Effect of bubble size and frequency on mass transfer in flat sheet MBR

The major current application of bubbling in membrane processing is in membrane bioreactors (MBR), but limited data of a fundamental nature have been published for bubble interactions with submerged flat sheet membranes. There is a need to identify the quantitative relationship between bubble population properties and enhancement of mass transfer coefficient. Herein the transient mass transfer coefficient and average mass transfer coefficient were measured by an electrochemical method for a rectangular channel with and without air sparging. The dimensions (height = 1000 mm, width = 300 mm, gap = 20 mm) are similar to those of a flat sheet membrane system. A solenoid valve and associated time controller were used to generate bubbles with defined frequency and bubble volume. Bubble size and frequency were found to have a strong influence on the hydrodynamic characteristics. It was found that over the range of experiments that the dependency of mass transfer on bubble size and frequency has two regions, an increasing region and a plateau region. For water and the present gap, the enhancement increased with bubble size up to a value of 60 ml but was insensitive to size beyond that. The enhancement was relatively insensitive for a fixed bubble size provided a threshold frequency of 0.4 Hz was achieved. Further work is required with different gaps and at different viscosities. However the results already suggest that periodic slug bubbling in flat sheet system should give an effective method of fouling control at a substantial reduced air usage. The main air usage would then be fine bubbles for aeration. © 2009 Elsevier B.V. All rights reserved.