The relationship between performance of submerged hollow fibers and bubble-induced phenomena examined by particle image velocimetry

Aeration is used in many membrane processes, especially in submerged membrane bio-reactors (MBRs). In order to reduce the costs of operation, it is important to optimize the bubble size and frequency. For a given airflow rate, it is possible to have many small bubbles at a high frequency, or a few larger bubbles at a lower frequency. The objective of this study was to determine the effects of bubbling flow on the performance of tight and loose submerged fibers. Parameters varied included the bubble size, bubble frequency, degree of looseness and the channel width. Particle image velocimetry was used to observe liquid flow patterns and measure the surface shear stresses as well as characterize the movement of the hollow fibers. Filtration behavior was examined for a model bentonite suspension at constant flux by monitoring transmembrane pressure (TMP) rise. For tight fibers, it was found that similar rates of increase of TMP can be achieved by using up to 10 times less air if small bubbles are used at a higher frequency. The reason for this was that although smaller bubbles produced a lower average shear stress, the fluctuations in the shear stress (characterized by the standard deviation) was a key factor in improving performance. The performance of the fiber was enhanced when a degree of looseness was allowed. For loose fibers, it was also found that using smaller bubbles at high frequency could provide the same performance at lower airflow rates. There was an optimum looseness for a single fiber when bubbling was applied. The bubble-induced phenomena that controlled performance for loose fibers appeared to be a combination of fiber acceleration and the standard deviation of the shear stress.