Microbubbles and their application to ozonation in water treatment: A critical review exploring their benefit and future application

Abstract

Ozonation is a widely applied water treatment process, used for oxidation of contaminants, as well as for the disinfection of water. However, the conventional ozonation process demands a high energy requirement and deep tanks to ensure effective mass transfer and oxidation.Microbubble technologies have emerged which have the potential to improve gas-liquid contacting. Microbubbles have diameters of 1-100 µm, while conventional bubbles used in ozonation are between 2 -6 mm. Microbubbles have many favorable characteristics that make them suitable for ozonation. In this review, the attributes of microbubbles for ozonation have been compared with those of conventional bubbles. The higher interfacial area and lower rise velocity of microbubbles compared with conventional bubbles means that ozone in the gas phase can be more efficiently transferred into the liquid phase. This is due to a higher contact time and increased contact area of the bubble with the bulk liquid. The analysis reveals that the volumetric mass transfer coefficient can be significantly enhanced through the use of microbubbles. In addition, the steady state dissolved ozone concentration was positively impacted by the use of microbubbles. Microbubbles were shown to be able to oxidize a broader range of organic compounds more quickly than for conventional bubbles. However, the review highlighted that comparison of microbubbles with conventional bubbles is not always carried

Are microbubbles magic or just small? a direct comparison of hydroxyl radical generation between microbubble and conventional bubble ozonation under typical operational conditions

“…Previous research has shown that the use of microbubbles in place of conventionally produced fine bubbles can enhance mass transfer [25], increase gas utilisation efficiency [38] and increase the rate and extent of compound removal [2] during ozonation. As such, they are now being considered to replace conventional bubble (2 -6 mm) systems [9].…”

“…This size difference results in a significantly higher interfacial area [22] and a decrease in rise velocity leading to an extended contact time for the microbubble systems compared to conventional systems [24]. These features allow for a faster and more extensive transfer of gas into the liquid phase [27] which can lead to enhanced treatment [9].…”

“…When ozone is dissolved in water it undergoes a series of pH dependent, complex self-decomposition reactions that lead, in part, to the formation of desirable hydroxyl radicals ( • OH). • OH are highly oxidising and non-selective and have been shown to be beneficial to the degradation of recalcitrant compounds [9]. A number of recent papers have proposed enhanced • OH generation from microbubbles as an additional benefit [11,15,18,25,31,23,39].…”

“…To test this a set of controlled experiments directly comparing • OH generation from microbubble and conventional bubbles were trialled in the same setup at three pH's which are most representative of operational ozonation pH's (6,7,8). The majority of research on microbubble ozonation is conducted in relatively short column (<1 m) laboratory trails at fixed ozone input doses which generates a positive bias toward microbubble systems [9]. The faster mass transfer associated with microbubble ozonation generates near complete transfer of the available ozone within 1 m and hence delivers a higher dissolved ozone concentration, generating a proportional increase in absolute • OH concentration.…”

“…The faster mass transfer associated with microbubble ozonation generates near complete transfer of the available ozone within 1 m and hence delivers a higher dissolved ozone concentration, generating a proportional increase in absolute • OH concentration. However, this does not translate to full scale systems where greater water depths are used (3 -7 m) resulting in less difference in overall transfer [9]. To obviate such bias the microbubble and conventional bubble systems were operated to achieve equivalent effective dissolved ozone concentrations and thus enable direct observation of any differential • OH generation.…”