Journal of Membrane Science 318 (2008) 435–440 Contents lists available at ScienceDirect Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci The effect of a non-woven scrim at the surface of a flat-sheet, microporous membrane on gas–liquid mass transfer Jos ´ e R. ´ Alvarez a , John W. Shanahan b , Michael J. Semmens b,∗ a University of Oviedo, Department of Chemical and Environmental Engineering, 33071 Oviedo, Spain b University of Minnesota, Department of Civil Engineering. 500 Pillsbury Drive SE, Minneapolis, MN 55455, USA article info Article history: Received 6 November 2007 Received in revised form 4 March 2008 Accepted 7 March 2008 Available online 19 March 2008 Keywords: Gas transfer Membrane Non-woven scrim Microelectrode abstract Non-woven scrims can be attached to membranes to provide a protective support for biofilm develop- ment and discourage complete biofilm sloughing. This work characterizes the impact of a ∼400-m thick, non-woven scrim at a membrane surface on gas–liquid oxygen transfer. The gas transfer characteristics of a conventional microporous membrane, with and without scrim attached to its surface, were measured experimentally. In addition, local changes in gas transfer behavior were characterized using microelec- trodes to measure dissolved oxygen profiles above each membrane. Reynolds numbers were varied from 50 to 1400 and a high gas flow rate was used to ensure a negligible drop in oxygen partial pressure across the membrane module. Higher mass transfer coefficients were generally obtained for the membrane without scrim. Indeed, the experimental performance dramatically exceeded that expected from theory, and we believe this was attributable to the module inlet design, and a localized boundary layer disruption. Dissolved oxygen pro- files along the membrane confirmed the expected boundary layer development. When a scrim was added it created a stagnant film close to the membrane surface but it appeared to interrupt the development of an overlying boundary layer. As a result, the scrim-covered membrane performed as well as the theo- retical predictions for a flat-sheet membrane. These data suggest that the presence of the scrim did not significantly impede gas transfer and may actually enhance gas transfer in long membrane modules. © 2008 Elsevier B.V. All rights reserved. 1. Introduction The use of gas-permeable membranes for oxygen delivery in wastewater applications has generated interest in recent years since they have a number of potential advantages over conven- tional bubble-aeration. Important potential advantages include lower energy requirements and a higher oxygen transfer efficiency. However, the design of a membrane-aeration system for wastewa- ter is complicated by bacterial colonization of membrane surfaces, which results in the formation of a membrane-aerated biofilm (MAB). These MABs can improve gas delivery by the membranes and recent modeling studies suggest that a biofilm thickness of about 400 m encourages the fastest oxygen transfer rates across the membrane [1]. Trying to control biofilm thickness to maintain optimal process performance has proven difficult however. Shear flow conditions, often employed to control biofilm thickness, can cause com- plete sloughing of MABs since the adhesive forces between the ∗ Corresponding author. Tel.: +1 612 625 9857; fax: +1 612 626 7750. E-mail address: semme001@umn.edu (M.J. Semmens). biofilm and the underlying membrane supports are weak [2]. Complete sloughing is detrimental since it causes a significant decline in gas transfer rate [1]. Sloughing may also cause the loss of slow growing aerobic bacteria, such as nitrifiers, near the membrane. Furthermore, depending upon the operating condi- tions, the biofilm that grows back may have a different microbial ecology. If a porous structure could be attached to the membrane sur- face to protect and secure a thin biofilm, then complete sloughing might be avoided. It is reasonable to assume, however, that adding a porous support structure to the membrane surface will impede gas transfer since the non-woven support creates a longer diffu- sion path. Previous investigators have tried to encourage and retain nitrifying biofilms by, surrounding hollow fiber membranes with a porous braided support for example [3], but they never character- ized the impact of the external support on gas transfer. If we are to understand how biofilms develop in the modified membranes, it is important to first understand how the addition of a support structure impacts the gas transfer behavior of the membrane itself. In this paper, we explore the impact of attaching a ∼400-m thick, non-woven scrim at the surface of a membrane on the gas transfer behavior of the membrane in a clean water environment. 0376-7388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2008.03.018