3.1.1 Morphological characteristics of microbubbles under
different process parameters
Smaller bubbles have a higher surface area to volume ratio, which leads
to higher probabilities of collision and attachment, a lower detachment
probability, lower ascending rate and higher free surface energy (Garg
et al., 2014; Hanotu et al., 2012). The optimal bubble size has been
reported as approximately 50 μm, and efficiency drops significantly with
larger bubbles (Cassell et al., 1975). This was attributed to the
reduced number of bubbles, and thus each bubble is required to remove
more cells resulting in decreased efficiency (Henderson et al., 2008).
Thus, the proportion of microbubbles within 50 μm was analyzed.
The effects of pressure, gas-liquid ratio, and length of the release
pipe on the particle size of microbubbles are shown in Table 2. The
particle size of the microbubbles produced under the working condition
2/3/4/5/7 are all within 100 μm, and the proportion of microbubbles
within 50 μm is more than 74%. The highest (80.58%) was obtained under
working condition 2. On the contrary, the particle size of microbubbles
produced by working condition 1/6/8/9 exceeds 100 μm, and more than 50%
of the microbubbles have a particle size greater than 50 μm. In
particular, the particle size distribution range of the microbubbles
produced under working condition 6 is the widest, with more than 80% of
the microbubbles exceeding 50 μm in size. When the
length of the release pipe is 10
cm, that is, working condition 3/5/7, the proportion of microbubbles ≤50
μm is more than 74%; When the length of the release pipe is 100 cm,
i.e., working condition 1/6/8, the proportion of microbubbles ≤50 μm is
less than 41%.