As shown in Fig. 2, the frequency distribution of microbubbles particle size under 9 working conditions concentrated in the range of 35~45 μm and 55~70 μm. The particle size of the microbubbles produced in condition 2/3/4/5/7 is concentrated in the front segment, while the particle size of the microbubbles produced in condition 1/6/8/9 is concentrated in the rear segment. The main difference between the two groups mainly lies in the length of the release pipe (10 cm VS 100 cm). The closer the distance between the pressure reducing valve and the air floating column, the shorter the residence time of the released microbubbles in the pipeline. On the one hand, the collision and merger between the microbubbles are reduced. On the other hand, the microbubble movement pressure loss is small. The above two aspects make the microbubbles more stable and the particle size distribution range is narrow.