used in typical ozone contactors have diameters ranging from 2 – 6 mm. Due to their small size, microbubbles have a larger interfacial area and a lower rise velocity than conventional bubbles. Therefore, ozone in the gas phase may be transported more efficiently into the liquid phase. Despite the favourable properties of microbubbles, the mechanism by which microbubbles outperform conventional bubbles is not fully understood, with various conflicting interpretations having been presented in the literature. This work is comprised of several direct-comparison studies of microbubble and conventional bubble ozonation systems under identical conditions. Experiments were normalised for both input and effective ozone dose in order to determine a number of critical performance parameters including: hydroxyl radical production, volumetric mass transfer coefficient, ozone self-decomposition, rate and extent of compound removal and bromate formation. Overall, the observed performance enhancement was attributed to an increase in the volumetric mass transfer coefficient through the combination of an increase in bubble specific interfacial area and a decrease in the mass transfer coefficient. When normalised to effective ozone dose, no enhancement in hydroxyl radical production or increase in bromate formation was observed. In addition, the generation of microbubbles results in a distribution of bubbles containing both micro- and nanobubbles. It was concluded that in order to optimise the overall ozonation process, emphasis should be placed on understanding how to manage the size distribution of the microbubble fraction as the risk of residual ozone from nanobubble survival was deemed insignificant. These findings were then applied to the design of microbubble contactors to determine the economic viability of microbubble generation when applied to ozonation at full scale compared with a conventional bubble ozone contactor.
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    Are microbubbles magic or just small? a direct comparison of hydroxyl radical generation between microbubble and conventional bubble ozonation under typical operational conditions
    (Elsevier, 2022-01-24) John, Alexander; Carra, Irene; Jefferson, Bruce; Jodkowska, Monika; Brookes, Adam; Jarvis, Peter
    The application of microbubbles for water treatment is an emerging technology which has been shown to significantly enhance gas–liquid contacting processes. When applied to ozonation, microbubble technology has been shown to enhance mass transfer and the speed and extent of compound removal compared with conventional bubbling techniques. One explanation as to why microbubble systems outperform conventional systems is that microbubbles shrink, collapse and spontaneously generate hydroxyl radicals which is thought to enhance the speed of compound removal. This study compared microbubble (mean diameter 37 μm) and conventional bubble (mean diameter 5.4 mm) ozonation systems under identical conditions. The experiments were normalised for effective ozone dose to determine whether microbubble ozonation generated significantly more hydroxyl radicals than conventional bubble ozonation. 4-chlorobenzoic was used as the hydroxyl radical probe and the proportion of hydroxyl radicals generated for a given effective ozone dose was quantified. The •OH-exposure to O3-exposure (the ) was used to compare the systems. The ratio of the mean to mean was 0.73, 0.84 and 1.12 at pH 6, 7 and 8 respectively. Statistical assessment of the showed that there was no significant difference between the bubble systems. No evidence was found to support the hypothesis that microbubble systems generate more •OH. Instead, the level of •OH-exposure is linked to the effective dose and pH of the system and future designs should focus on those factors to deliver •OH based benefits.
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    Enhancement of ozonation using microbubbles – Micropollutant removal, mass transfer and bromate formation
    (Elsevier, 2023-10-13) John, Alexander; Carra, Irene; Jefferson, Bruce; Bertolaso, Lucie; Brookes, Adam; Jarvis, Peter
    Microbubble technology is a promising development in the optimisation of gas–liquid contacting processes. When applied to ozonation, microbubbles have demonstrated significant enhancements to mass transfer, dissolved ozone residual and the speed and extent of compound removal. However, the mechanism by which microbubbles enhance performance over conventional bubbles is not well understood and numerous explanations exist within the literature. To elucidate the critical components that drive such enhancements the performance of microbubbles (Sauter mean diameter 37 µm) and conventional bubbles (5.4 mm) were compared under identical conditions in terms volumetric mass transfer coefficient, steady state dissolved ozone concentration, rate constant for ozone self-decomposition and the rate constant for degradation of two pesticides: mecoprop and metaldehyde. Overall, the improvement observed in performance can be attributed to the increase in the volumetric mass transfer coefficient through the combination of an increase in specific interfacial area and a decrease in the mass transfer coefficient. The increase in area outweighed the decrease in mass transfer coefficient such that an overall improvement factor of 1.6 was observed for microbubbles over conventional bubbles. All other differences were an artefact of the enhanced mass transfer leading to higher dissolved ozone concentrations when operating at a fixed input dose. For the first time it has been shown that when normalised to the amount of ozone transferred to the water, no enhancement in hydroxyl radical production, bromate formation or impact from the background constituents could be observed.
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    Understanding the difference between the nano and micro bubble size distributions generated by a regenerative turbine microbubble generator using ozone
    (Elsevier, 2025-02-01) John, Alexander; Brookes, Adam; Carra, Irene; Jefferson, Bruce; Jarvis, Peter
    There is a genuine paucity of data concerning the relative significance of the nano and the microbubble size distributions that are collectively generated when operating microbubble generation devices. Accordingly, the current work aimed to address this knowledge gap by measuring the two size distributions generated by a regenerative turbine microbubble generator using ozone and assess the relative significance of the nanobubble fraction. The microbubble fraction was measured with a focus-beam reflectance measurement device and the nanobubble fraction with a nano particle tracking instrument. The latter was calibrated using latex spheres to understand method uncertainty and to optimise the measurement approach. Sauter mean diameters of 217 nm and 37 μm were reported for the nano and microbubble fractions, respectively, with half of the microbubbles being <5000 nm in size. A comparison of the size and number concentrations of the different bubble types revealed that the majority of the gas was contained within the microbubble fraction, and hence, this controlled the overall mass transfer performance of the system. Further, the nanobubbles were observed to be stable for 18 h with little change in their size or number, indicating there was no net transfer of their gaseous contents. Overall, the work revealed that when considering enhancing gas-liquid mass transfer processes with micro-nano bubble generators, the microbubble fraction is key.