Effect of bubble size

In early studies, coarse bubbles generated from coarse bubble diffusers with 6-8 mm pores showed the better membrane scouring performances than finer bubbles. It is because large bubbles rise faster than small bubbles as they experience less drag force per volume than smaller bubbles. Therefore, coarse bubbles have been predominantly used in MBR to scour membrane since horizontally mounted submerged hollow fiber membranes (SteraporeTM) were introduced to the market in 1990’s by Mitsubishi Rayon Co. of Japan.

Meanwhile some of the later studies have revealed that small bubbles were more effective than large bubbles in certain conditions. This implies that the greater number of small shear events is superior to the less number of large shear events while total air flow rate stays same. This observation was made particularly with flat sheet configuration (Sofia, 2004) and hollow fiber configuration with short fiber length (Fane, 2005a).

Due to the complexities of hydrodynamics in immersed membrane process, many conflicting information exist in literature. But, the optimum bubbles for commercial scale hollow fiber membranes appear consistently bigger than those for flat sheet membranes in general. It can be attributed to the different membrane scouring mechanisms of the two membrane configurations as will be discussed in this section. More details are described separately for flat sheet and hollow fiber membranes in this section.  

 

© Seong Hoon Yoon