Enhancing Gas-Liquid Mass Transfer and (Bio) Chemical Reactivity using ultrafine/Nanobubble in Water and Wastewater Treatments

Abstract

Enhancing Gas-Liquid Mass Transfer and (Bio) Chemical Reactivity using ultrafine/Nanobubble in Water and Wastewater Treatments In the four major environmental spheres, phase involving processes are mostly responsible for the mass transfer of a given matter. The interaction between different phases mostly detect the outcome of a given natural or artificial process. Among the phase-related processes, gas–liquid phase interactions are often encountered in many environmental processes industries, water and wastewater treatment plants, gas absorption systems, aquatic system restoration techniques, aqua farming, surface cleaning and other chemical as well as petrochemical industries. In most of this processes the gas-liquid mass transfer efficiency is the process limiting factor. In this research, the possibility of intensifying mass transfer efficiency by applying ultrafine/nano bubbles is investigated giving a prior emphasis to its application in water and wastewater treatment techniques. To achieve this target, aeration, O3 based advanced oxidation and aerobic MBR water treatment techniques all having different gas-liquid mass transfer interaction theories are selected. Through studying the application of nanobubbles in these three different processes, the influence of ultrafine bubbles in: 1- systems with pure gas liquid mass transfer (no reaction), 2- systems with fast reaction in the liquid film and, 3- systems involving slow reactions in the bulk liquid are studied in different sections of the research. The influence of nanobubbles on the improvement of each system is addressed by comparing the final outcomes of applying ultrafine bubbles with that of the bubble types currently in use for the respective technologies. In the first chapter of the thesis, general information about the existing knowledge gap in the area of nanobubbles application in water technologies is discussed giving due emphasis on mass transfer. A brief summary of works relating bubble and the selected water treatment technologies is covered. Clear objectives and specific targets are described and presented in detail. Theoretical framework relating both bubble technology and mass transfer are summarized in the second chapter of the dissertation. In this section, clear definition on the size range of the different bubble categories is proposed. The proposal was necessary considering the lack of clear size range definition or overlapping of currently existing representations of the different bubble types. This definition is proposed from an intensive review work based on the agreement of most researchers on the properties shared by the range of bubble sizes. It is believed that at least it will help to clearly communicate among the chapters of this thesis if not used beyond to clarify the current existing overlapping definitions. Relevant bubble properties related to bubble mass transfer properties and that will be used in successive chapters of this dissertation are discussed briefly. Basic mass transfer theories and assumptions, for the selection of the two film theory for this research is also justified. The effect of nanobubbles in pure gas-liquid mass transfer is studied in chapter three. In this section, hydrodynamic splitting technique was applied for bubble generation. The influence of the design and operation parameters of the bubble generation unit on the bubble size reduction is studied. By determining the effect of design and operation parameters on the mass transfer indicators, an indirect analysis of the bubble size effect on mass transfer was induced. Furthermore, a regression model is developed to relate the design parameters of the bubble splitter with the volumetric mass transfer coefficient to better express the contributions of the major design and operational parameters. By the regression model, it was confirmed that bubble splitter flow path length, flow area and recycled water flow rate are the dominant contributors for bubble size change using hydrodynamic splitting. An increase in flow path length and gas-liquid mixture flow rate as well as reduction in flow area decrease the bubble size to nano scale. This in turn improved the volumetric mass transfer coefficient of a nanobubbled system up to 600% more compared to the conventional bubbled and/or 100-200 % compared to microbubbled systems. Process improvement possibilities for mass transfer involving fast reactions in ozone based advanced oxidation processes is covered in chapter four. For the semibatch system tested, nano bubbles show the tendency of suppressing the negative effects of temperature and pH on ozonation efficiency. The effect of increasing process temperature on reduction in solubility of ozone gas was reduced by application of nanobubbles. Similarly, ozone nanobubbles show high tendency of generating hydroxyl radical in acidic environment with higher concentration than microbubbles. This was not possible in previous studies using big bubbles, without addition of other hydroxyl radical formation reaction initiators. This indicates nanobubbles potential in initiating radical formation reactions which improve advanced oxidation processes in low pH medium. Moreover, mathematical model was developed based on the two-film theory to predict the volumetric mass transfer coefficient. The values of mass transfer coefficients showed a relative higher value for nanobubbles compared to microbubbles. Similarly, the ozone consumption rate was too high for nanobubbles even at low pH value. Based on this outcome, the concentration of hydroxyl radicals was checked and the result confirmed that the application of nanobubbles generate high concentration of hydroxyl radicals in all pH zones tested. This proofs that application of nanobubbles could improve advanced oxidation process. Finally, the benefit of nanobubbles over conventional was tested for aerobic wastewater treatment which involves a slow biological reaction process in the liquid phase. Theoretical floc models were adopted and modified to express the process of mass transfer in gas-liquid-solid phases. These models are supported by the results from different experiments on comparison of the rate of mass transfer, biological waste degradation rate, biomass growth rate and membrane filteration. In the nanobubble supported aerobic digester, the volumetric mass transfer coefficient was found to be 0.13 which is double to that of the conventional bubble supported system with the value of 0.07. Similarly, the oxygen uptake rate was also double to that of conventional bubble system. This improvement in mass transfer boosted the supply of oxygen to the biological reaction, reducing the limiting effect of oxygen shortage in the conventional aerobic digesters. Finally, because of the influence of nanobubbles, better efficiency of biomass growth, biological waste degradation, reduction in excess sludge production and reduction in membrane fouling possibility was achieved.