Hydrodynamic effects of air sparging on hollow fiber membranes in a bubble column reactor Lijun Xia a,b, *, Adrian Wing-Keung Law a,b , Anthony G. Fane a,b a Singapore Membrane Technology Centre, 50 Nanyang Avenue, 639798 Singapore, Singapore b School of Civil and Environmental Engineering, Nanyang Technological University, 50 Nanyang Avenue, 639798 Singapore, Singapore article info Article history: Received 4 December 2012 Received in revised form 19 April 2013 Accepted 21 April 2013 Available online 2 May 2013 Keywords: Air sparging Alternate aeration Bubble column reactor Continuous aeration Hollow fiber membranes Hydrodynamics Superficial air velocity abstract Air sparging is now a standard approach to reduce concentration polarization and fouling of membrane modules in membrane bioreactors (MBRs). The hydrodynamic shear stresses, bubble-induced turbulence and cross flows scour the membrane surfaces and help reduce the deposit of foulants onto the membrane surface. However, the detailed quantitative knowledge on the effect of air sparging remains lacking in the literature due to the complex hydrodynamics generated by the gaseliquid flows. To date, there is no valid model that describes the relationship between the membrane fouling performance and the flow hy- drodynamics. The present study aims to examine the impact of hydrodynamics induced by air sparging on the membrane fouling mitigation in a quantitative manner. A modelled hollow fiber module was placed in a cylindrical bubble column reactor at different axial heights with the trans-membrane pressure (TMP) monitored under constant flux conditions. The configuration of bubble column without the membrane module immersed was identical to that studied by Gan et al. (2011) using Phase Doppler Anemometry (PDA), to ensure a good quantitative understanding of turbulent flow conditions along the column height. The experimental results showed that the meandering flow regime which exhibits high flow instability at the 0.3 m is more beneficial to fouling alleviation compared with the steady flow circulation regime at the 0.6 m. The filtration tests also confirmed the existence of an optimal superficial air velocity beyond which a further increase is of no significant benefit on the membrane fouling reduction. In addition, the alternate aeration provided by two air stones mounted at the opposite end of the diameter of the bubble column was also studied to investigate the associated flow dynamics and its influence on the membrane filtration performance. It was found that with a proper switching interval and membrane module orientation, the membrane fouling can be effectively controlled with even smaller superficial air velocity than the optimal value provided by a single air stone. Finally, the testing results with both inorganic and organic feeds showed that the solid particle composition and par- ticle size distribution all contribute to the cake formation in a membrane filtration system. ª 2013 Elsevier Ltd. All rights reserved. 1. Introduction Air sparging is now a standard approach to reduce concen- tration polarization and fouling of membrane modules in membrane bioreactors (MBRs). In the air sparged membrane system, shear stress is the critical parameter governing the foulant removal and enhancing the permeate flux. Shear stress is related to the viscosity and the velocity gradient at * Corresponding author. Singapore Membrane Technology Centre, 50 Nanyang Avenue, 639798 Singapore, Singapore. Tel.: þ65 96238187. E-mail address: XIAL0002@e.ntu.edu.sg (L. Xia). Available online at www.sciencedirect.com journal homepage: www.elsevier.com/locate/watres water research 47 (2013) 3762 e3772 0043-1354/$ e see front matter ª 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.watres.2013.04.042